Mitosis – growing & replacing cells

Meiosis – producing gametes Cell Division

Always follows Replication

Observations • Late 1800’s; cell

division • Researchers saw

thread-like structures called chromosomes (chromo – colored; soma – body) pairing up and separating just prior to cell division

Observations on chromosome # • Total number of chromosomes differ

between species – Roundworms = 4; Peas = 14; Humans = 46

• Same between individuals within a species – All roundworms = 4

• Same between cells within an individual

In Eukaryote cell

• Unique types of

chromosomes – Based on size & shape

• Each distinct type has a twin (homolog)

• N= # of unique types – N = ?

• Ploidy = How many sets of unique chromosomes

In Eukaryote cells

• 1 set of unique types

= haploid cell • 2 sets of unique

types = diploid • 3 set = ?

Clicker Q • If n = 15, how many chromosomes

does a triploid organism have? 1. 15 2. 30 3. 45 4. 60 How many complete sets of chromosomes does it have?

Chromosome terminology

• Chromosome* • Centromere • Homolog

Chromosome terminology

• Chromosome* • Centromere • Homolog • Sister chromatids

(exact replicas)

• Nope. Usually exist as unwound, invisible, stringy, chromatin

Do chromosomes always look like that?

Chromatin

• Unwound, unreplicated DNA with genes on them

When does Mitosis occur? • Fraction a of cell’s life • Chromosome

replication (DNA synthesis) is separate from Mitosis

• Significant time spent in Gap phases (G1, G2)…duplicating organelles, growing, etc.

Major events of Mitosis • Chromosomes have already replicated!! • Sister chromatids separate --> equal

allocation of chromosomes to daughter cells – Remember, the point is to make a cell that

is an exact replica of the parent

Stages of Mitosis

1. Chromosomes replicate (S)

2. Centrosomes replicate (M)

3. Chromosomes condense (M)

Interphase

1. Chromosomes replicate (S)

2. Centrosomes replicate (M)

3. Chromosomes condense (M)

Prophase

Prometaphase

Metaphase

Anaphase

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Summary • Homologs do not associate. • All chromosome align linearly on

metaphase plate. • 2 identical cells result

Gamete fusion (fertilization) • To create an embryo, gametes (egg +

sperm) must fuse. • Chromosomes in gametes number 1/2 of

those in somatic cells; 1883 – All roundworm somatic cells = 4 – All roundworm gametes = 2

• A special type of division produces sperm & egg cells = Meiosis; 1887

• So, two types of cell divisions? – One creates daughter cells with same

number of chromosomes as parent cells

– The other creates daughter cells with 1/2 that number. Original # is restored when gametes fuse.

Major events of Meiosis • 2 cell divisions: Meiosis I & Meiosis II • Meiosis I:

– Homologs form tetrads – Non-sister chromatids cross over! – Homologs associate on metaphase plate, then

separate. • Meiosis II:

– Sister chromatids separate! • Full compliment (diploid #) restored upon

fertilization

Homologs replicate

1. Homologous pairs separate 2. Sister chromatids separate

Crossing over (Recombination)

DNA strands break at same spot on non-sister chromatids

Cross over

And reanneal to non-sister chromatid

Meiosis I: Phases

Meiosis II: Phases

How is heritable variation produced?

1. Homologous chromosomes align independently on the metaphase plate

• Varies the combination of chromosomes in daughter cells

2. Crossing over (recombination) between non-sister chromatids at chiasmata shuffles alleles

• Varies combinations of alleles on chromosomes in daughter cells

Independent assortment

Overhead • How many different chromosome

arrangements (at the metaphase plate of Meiosis I) could a cell have that is 2n = 6?

2. 2 3. 3 4. 4 5. 5

Even selfing produces variation