Cell Division

Sexual Reproduction =

OR

Asexual Reproduction =

Cell Division: reproduction of cells; “cells come from cells”

* Basis of all life2 Main Roles:

1) development of fertilized egg

2) continuation of life (growth, repair)

Prokaryotes = binary fission (split in half)

OR

Eukaryotes = more complex; more genetic material

Prokaryotes = binary fission (split in half)

OR

Eukaryotes = more complex; more genetic material

chromosome: structure which contains DNA (deoxyribonucleic acid)

chromatin: long, thin fibers of DNA & protein clumping together to form chromosomes

gene:

somatic cell: all body cells except egg & sperm; contain chromosomes

(humans= 46)Human egg & sperm (gametes) have 23

chromosomesPrior to Cell Division…

* All chromosomes duplicate…result in 2 identical parts = sister

chromatids (X-shaped)

* joined at centromere

Sister Chromatids

A chromosome and its identical replicated copy, joined at the

centromere.

When Cells Divide

* sister chromatids separate..each goes to separate cell (daughter

cell)

*

Overview of Cell Division

* eukaryotic cells divide according to cell cycle

cell cycle: sequence of events including time a cell divides until its daughter cell divide

6.4 A time for everything: the cell cycle.

Phases in the Cell Cycle

1) Interphase: most of cycle here

- chromosomes duplicate

- cell grows

2) Mitotic Phase: cell division phase

Includes Mitosis & Cytokinesis

* Mitosis unique to eukaryotes* Mitosis = continuous process but separated into defined stages

Stages of Mitosis

1) Prophase

- chromatin fibers coil to form discrete chromosomes

- sister chromatids

- nuclear membrane breaks near end2) Metaphase

- sister chromatids line up along center of cell

Stages of Mitosis

3) Anaphase

- sister chromatids separate & migrate to opposite ends of cell4) Telophase

- nuclear membrane reforms around chromosomes

Cytokinesis: division of cytoplasm

- usually occurs along with telophase

- daughter cells separate

Mitosis has just one purpose:

homologous chromosome: matched pair of chromosomes; same length, genes for same traits at same loci

e.g., each chromosome has gene for hair color at same loci, but the gene may be for any color of hair … impt pt = gene results in some color of hair

locus (loci = plural):

• homologous chromosomes have matching loci &

• One chromosome of each pair inherited from mother & father

Human Example

Somatic cells = 46 chromosomes

Sex Chromosomes

Human females 1 pair (2 XX)

Human male 1 pair (1X, 1Y)

• Are human male sex chromosomes homologous?

diploid cells: cells with 2 homologous sets of chromosomes in nucleus

total # chromosomes = diploid # = 2n

human diploid # = 46 (2x23=46)

• Humans = diploid animals because most of our cells = diploid (e.g., somatic cell)

• But, eggs & sperm are not diploid

gametes: egg & sperm cells (sexual reproduction only)

haploid cells: cells with 1 homologous set of chromosomes

haploid # = n

human haploid # = 23

• Human gametes are haploid

6.11 Sperm and egg are produced by meiosis: the details, step-by-step.

Mitosis occurs almost everywhere in an animal’s body. Meiosis only occurs

in one place. Where?

Meiosis starts with a diploid cell.

one of the specialized diploid cells in the gonads

Meiosis starts with a diploid cell.

a homologous pair, or homologues•the maternal and paternal copies of a

chromosome

Chromosomes are duplicated.

sister chromatids•Each strand and its identical

duplicate, held together at the centromere.

Cells undergoing meiosis divide twice.

There are two major parts to meiosis:

Meiosis Division 1

Separating the homologues

1. Prophase I

The most complex of all of the phases of meiosis

Crossing over

2. Metaphase I

Each pair of homologous chromosomes moves to the equator of the cell.

3. Anaphase I

Beginning of the first cell division that occurs during meiosis

The homologues are pulled apart toward opposite sides of the cell.

The maternal and paternal sister chromatids are pulled to the ends of the cell in a random fashion.

3. Anaphase I

4. Telophase I and Cytokinesis

This phase is marked by the chromosomes arriving at the two poles of the cell.

The cytoplasm then divides and the cell membrane pinches the cell into two daughter cells.

4. Telophase I and Cytokinesis

Meiosis Division 2

Separating the sister chromatids

5. Prophase II

The genetic material once again coils tightly making the chromatids visible under the microscope.

It is important to note that in the brief interphase prior to prophase II, there is no replication of any of the chromosomes.

6. Metaphase II

The sister chromatids (each appearing as an X) move to the center of the cell.

7. Anaphase II

The fibers attached to the centromere begin pulling each chromatid in the sister chromatid pair toward opposite ends of each daughter cell.

8. Telophase II

The cytoplasm then divides, the cell membrane pinches the cell into two new daughter cells, and the process comes to a close.

Outcome of Meiosis

the creation of four haploid daughter cells, each with just one set of chromosomes which contains a completely unique combination of traits

Why is there so much variety among species? (e.g., diversity in humans)

1) Independent orientation of chromosomes

- in Metaphase I --- way that tetrads line up is due to chance (random)

- Results in different possible combinations of chromosomes in gametes

- For humans = 8 million possible combos.!

2) Random fertilization (1 egg & 1 sperm)

What is probability that 1 of 8 million possible sperm fertilizes 1 of 8 million possible eggs????

Humans = (8 M) * (8 M) = 64 trillion possible combinations of chromosomes due to random fertilization!

3) Crossing Over

- can result in genetic recombination

genetic recombination: producing gene combinations different from those carried by original chromosomes

* During synapsis, tetrad formed – crossing over possible

1) homologous chromatids break at similar locations & chromatids join

2) h. chrom. separate at Anaphase I – crossing over

3) Meiosis II, sister chromatids separate

Mendelian Genetics

genetics = science of heredity

gene: specific region of genetic material (DNA) that provides provides the cell with a “map”

Goal: determine patterns of inheritance

Mendelian Genetics

Gregor Mendel – 1860’s monk

significant findings = offspring obtain discrete heritable factors (genes) from their parents

Mendelian Genetics

Gregor Mendel – 1860’s monk

-carefully chose organisms to study (garden pea), controlled pollinations, chose traits that were easy to observe, used statistical methods to analyze data

-significant findings = offspring obtain discrete heritable factors (genes) from their parents

Terms

self-fertilization: plant’s egg fertilized by it’s own pollen

cross-fertilization: plant’s egg fertilized by another plant’s pollen (hybridization)

P generation: parental generation

F1 generation: filial generation; hybrid offspring of the P generation

F2 generation: offspring produced by F1 generation via self-fertilization

Mendel’s Principles

1) Principle of Segregation – pairs of genes segregate during gamete formation; fertilization pairs genes again

monohybrid cross: cross of 2 individuals that differ in 1 trait

allele: alternate form of a gene found at same loci of homologous chromosomes

1) Principle of Segregation

Ex: Flower color (P = purple, p = white)

P = 1 Purple (PP) & 1 white (pp)

F1 = all Purple (Pp)

F2 = ¾ Purple (PP & Pp) ¼ white (pp)

homozygous: identical pair of alleles

heterozygous: 2 different alleles for a trait

phenotype: physical trait; appearance of organism; expressed as phenotypic ratio

genotype: genetic makeup of organism; expressed as genotypic ratio

• In the flower color example…..

What is the phenotypic ratio?

What is the genotypic ratio?

** For monohybrid cross… phenotypic ratio is always 3:1 & genotypic ratio is always 1:2:1

2) Principle of Independent Assortment

• each pair of alleles segregates independently during gamete formation

dihybrid cross: cross of 2 individuals that differ in 2 traits

2) Principle of Independent Assortment

ExampleP generation: Round (RR) & Yellow (YY) seeds = RRYY

Wrinkled (rr) & Green (yy) seeds = rryy

Gametes = RY and ry

F1 gen: All RrYy (Round & Yellow seeds)

Gametes = RY, Ry, rY, ryFemale

Male

RY

ry RrYy

2) Principle of Independent Assortment

Example (continued)F2 gen: (Do Punnett Square

Female

Male

RY ryRy rY

RY

Ry

rY

ry

2) Principle of Independent Assortment

Example (continued)F2 gen: (Do Punnett Square

Female

Male

RY ryRy rY

RY

Ry

rY

ry

RRYY

2) Principle of Independent Assortment

Example (continued)F2 gen: (Do Punnett Square

Female

Male

RY ry

RRYy

Ry rY

RY

Ry

rY

ry

RRYY RrYY RrYy

2) Principle of Independent Assortment

Example (continued)F2 gen: (Do Punnett Square

Female

Male

RY ry

RRYy

Ry rY

RY

Ry

rY

ry

RRYY

RRYy

RrYY RrYy

RRyy RryyRrYy

RrYY

rryyrrYyRryyRrYy

rrYyrrYYRrYy

Probabilities• Probability (chance) of an event occurring ranges

from 0 to 1

Probability = 0 = event will not occur

Probability = 1 = event will occur always

Tossing a Coin

What is the probability of getting a “tails”?

= 0.5 (1/2)

What is the probability of getting a “heads”?

= 0.5 (1/2)What is the probability of getting a “heads” or a “tails”?

= P(heads) + P(tails) = 0.5 + 0.5 = 1.0

Tossing 2 Coins

What is the probability of getting a “heads” on both coins?

= P(heads) x P (heads) = (0.5)*(0.5) = 0.25

Flower Color Example

F1 = Pp = 0.5 P & 0.5 p gametes

F2 = Pp x Pp

1 P (female) x 1 P (male) = 0.5 * 0.5 = 0.25 PP

1 P (female) x 1 p (male) = 0.5 * 0.5 = 0.25 Pp

1 p (female x 1 P (male) = 0.5 * 0.5 = 0.25 Pp

1 p (female) x 1 p (male) = 0.5 * 0.5 = 0.25 pp

• What is the probability of getting a heterozygote?• What is the probability of getting a homozygote?

Why are some flowers pink?

• Complete dominance = dominant & recessive alleles

• Incomplete dominance = F1 offspring have phenotype somewhere between that of the 2 parents = both alleles expressed

Ex: Flower color (R = red, r = white)

P = 1 Red (RR) & 1 white (rr)

F1 = all Reddish-White = Pink (Rr)

F2 = ¼ Red (RR), ¼ white (rr), ½ pink (Rr)

Incomplete Dominance

Pleiotropy vs. Polygenic Inheritance

• pleiotropy = 1 gene influence many traits

e.g., sickle-cell anemia = homozygous recessive disease

sickle-cell gene influences:

- shape of RBC’s

- health of heart, brain, spleen, kidneys• polygenic inheritance = many genes

influence 1 trait, e.g., skin color

- many genes interact to give diverse skin color ranging very dark to very light

Chromosomal Basis

Study Slide

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