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