Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetics is the scientific study of heredity and variation Heredity i s the transmission

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Genetics is the scientific study of heredity and variation

• Heredity is the transmission of traits from one generation to the next

• Variation is demonstrated by the differences in appearance that offspring show from parents and siblings.

Figure 13.1


Inheritance of Genes

• Genes

– Are the units of heredity

– Are segments of DNA

– Recall that DNA is made up of repeating subunits called nucleotides (A C T G).

• Locus

– Each gene in an organism’s DNA has a specific locus (location) on a certain chromosome


• Gametes

– Are the reproductive cells that are the vehicles that transmit genes from one generation to the next

– Male Gamete=sperm, Female Gamete=egg

• We inherit

– One set of chromosomes from our mother and one set from our father


Comparison of Asexual and Sexual Reproduction

• In asexual reproduction (usually results in cloned closed individuals)

– One parent produces genetically identical offspring by mitosis

– Occurs in single celled and simple multi-cellular organisms such as the hydra

• In sexual reproduction

– Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

ParentBud

0.5 mm


Sets of Chromosomes in Human Cells

• In humans

– Each somatic cell has 46 chromosomes, made up of two sets (one set from each parent)

• A chromosome

– Is DNA that has been condensed and coiled


• A somatic cell

– Is any cell in a multi-cellular organism except for sperm or egg cells (e.g. liver and muscle cells)

– Have 23 pairs of chromosomes, one set from the father, the other from the mother, for a total of 46 chromosomes (2n=46)

– Each set of 23 consists of 22 sets of autosomes (a chromosome not directly involved in determining sex) and a single set of sex chromosomes.

• Males XY, Females XX


• Gametes (e.g. sperm and egg cells)

– Have only 23 chromosomes (n=23)

– 22 are autosomes and 1 is a sex chromosome.

• Male sperm cell can have X or Y

• Female egg cell can only have X


• What type of cell?

• Male or female?


Explanation # of Sets in Human Cells

Sex Chromosome Chromosome directly involved in determining sex

1

Autosome Chromosome not directly involved in determining sex

22

Y

X


5 µmPair of homologouschromosomes

Centromere

Sisterchromatids

• A karyotype

– Is an ordered, visual representation of the chromosomes in a cell

– Prepared from isolated somatic cells that are treated with drugs that stimulate mitosis

– Cells are then arrested in metaphase and stained

– Photographs are taken and scanned into the computer, and then chromosomes are electronically arranged according to size, shape and centromere position


• A karyotype shows:

– The two chromosomes in each pair (called homologous chromosomes or homologs)

– Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited haracters

– The sex chromosomes called X and Y


• Homologous chromosomes

– Are the two chromosomes that have the same staining pattern, centromere position and length

– Have the same characteristics, one from mother, one from father

– Autosomes are considered to be homologous, but sex chromosomes are not

• Only parts of the X and Y are homologous

• Most genes on X do not have counterparts on Y and vice versa


• A haploid cell (haploid number, n)

– Has only one homologous pair

– Human: n=23

• A diploid cell (diploid number, 2n)

– Has two sets of each homologous pair

– A human has 46 chromosomes (2n = 46)


• In a cell in which DNA synthesis (Interphase) has occurred

– All the chromosomes are duplicated and thus each consists of two identical sister chromatids

Figure 13.4

Key

Maternal set ofchromosomes (n = 3)

Paternal set ofchromosomes (n = 3)

2n = 6

Two sister chromatidsof one replicatedchromosome

Two nonsisterchromatids ina homologous pair

Pair of homologouschromosomes(one from each set)

Centromere


Behavior of Chromosome Sets in the Human Life Cycle

• At sexual maturity in animals

– The ovaries (eggs) and testes (sperm) produce haploid gametes by meiosis

• Gametes are the only types of human cells produced by meiosis, rather than mitosis

• Meiosis results in one set of chromosomes in each gamete


• During fertilization

– These gametes, sperm and ovum, fuse, forming a diploid zygote (2n)

• The zygote

– Develops into an adult organism by mitosis

– At differing points during development, cells will specialize

• Those cells that are destined for the testicles or eggs will undergo meiosis to develop gametes



Interesting…


Figure 13.5

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

The Human Life Cycle

• The purpose of meiosis:

– To produce 1 set of chromosomes in each gamete instead of two to compensate for the doubling that occurs at fertilization

– Produce genetic variability


• In animals

– Life cycle includes only diploid multi-cellular stages

– Gametes are the only haploid cells (unicellular)

– Meiosis occurs only during gamete formation

– Mitosis occurs only in multi-cellular organisms

Gametes

Figure 13.6 A

Diploidmulticellular

organism

Key


n

n

n

2n2nZygote

Haploid

Diploid

Mitosis

(a) Animals


The Variety of Sexual Life Cycles

• Plants and some algae exhibit an alternation of generations

– The life cycle includes both diploid and haploid multicellular stages

– The two generations are: gametophyte (n-haploid) and sporophyte (2n-diploid)


nn

n

nn

2n2n

Haploid multicellularorganism (gametophyte)

Mitosis Mitosis

SporesGametes

Mitosis

Zygote

Diploidmulticellularorganism(sporophyte)(b) Plants and some algae


• Polytrichum Moss

Sporophyte

Gametophyte


MitosisSpore making a gametophyte

MitosisGametophyte making gametes

MitosisSporophyte development


Alternation of generations

• Plants and some algae

– Plants have a life cycle that involves spores, which form as a result of meiosis. These spores are haploid. Notice that both haploid and diploid cells can divide by mitosis.

– Meiosis always begins with cells that are diploid (2n), and a result, daughter cells formed are always haploid (n).


The Stages of Meiosis

• An overview of meiosis

• Meiosis I

– Reduces the number of chromosomes from diploid to haploid

• Meiosis II

– Produces four haploid daughter cells

Figure 13.7

Interphase

Homologous pairof chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicatedchromosomes

1

2

Homologous chromosomes separate

Haploid cells withreplicated chromosomes

Sister chromatids separate

Haploid cells with unreplicated chromosomes

Meiosis I

Meiosis II


Centrosomes(with centriole pairs)

Sisterchromatids

Chiasmata

Spindle

Tetrad

Nuclearenvelope

Chromatin

Centromere(with kinetochore)

Microtubuleattached tokinetochore

Tertads line up

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Pairs of homologouschromosomes split up

Chromosomes duplicateHomologous chromosomes

(red and blue) pair and exchangesegments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

• Interphase and meiosis I

Figure 13.8


TELOPHASE I ANDCYTOKINESIS

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavagefurrow Sister chromatids

separate

Haploid daughter cellsforming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Two haploid cellsform; chromosomesare still doubleFigure 13.8

• Telophase I, cytokinesis, and meiosis II


Prophase I

• Synapsis: Duplicated homologous chromosomes line up and are held together by a protein called the synaptonemal complex. This eventually dissassembles in late prophase.

• Crossing Over: Non-sister chromatids exchange DNA segments

• Each pair of chromosomes forms a tetrad, a group of four chromatids.

• Each tetrad usually has one more more chiasmata or X shaped criss-cross regions where crossing over has occurred.


Metaphase – Mitosis vs. Meiosis

• In metaphase of mitosis, the sister chromatids are aligned at the metaphase plate, whereas in metaphase I of meiosis, the homologous chromosomes (tetrads) are lined up.


• Separation of homologues

– At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell

• Sister chromatids remain attached

• Resultant daughter cells (after anaphase I and telophase I) are haploid (n)

– In anaphase II of meiosis, the sister chromatids separate

– Chromosomes never duplicate in meiosis! (Interphase is not technically part of meiosis…or mitosis!)


Figure 13.9

MITOSIS MEIOSIS

Prophase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

Chromosomereplication

Parent cell(before chromosome replication)

Chiasma (site ofcrossing over)

MEIOSIS I

Prophase I

Tetrad formed bysynapsis of homologouschromosomes

Metaphase

Chromosomespositioned at themetaphase plate

Tetradspositioned at themetaphase plate

Metaphase I

Anaphase ITelophase I

Haploidn = 3

MEIOSIS II

Daughtercells of

meiosis I

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Daughter cells of meiosis II

n n n n

Sister chromatids separate during anaphase II

AnaphaseTelophase

Sister chromatidsseparate duringanaphase

2n 2nDaughter cells

of mitosis

2n = 6

• A comparison of mitosis and meiosis


Mitosis Meiosis

Role in Animal Body •Enables multi-cellular adult to arise from a zygote•Produces cells for growth and repair•Asexual reproduction (in some species)

•Produces gametes•Reduces number of chromosomes in half•Introduces variability among gametes

Number of DNA replications

0 0

Number of divisions 1 2

Number of Daughter Cells 2 4*

Chromosome Number of Daughter Cells

Diploid (2n) Haploid (n)


Spermatogenesis vs. Oogenesis


• http://www.youtube.com/watch?v=zGVBAHAsjJM


• The original source of genetic variation in individuals are mutations

– Most of these are repaired

• The vast majority of genetic variability is as a result of the “reshuffling of genetic material” that occurs in meiosis

– Independent Assortment

– Crossing Over

– Random Fertilization


Independent Assortment of Chromosomes

• Independent Assortment

– Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

– Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs


• Number of chromosomes possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

Key

Maternal set ofchromosomesPaternal set ofchromosomes

Possibility 1

Two equally probable arrangements ofchromosomes at

metaphase I

Possibility 2

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

n=2

22=4 combos of gametes


Crossing Over

• Crossing over

– Produces recombinant chromosomes that carry genes derived from two different parents

Figure 13.11

Prophase Iof meiosis

Nonsisterchromatids

Tetrad

Chiasma,site ofcrossingover

Metaphase I

Metaphase II

Daughtercells

Recombinantchromosomes


Random Fertilization

• Random Fertilization

• Fertilization: the fusion of gametes

– Any sperm can fuse with any ovum (egg), each containing different combination of genes


In humans…

• Number of possible different combinations of chromosomes in sperm:

223 = approx 8 million combos

• Number of possible different combinations of chromosomes in egg:

223 = approx 8 million combos

• Number of possible diploid combinations as a result of fertilization:

8 million x 8 million = 70 trillion

WE ARE SO UNIQUE AND SPECIAL!


Mutations!

Documents

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetics is the scientific study of heredity and variation Heredity i s the transmission