1. GAMETE PRODUCTION IN SEXUAL REPRODUCTION
MEIOSIS
GAMETE PRODUCTION IN SEXUAL REPRODUCTION

2. What are chromosomes?
Human cell = >6 billion nucleotide base pairs (~2 meters)
Wrapped around protein = chromatin
DNA/protein = chromosome
E. Coli bacteria have approx 5 million base pairs. Human cells have approximately 1000X as much DNA.

3. How many chromosomes do humans have?
Strait hair allele
Curly hair allele
Humans are diploid (2n)
Two of each chromosome, one from each parent.
n = 23 unique chromosomes (haploid #)
2(n) = 46 total chromosomes
The two copies of each chromosome in human cells are homologous

4. Families: similar yet different. Why?
Asexual Reproduction
Single parent
Offspring identical to each other and parent
Sexual Reproduction
Two parents
Offspring are unique
Offspring are similar to each other and parents
Combine DNA from two individuals
Combines characteristics of both individuals

5. MEIOSIS
While mitosis conserves chromosome number, meiosis reduces the chromosome number by half
Meiosis has a few stages:
Interphase,
Meiosis I, and
Prophase I
Metaphase I
Anaphase I
TelophaseI
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II

6. MEIOSIS INTERPHASE Interphase is similar to the process in mitosis
The process results in two genetically identical sister chromatids
The chromatids are connected at their centromeres
Centrosomes also replicate

7. PROPHASE I Chromosomes begin to condense
Homologues (consisting of two sister chromatids) pair up
When the chromosome pairs become visible, they show up as a tetrad, a cluster of four chromatids
Homologous chromosomes can criss-cross, allowing the chromatids to switch segments, called crossing- over

8. PROPHASE I continued Centrosomes move away from each other
Microtubules form between the centrosomes
Nuclear envelope and nucleoli disperse
Prophase I can last for days or longer, and takes up 90% of the time required for meiosis

9. METAPHASE I
Chromosomes arrange on the metaphase plate (equator), still in homologous pairs
Microtubules from one pole attach to one chromosome from each pair
The other chromosome (homologous pair) is attached to a microtubule from the other pole

10. ANAPHASE I
The spindle apparatus guides the chromosomes towards the poles
Sister chromatids remain attached at their centromeres
The sister chromatids move together towards the same pole

11. TELOPHASE I
The homologous chromosome pairs continue to move apart to each pole
Each pole contains a haploid (n) number of a chromosome set
Cleavage furrows form in animal cells, and cell plates appear in plant cells

12. TELOPHASE I AND CYTOKINESIS
Cytokinesis usually occurs simultaneously with telophase I, forming two daughter cells
There is no further replication of genetic material
Depending on the species, the chromosomes de- condense and the nuclear membranes and nucleoli re-form.

13. PROPHASE II A spindle apparatus forms
The chromosomes progress towards the metaphase II plate (equator of the cell)

14. METAPHASE II
The chromosomes are positioned on the metaphase plate in mitosis-like fashion
The microtubules attach to the centromeres of the sister chromatids

15. ANAPHASE II The centromeres of the sister chromatids finally separate
The sister chromatids of each pair, now individual chromosomes, move toward opposite poles of the cell

16. TELOPHASE II AND CYTOKINESIS
Nuclei form at opposite poles of the cell
Cytokinesis occurs
In the end, there are four daughter cells, each with the haploid number of un-replicated chromosomes