Chapter 8:Chapter 8:

The Cellular Basis of Reproduction and The Cellular Basis of Reproduction and

InheritanceInheritance

IntroductionIntroduction

Stages of an Organism’s Life Cycle:Stages of an Organism’s Life Cycle:

DevelopmentDevelopment: : All changes that occur from a All changes that occur from a fertilized egg or an initial cell to an adult fertilized egg or an initial cell to an adult organism. organism.

ReproductionReproduction: : Production of offspring that Production of offspring that carry genetic information in the form of carry genetic information in the form of DNA, from their parents.DNA, from their parents.

Two types of reproduction:Two types of reproduction:

1. Sexual Reproduction1. Sexual Reproduction

2. Asexual Reproduction2. Asexual Reproduction

Types of reproduction:Types of reproduction:1. Sexual Reproduction: 1. Sexual Reproduction: Most common type of animal reproduction.Most common type of animal reproduction. Male and female Male and female gametesgametes or sex cells (sperm and egg cell) or sex cells (sperm and egg cell)

join together to create a fertilized egg or join together to create a fertilized egg or zygotezygote.. The offspring has genetic information from both parents.The offspring has genetic information from both parents. Offspring are genetically different from each parents and Offspring are genetically different from each parents and

their siblings.their siblings. AdvantagesAdvantages: :

Ensures Ensures genetic diversitygenetic diversity of offspring. of offspring. • Population Population more likelymore likely to survive changing environment. to survive changing environment.

DisadvantagesDisadvantages: : Cannot reproduce without a partner of opposite sex.Cannot reproduce without a partner of opposite sex. Considerable time, energy, and resources spent to find a suitable Considerable time, energy, and resources spent to find a suitable

mate. mate. Parents only pass on 1/2 (50%) of their genetic information to each Parents only pass on 1/2 (50%) of their genetic information to each

offspring. offspring.

2. Asexual Reproduction: 2. Asexual Reproduction: Production of offspring by a Production of offspring by a single parentsingle parent through: through:

SplittingSplitting: Binary fission in bacteria.: Binary fission in bacteria. BuddingBudding: Yeasts, plants: Yeasts, plants FragmentationFragmentation: Sea stars: Sea stars ParthenogenesisParthenogenesis: “Virgin birth”. Several insect species.: “Virgin birth”. Several insect species.

Offspring inherit DNA form one parent only.Offspring inherit DNA form one parent only. Offspring are Offspring are genetically identical to parent and siblingsgenetically identical to parent and siblings, ,

unless mutations occur.unless mutations occur. AdvantagesAdvantages: :

Can reproduce without a partner of opposite sex.Can reproduce without a partner of opposite sex. Don’t spend time, energy, and resources to find a suitable mate. Don’t spend time, energy, and resources to find a suitable mate. Parents pass on 100% of their genetic information to each Parents pass on 100% of their genetic information to each

offspring. offspring.

DisadvantageDisadvantage: : No genetic diversity of offspring.No genetic diversity of offspring.

• Population Population less likelyless likely to survive changing environment. to survive changing environment.

Cells Only Arise from Preexisting CellsCells Only Arise from Preexisting CellsNew cells are made through New cells are made through cell division:cell division:

Unicellular organismsUnicellular organisms (Bacteria, protozoa): (Bacteria, protozoa):Division of one cell into two new organisms through Division of one cell into two new organisms through binary fission or mitosis.binary fission or mitosis.

Multicellular organismsMulticellular organisms (Plants, animals): (Plants, animals):If sexual reproduction:If sexual reproduction:

1.1. Growth and developmentGrowth and development from from zygote or fertilized egg.zygote or fertilized egg. Original cell divides by Original cell divides by mitosismitosis to produce many cells, to produce many cells,

that are genetically identical to first cell.that are genetically identical to first cell.

Cells later develop specific functions (differentiation).Cells later develop specific functions (differentiation).

2.2. ReproductionReproduction requires: requires: Meiosis:Meiosis: Special type of cell division that will generate Special type of cell division that will generate

gametes or sex cells, with 50% of individual’s genetic gametes or sex cells, with 50% of individual’s genetic material.material.

BacteriaBacteria (Procarytoes) Reproduce Asexually by (Procarytoes) Reproduce Asexually by Binary FissionBinary Fission

Features of Bacterial DNAFeatures of Bacterial DNA

Single, relatively small circular chromosome:Single, relatively small circular chromosome:

About 3-5 million nucleotide base pairs About 3-5 million nucleotide base pairs Contains only about 5-10,000 genesContains only about 5-10,000 genes

Binary fissionBinary fission

Single circular DNA is Single circular DNA is replicatedreplicated Bacterium grows to twice normal sizeBacterium grows to twice normal size Cell divides into two Cell divides into two daughter cellsdaughter cells Each daughter cell with an identical copy of DNAEach daughter cell with an identical copy of DNA Rapid process, as little as 20 minutes.Rapid process, as little as 20 minutes.

Bacteria Reproduce Asexually by Binary Fission

Eucaryotic cell division is a more complex and time Eucaryotic cell division is a more complex and time consuming process than binary fissionconsuming process than binary fission

Features of Eucaryotic DNAFeatures of Eucaryotic DNA

1. DNA is in 1. DNA is in multiplemultiple linear linear chromosomeschromosomes.. Unique number for each species:Unique number for each species:

• Humans have 46 chromosomes.Humans have 46 chromosomes.

• Cabbage has 20, mosquito 6, and fern over 1000.Cabbage has 20, mosquito 6, and fern over 1000.

2. Large Genome: Up to 3 billion base pairs (humans) 2. Large Genome: Up to 3 billion base pairs (humans) Contains up to 50,000-150,000 genesContains up to 50,000-150,000 genes Human genome projectHuman genome project is determining the sequence of entire is determining the sequence of entire

human DNA.human DNA.

3. DNA is enclosed by 3. DNA is enclosed by nuclear membranenuclear membrane..

Correct distribution of Correct distribution of multiplemultiple chromosomes in each chromosomes in each daughter cell requires a much more elaborate process daughter cell requires a much more elaborate process than binary fission.than binary fission.

Human Body Cells Have 46 Chromosomes

ChromosomesChromosomes ChromatinChromatin

Tightly packaged DNATightly packaged DNA Unwound DNAUnwound DNA

Found only during Found only during cell cell FoundFound throughout cell throughout cell divisiondivision cyclecycle

DNA is DNA is notnot being used being used DNA is being used DNA is being used

for macromolecule for macromolecule for macromolecule for macromolecule

synthesis.synthesis. synthesis. synthesis.

DNA: Found as Chromosomes or Chromatin

Eucaryotic Chromosomes Duplicate Before Each Cell Division

Cell Cycle of Eucaryotic Cells Sequence of events from the time a cell is formed, Sequence of events from the time a cell is formed,

until the cell divides once again.until the cell divides once again. Before cell division, the cell must:Before cell division, the cell must:

Precisely copy genetic material (DNA)Precisely copy genetic material (DNA) Roughly double its cytoplasmRoughly double its cytoplasm Synthesize organelles, membranes, proteins, and other Synthesize organelles, membranes, proteins, and other

molecules.molecules.

Cell cycle is divided into two main phases:Cell cycle is divided into two main phases: InterphaseInterphase: Stage between cell divisions: Stage between cell divisions Mitotic PhaseMitotic Phase: Stage when cell is dividing: Stage when cell is dividing

Eucaryotic Cell Cycle: Interphase + Mitotic Phase

The Life Cycle of a Eucaryotic CellThe Life Cycle of a Eucaryotic Cell::

InterphaseInterphase: : Time between cell divisions. Time between cell divisions. Most cells spend about Most cells spend about 90% of their time90% of their time in interphase. in interphase. Cells actively synthesize materials they need to grow.Cells actively synthesize materials they need to grow. Chromosomes are duplicated.Chromosomes are duplicated. Interphase can be divided into three stages:Interphase can be divided into three stages:

1. G1. G11 phase: phase: Just Just after after cell division. cell division.

Cell grows in size, increases number of organelles, and Cell grows in size, increases number of organelles, and makes proteins needed for DNA synthesis.makes proteins needed for DNA synthesis.

2. S phase2. S phase: : DNA replication. DNA replication.

Single Single chromosomes are duplicatedchromosomes are duplicated so they contain two so they contain two sister chromatidssister chromatids..

3. G3. G22 phase: phase: J Just ust beforebefore cell division. cell division.

Protein synthesis increases in preparation for cell division.Protein synthesis increases in preparation for cell division.

Duplication of Chromosomes During S stage of Interphase

Single chromosome Two identical sisterchromatids joined by a centromere ( )

DNA replication duringS stage of Interphase

The Life Cycle of a Eucaryotic CellThe Life Cycle of a Eucaryotic Cell::

MitosisMitosis:: The process of eucaryotic cell division.The process of eucaryotic cell division.

Most cells spend less than 10% of time in mitosis.Most cells spend less than 10% of time in mitosis.

Mitosis is divided into four stages:Mitosis is divided into four stages:

1. Prophase1. Prophase: : Cell prepares for division.Cell prepares for division.

2. Metaphase2. Metaphase: C: Chromosomes line up in “hromosomes line up in “middlemiddle”” of cell.of cell.

3. Anaphase3. Anaphase: : Sister chromatids split and migrate Sister chromatids split and migrate to to opposite sides of the cell.opposite sides of the cell.

4. Telophase4. Telophase: : DNA is equally divided into two new DNA is equally divided into two new daughter cells. Cytokinesis usually occurs. daughter cells. Cytokinesis usually occurs.

CytokinesisCytokinesis: Division of cytoplasm.: Division of cytoplasm.

Mitotic PhaseMitotic Phase: : Mitosis + CytokinesisMitosis + Cytokinesis

Mitotic Phase: Mitosis + Cytokinesis

Mitosis:Mitosis: The Stages of Cell Division The Stages of Cell Division

1. Prophase1. Prophase Chromatin condenses into Chromatin condenses into chromosomeschromosomes, which appear , which appear

as two sister chromatids joined by a as two sister chromatids joined by a centromerecentromere..

Nucleoli disappear.Nucleoli disappear.

Nuclear envelope breaks apart. Nuclear envelope breaks apart.

In animal cells, In animal cells, mitotic spindlemitotic spindle begins to form as begins to form as

mictotubulesmictotubules grow out of two grow out of two centrosomescentrosomes or or

microtubulemicrotubule organizingorganizing centerscenters (MTOCs). (MTOCs).

• Each centrosome is made up of a pair of Each centrosome is made up of a pair of centriolescentrioles. .

Microtubules attach to Microtubules attach to kinetochoreskinetochores on chromatids and on chromatids and

begin to move chromosomes towards center of cell.begin to move chromosomes towards center of cell.

Centrosomes begin migrating to opposite poles of cell.Centrosomes begin migrating to opposite poles of cell.

Interphase and Prophase of Mitosis in Animal Cell

Mitosis:Mitosis: The Stages of Cell Division The Stages of Cell Division

2. Metaphase2. Metaphase

Short period in which chromosomes line up along Short period in which chromosomes line up along

equatorial plane of cell (equatorial plane of cell (metaphase platemetaphase plate).).

Chromosomes are completely condensed and easy to Chromosomes are completely condensed and easy to

visualize.visualize.

Mitotic spindle is fully formed.Mitotic spindle is fully formed.

Kinetochores of sister chromatids face opposite sides Kinetochores of sister chromatids face opposite sides

and are attached to spindle microtubules at opposite and are attached to spindle microtubules at opposite

ends of the cell.ends of the cell.

Metaphase, Anaphase, and Telophase of Mitosis in an Animal Cell

Mitosis:Mitosis: The Stages of Cell Division The Stages of Cell Division

3.Anaphase3.Anaphase

Centromeres of sister chromatids begin to Centromeres of sister chromatids begin to separateseparate..

Each chromatid is now an Each chromatid is now an independent daughter independent daughter

chromosomechromosome..

The separate chromosomes are pulled toward opposite The separate chromosomes are pulled toward opposite

ends by spindle microtubules, attached to the ends by spindle microtubules, attached to the

kinetochores.kinetochores.

Cell elongates as poles move farther apart.Cell elongates as poles move farther apart.

Anaphase ends when a complete set of chromosomes Anaphase ends when a complete set of chromosomes

reaches each pole.reaches each pole.

Mitosis:Mitosis: The Stages of Cell Division The Stages of Cell Division

4. Telophase4. Telophase

Cell continues to elongate.Cell continues to elongate.

Cell returns to interphase conditions:Cell returns to interphase conditions:

• A nuclear envelope forms around each set of A nuclear envelope forms around each set of

chromosomes.chromosomes.

• Chromosomes uncoil, becoming chromatin threads.Chromosomes uncoil, becoming chromatin threads.

• Nucleoli reappear.Nucleoli reappear.

• Spindle microtubules disappear.Spindle microtubules disappear.

CytokinesisCytokinesis usually occurs at the end of this stage usually occurs at the end of this stage

Mitotic PhaseMitotic Phase: Mitosis + Cytokinesis: Mitosis + Cytokinesis

CytokinesisCytokinesis

The division of cytoplasm to produce two daughter The division of cytoplasm to produce two daughter

cells. Usually begins during cells. Usually begins during telophasetelophase..

• In In animal cellsanimal cells: Division is accomplished by a : Division is accomplished by a

cleavage furrowcleavage furrow that encircles the cell like a ring in that encircles the cell like a ring in

the equator region.the equator region.

• In In plant cellsplant cells: Division is accomplished by the : Division is accomplished by the

formation of a formation of a cell platecell plate between the daughter cells. between the daughter cells.

Each cell produces a plasma membrane and a cell Each cell produces a plasma membrane and a cell

wall on its side of the plate.wall on its side of the plate.

Cytokinesis in Animal and Plant Cells

Animal Cell Plant Cell

External Factors Control MitosisExternal Factors Control Mitosis1. Anchorage1. Anchorage

Most cells cannot divide unless they are attached Most cells cannot divide unless they are attached to a solid surface.to a solid surface. May prevent inappropriate growth of detached cellsMay prevent inappropriate growth of detached cells

2. Nutrients and growth factors2. Nutrients and growth factors

Lack of nutrients can limit mitosisLack of nutrients can limit mitosis Growth factorsGrowth factors: Proteins that stimulate cell : Proteins that stimulate cell

division.division.

3. Cell density3. Cell density

Density-dependent inhibitionDensity-dependent inhibition:: Cultured cells will Cultured cells will stop dividing after a single layer covers the petri stop dividing after a single layer covers the petri dish. Mitosis is inhibited by high cell density.dish. Mitosis is inhibited by high cell density. Cancer cellsCancer cells do not demonstrate density inhibition do not demonstrate density inhibition

Density Dependent Inhibition of Mitosis

Normal Cells Stop Dividing at High Cell DensityCancer Cells are Not Inhibited by High Cell Density

Cell-Cycle Control SystemThere are three critical points at which the cell cycle There are three critical points at which the cell cycle

is controlledis controlled**::

1. G1 Checkpoint1. G1 Checkpoint: Prevents cell from entering S phase : Prevents cell from entering S phase and duplicating DNA.and duplicating DNA. Most important checkpoint.Most important checkpoint. Amitotic cells (muscle and nerve cells) are frozen here.Amitotic cells (muscle and nerve cells) are frozen here.

2. G2 Checkpoint:2. G2 Checkpoint: Prevents cell from entering mitosis. Prevents cell from entering mitosis.

3. M Checkpoint:3. M Checkpoint: Prevents cell from entering Prevents cell from entering cytokinesis.cytokinesis.

*Cells must have proper growth factors to get *Cells must have proper growth factors to get through each checkpoint.through each checkpoint.

Cell Division is Controlled at Three Key Stages

Growth factors arerequired to passeach checkpoint

Cancer is a Disease of the Cell Cycle

Cancer kills 1 in 5 people in the United States.Cancer kills 1 in 5 people in the United States. Cancer cells divide excessively and invade other Cancer cells divide excessively and invade other

body tissues.body tissues. Tumor: Tumor: Abnormal mass of cells that originates Abnormal mass of cells that originates

from uncontrolled mitosis of a single cell.from uncontrolled mitosis of a single cell. Benign tumor:Benign tumor: Cancer cells remain in original site. Can Cancer cells remain in original site. Can

easily be removed or treatedeasily be removed or treated

Malignant tumor:Malignant tumor: Cancer cells have ability to “detach” Cancer cells have ability to “detach” from tumor and spread to other organs or tissuesfrom tumor and spread to other organs or tissues

Metastasis:Metastasis: Spread of cancer cells form site of origin to Spread of cancer cells form site of origin to another organ or tissue. another organ or tissue. Tumor cells travel through blood vessels or lymph nodes.Tumor cells travel through blood vessels or lymph nodes.

Metastasis: Cancer Cells Spread Throughout Body

Functions of Mitosis in Eucaryotes: Functions of Mitosis in Eucaryotes:

1.1. Growth:Growth: All All somaticsomatic cells that originate after a cells that originate after a new individual is created are made by mitosis.new individual is created are made by mitosis.

2. 2. Cell replacement:Cell replacement: Cells that are damaged or Cells that are damaged or destroyed due to disease or injury are replaced destroyed due to disease or injury are replaced through mitosis.through mitosis.

3. 3. Asexual Reproduction:Asexual Reproduction: Mitosis is used by Mitosis is used by organisms that reproduce asexually to make organisms that reproduce asexually to make offspring.offspring.

Mitosis Replaces Dead Skin Cells

Chromosomes are matched in homologous pairsChromosomes are matched in homologous pairs

Homologous Chromosomes:Homologous Chromosomes:

Eucaryotic chromosomes come in pairs. Eucaryotic chromosomes come in pairs.

Normal humans have 46 chromosomes in 23 pairs.Normal humans have 46 chromosomes in 23 pairs.

One chromosome of each pair comes from an One chromosome of each pair comes from an

individual’s individual’s mothermother, the other comes from the , the other comes from the fatherfather..

Homologous chromosomes Homologous chromosomes carry genes that control the carry genes that control the

samesame characteristics characteristics.. Examples: Eye color, blood type, flower color, or heightExamples: Eye color, blood type, flower color, or height

LocusLocus: Physical site on a chromosomes where a given : Physical site on a chromosomes where a given gene is located.gene is located.

AlleleAllele: Different forms of the same gene.: Different forms of the same gene. Example: Alleles for blood types A, B, or O.Example: Alleles for blood types A, B, or O.

Homologous Pair of Chromosomes: One Comes From Each Parent

Homologous Chromosomes: Code for the Same Genetic Traits, but Have Different Alleles

There are two types of chromosomes:There are two types of chromosomes:

1. 1. AutosomesAutosomes: : Found in both males and females. Found in both males and females. In humans there are 22 pairs of autosomes.In humans there are 22 pairs of autosomes. Autosomes are of the same size and are homologous.Autosomes are of the same size and are homologous.

2. 2. Sex Chromosomes:Sex Chromosomes: Determine an individual’s gender. Determine an individual’s gender.

One pair of chromosomes (X and Y).One pair of chromosomes (X and Y). The X and Y chromosomes are not homologous. The X and Y chromosomes are not homologous.

The X chromosome is much larger than the Y chromosome and The X chromosome is much larger than the Y chromosome and

contains many genes. contains many genes.

The Y chromosome has a small number of genes. The Y chromosome has a small number of genes.

In Humans and other mammals females are XX and In Humans and other mammals females are XX and

males are XY.males are XY.

Chromosomes of Normal Human Male:44 (22 Pairs) Autosomes + XY

Normal Genetic Complement of Humans:Normal Genetic Complement of Humans:

Females: Females: 44 autosomes (22 pairs) + XX44 autosomes (22 pairs) + XX

Males: Males: 44 autosomes (22 pairs) + XY44 autosomes (22 pairs) + XY

NoteNote: In most cases, having additional or missing : In most cases, having additional or missing

chromosomes is usually fatal or causes serious defects.chromosomes is usually fatal or causes serious defects.

Down’s syndromeDown’s syndrome: Trisomy 21. Individual’s with an extra : Trisomy 21. Individual’s with an extra

chromosome 21. Most common chromosomal defect (1 in chromosome 21. Most common chromosomal defect (1 in

700 births in U.S.). Mental retardation, mongoloid facial 700 births in U.S.). Mental retardation, mongoloid facial

features, heart defects, etc.features, heart defects, etc.

Gametes have a single set of chromosomesGametes have a single set of chromosomes

Humans have Humans have two sets of chromosomestwo sets of chromosomes, , one one inherited from each parent.inherited from each parent.

Diploid Cells:Diploid Cells: Cells whose nuclei contain two Cells whose nuclei contain two homologous sets of chromosomes (2n). homologous sets of chromosomes (2n). Somatic cells are diploid (almost all cells in our body).Somatic cells are diploid (almost all cells in our body). In humans the diploid number (2n) is 46.In humans the diploid number (2n) is 46.

Haploid Cells:Haploid Cells: Cells whose nuclei contain a single Cells whose nuclei contain a single set of chromosomes (n).set of chromosomes (n). Gametes are haploid (egg and sperm cells).Gametes are haploid (egg and sperm cells). In humans the haploid number (n) is 23.In humans the haploid number (n) is 23.

Fertilization:Fertilization: Haploid egg fuses with a haploid Haploid egg fuses with a haploid sperm to form a diploid sperm to form a diploid zygotezygote (fertilized egg). (fertilized egg).

Meiosis Produces Haploid Gametes From Diploid Parents

Fertilization Produces Diploid Offspring from Haploid Gametes

Mitosis versus MeiosisMitosis versus MeiosisMitosisMitosis MeiosisMeiosis

OneOne cell divisioncell division TwoTwo successive cell divisionssuccessive cell divisions

Produces Produces twotwo (2) cells(2) cells ProducesProduces four four (4) cells(4) cells

Produces Produces diploiddiploid cellscells ProducesProduces haploidhaploid gametesgametes

Daughter cells are geneticallyDaughter cells are genetically Cells are geneticallyCells are genetically differentdifferent fromfromidenticalidentical to mother cellto mother cell mother cell and each othermother cell and each other

No crossing overNo crossing over Crossing over*Crossing over*

Functions:Functions: Growth, Growth, Functions:Functions: Sexual reproduction Sexual reproductioncell replacement, andcell replacement, andasexual reproductionasexual reproduction

**Crossing overCrossing over: : Exchange of DNA between homologous chromosomesExchange of DNA between homologous chromosomes..

Meiosis: Generates haploid gametesMeiosis: Generates haploid gametes Reduces the number of chromosomes by half, Reduces the number of chromosomes by half,

producing producing haploidhaploid cells from diploid cells. cells from diploid cells. Also produces Also produces genetic variabilitygenetic variability, each gamete is , each gamete is

different, ensuring that two offspring from the different, ensuring that two offspring from the same parents are never identical.same parents are never identical.

Two divisions: Meiosis I and meiosis II. Two divisions: Meiosis I and meiosis II. Chromosomes are duplicated in Chromosomes are duplicated in interphaseinterphase prior prior to Meiosis I. to Meiosis I. Meiosis IMeiosis I:: SeparatesSeparates the members of each the members of each homologous homologous

pair of chromosomespair of chromosomes. . ReductiveReductive division. division. Meiosis IIMeiosis II: : Separates chromatidsSeparates chromatids into individual into individual

chromosomes.chromosomes.

Interphase:Chromosomesreplicate

Meiosis I: Reductive division. Homologouschromosomes separate

Meiosis II: Sister chromatidsseparate

STAGES OF MEIOSIS

Meiosis I:Meiosis I: Separation of Homologous ChromosomesSeparation of Homologous Chromosomes

1. 1. Prophase I:Prophase I: Most complex phase of meiosis (90% of time)Most complex phase of meiosis (90% of time) Chromatin condenses into chromosomes. Chromatin condenses into chromosomes. Nuclear membrane and nucleoli disappear.Nuclear membrane and nucleoli disappear. Centrosomes move to opposite poles of cell and Centrosomes move to opposite poles of cell and

microtubules attach to chromatids.microtubules attach to chromatids. Synapsis:Synapsis: Homologous chromosomes pair up Homologous chromosomes pair up

and form aand form a tetradtetrad of 4 sister chromatids.of 4 sister chromatids. Crossing overCrossing over:: DNA is exchanged between DNA is exchanged between

homologous chromosomes, resulting inhomologous chromosomes, resulting in genetic genetic recombinationrecombination. . Unique to meiosisUnique to meiosis. .

ChiasmataChiasmata: : Sites of DNA exchange.Sites of DNA exchange.

Prophase I: Crossing Over Between Homologous Chromosomes

Meiosis I:Meiosis I: Separation of Homologous Separation of Homologous

ChromosomesChromosomes

2. 2. Metaphase I:Metaphase I: Chromosome tetrads (homologous Chromosome tetrads (homologous

chromosomes) line up in the middle of the cell.chromosomes) line up in the middle of the cell. Each homologous chromosome faces opposite Each homologous chromosome faces opposite

poles of the cell.poles of the cell.

Meiosis I: Homologous Chromosomes Separate

Stages of Meiosis: Meiosis IStages of Meiosis: Meiosis I 3. 3. Anaphase I:Anaphase I: Chromosome tetrads split up.Chromosome tetrads split up. Homologous chromosomesHomologous chromosomes of each pairof each pair separateseparate, ,

moving towards opposite poles.moving towards opposite poles. Random assortmentRandom assortment: One chromosome from each : One chromosome from each

homologous pair is shuffled into the two daughter homologous pair is shuffled into the two daughter cells, randomly and independently of the other pairs. cells, randomly and independently of the other pairs.

Random assortmentRandom assortment increases genetic diversityincreases genetic diversity of of offspring. Possible combinations:offspring. Possible combinations: 22nn..

One human cell can generate 2One human cell can generate 22323 or over 8.3 million or over 8.3 million different gametes by random assortment alone.different gametes by random assortment alone.

Random Assortment of Homologous Chromosomes During Meiosis I Generates Many Possible Gametes

Meiosis IMeiosis I: : Separation of Homologous Separation of Homologous

ChromosomesChromosomes

4. 4. Telophase I and Cytokinesis:Telophase I and Cytokinesis: Chromosomes reach opposite poles of the cell.Chromosomes reach opposite poles of the cell. Nucleoli reorganize, chromosomes uncoil, and Nucleoli reorganize, chromosomes uncoil, and

cytokinesis occurs.cytokinesis occurs. New cells are haploid.New cells are haploid.

Meiosis IIMeiosis II: : Separation of Sister ChromatidsSeparation of Sister Chromatids During interphase that follows meiosis I, no DNA During interphase that follows meiosis I, no DNA

replication occurs.replication occurs.

Interphase may be very brief or absent.Interphase may be very brief or absent.

Meiosis II is very Meiosis II is very similar to mitosissimilar to mitosis..

1. 1. Prophase II:Prophase II: Very brief, chromosomes reform.Very brief, chromosomes reform. No crossing over or synapsis.No crossing over or synapsis. Spindle forms and starts to move chromosomes Spindle forms and starts to move chromosomes

towards center of the cell.towards center of the cell.

Meiosis IIMeiosis II: : Separation of Sister ChromatidsSeparation of Sister Chromatids

2. 2. Metaphase II:Metaphase II: Very brief, individual chromosomes line up in Very brief, individual chromosomes line up in

the middle of the cell.the middle of the cell. Kinetochores of chromatids face opposite Kinetochores of chromatids face opposite

poles.poles.

3. 3. Anaphase II:Anaphase II: Chromatids separate and move towards Chromatids separate and move towards

opposite ends of the cell.opposite ends of the cell.

Meiosis II: Separation of Sister Chromatids

Meiosis IIMeiosis II: : Separation of Sister ChromatidsSeparation of Sister Chromatids

4. 4. Telophase II:Telophase II: Nuclei form at opposite ends of the cell.Nuclei form at opposite ends of the cell. Cytokinesis occurs.Cytokinesis occurs.

Product of meiosisProduct of meiosis: :

Four (4) haploid gametes, each genetically Four (4) haploid gametes, each genetically different from the other.different from the other.

Meiosis Produces Four Genetically Different Gametes

Meiosis in Males and Females Meiosis in Males and Females

Spermatogenesis:Spermatogenesis: Four sperm cells are made.Four sperm cells are made. Starts in puberty and occurs continuously.Starts in puberty and occurs continuously. Males produce millions of sperm cells a month.Males produce millions of sperm cells a month.

OogenesisOogenesis:: Only one large egg is produced. The other three Only one large egg is produced. The other three

cells are small polar bodies. cells are small polar bodies. Oogenesis starts before birth in females, stops at Oogenesis starts before birth in females, stops at

Prophase I, and resumes during puberty.Prophase I, and resumes during puberty. Meiosis is completed only after fertilization.Meiosis is completed only after fertilization. Females make one mature egg/month.Females make one mature egg/month.

Mitosis versus Meiosis (Review)Mitosis versus Meiosis (Review)MitosisMitosis MeiosisMeiosis

OneOne cell divisioncell division TwoTwo successive cell divisionssuccessive cell divisions

Produces Produces twotwo (2) cells(2) cells ProducesProduces four four (4) cells(4) cells

Produces Produces diploiddiploid cellscells ProducesProduces haploidhaploid gametesgametes

Daughter cells are geneticallyDaughter cells are genetically Cells are geneticallyCells are genetically differentdifferent fromfromidenticalidentical to mother cellto mother cell mother cell and each othermother cell and each other

No crossing overNo crossing over Crossing over*Crossing over*

Functions:Functions: Growth, Growth, Functions:Functions: Sexual reproduction Sexual reproductioncell replacement, andcell replacement, andasexual reproductionasexual reproduction

**Crossing overCrossing over: : Exchange of DNA between homologous chromosomesExchange of DNA between homologous chromosomes..

Crossing Over in Meiosis Increases Genetic Diversity

Sources of Genetic Variability in Sources of Genetic Variability in

Sexual ReproductionSexual Reproduction1. Crossing Over1. Crossing Over: After crossing over and synapsis, : After crossing over and synapsis,

sister chromatids are no longer identical.sister chromatids are no longer identical.

2. Independent Assortment2. Independent Assortment:: Each human can Each human can produce over 8.3 million different gametes by produce over 8.3 million different gametes by random shuffling of chromosomes in meiosis I.random shuffling of chromosomes in meiosis I.

3. Fertilization3. Fertilization:: A couple can produce over A couple can produce over 64 64 trilliontrillion (8.3 million x 8.3 million) different zygotes (8.3 million x 8.3 million) different zygotes during fertilization. This figure during fertilization. This figure does notdoes not take into take into account diversity created by crossing over.account diversity created by crossing over.

Accidents During Meiosis Can Cause Accidents During Meiosis Can Cause Chromosomal AbnormalitiesChromosomal Abnormalities

NondisjunctionNondisjunction: Chromosomes fail to separate.: Chromosomes fail to separate. Members of a pair of homologous chromosomes fail to Members of a pair of homologous chromosomes fail to

separate during meiosis I or:separate during meiosis I or: Sister chromatids fail to separate during meiosis II. Sister chromatids fail to separate during meiosis II.

Nondisjunction Nondisjunction increasesincreases with with ageage.. Gametes (and zygotes) will have an extra Gametes (and zygotes) will have an extra

chromosome, others will be missing a chromosome.chromosome, others will be missing a chromosome. TrisomyTrisomy: Individuals with one extra chromosome, three : Individuals with one extra chromosome, three

instead of pair. Have 47 chromosomes in cells.instead of pair. Have 47 chromosomes in cells. MonosomyMonosomy: Missing a chromosome, one instead of pair. : Missing a chromosome, one instead of pair.

Have 45 chromosomes in cells.Have 45 chromosomes in cells.

Nondisjunction of Chromosomes During Meiosis Produces Abnormal Gametes

Accidents During Meiosis Can Accidents During Meiosis Can Result in a Trisomy or MonosomyResult in a Trisomy or Monosomy

Most abnormalities in numbers of Most abnormalities in numbers of autosomesautosomes are are very serious or fatal.very serious or fatal. Down’s syndromeDown’s syndrome: Caused by a : Caused by a trisomytrisomy of of

chromosome number 21 (1 in 700 births). Mental chromosome number 21 (1 in 700 births). Mental retardation, mongoloid features, and heart defects.retardation, mongoloid features, and heart defects.

Most abnormalities of Most abnormalities of sex chromosomessex chromosomes do not do not affect survival.affect survival. Klinefelter SyndromeKlinefelter Syndrome: Males with an : Males with an extraextra sex sex

chromosome (XXY) (1 in 1000 male births).chromosome (XXY) (1 in 1000 male births). Turner SyndromeTurner Syndrome: Females : Females missingmissing one sex one sex

chromosome (XO) (1 in 2500 female births).chromosome (XO) (1 in 2500 female births).

Down’s Syndrome is More Common in Children Born to Older Mothers

Abnormal Numbers of Sex Chromosomes Usually Do Not Affect Survival

Klinefelter Syndrome (XXY) Turner Syndrome (XO)Incidence: 1:1000 male births Incidence: 1 in 2500 female births