1. Sexual Reproduction
Chapter 12

2. Sexual Reproduction
In asexual reproduction, a single individual gives rise to offspring that are identical to itself and others [MITOSIS]
Bacteria reproduce asexually
Most plants and some simple animals can reproduce both asexually and sexually
In sexual reproduction, two individuals mix their genetic material
Creates a diversity of traits which offers a better chance of surviving environmental change

3. Sexual Reproduction
Somatic (body) cells of sexually reproducing eukaryotes have pairs of homologous chromosomes
Carry genes of the same characteristics (one from dad, one from mom)
Humans have 23 pairs of homologous chromosomes
22 autosomal pairs, 1 sex pair

4. Sexual Reproduction Gametes are haploid, autosomal cells are diploid
Sexual reproduction mixes up traits from two parents through the fusion of gametes
Gamete: mature reproductive [sex] cells – sperm and egg
Gametes are haploid, autosomal cells are diploid
n = number of chromosomes
Haploid: n (cell has 1 copy of every chromosome)
Diploid: 2n (cell has 2 of every chromosome, female n + male n)

5. Meiosis
Meiosis: a nuclear division mechanism that occurs in reproductive cells of eukaryotes
DNA replication occurs prior to meiosis:
The nucleus is diploid (2n) with two sets of chromosomes, one from each parent
During meiosis, chromosomes of a diploid nucleus become distributed into four haploid nuclei

6. Meiosis
Meiosis halves the diploid (2n) chromosome number to the haploid (n) chromosome number for forthcoming gametes, ensuring the offspring have the same number of chromosomes as the parents
Diploid chromosome number is restored at fertilization

7. Fertilization
Fertilization: when one haploid gamete (egg) combines with another (sperm)
In humans, n = 23 and 2n = 46
This number is different for every species!
When two haploid gametes fuse, a zygote is formed
The first cell of a new individual

8. Fertilization
If meiosis didn’t happen before fertilization, the chromosome number would double with every generation
Chromosome number changes can have drastic consequences in animals
An individual’s set of chromosomes is like a fine-tuned blueprint that must be followed exactly to have normal functions

9. Meiosis Two consecutive nuclear divisions Reduction Division: 2nn
Interphase: DNA replication
Meiosis I
Prophase I, Metaphase I, Anaphase I, Telophase I
Meiosis II
Prophase II, Metaphase II, Anaphase II, Telophase II

10. Meiosis I Prophase I Metaphase I Anaphase I Telophase I
Homologous chromosomes condense, pair up, and swap segments, spindle microtubules attach to chromosomes as the nuclear envelope breaks up
Metaphase I
The homologous chromosome pairs are aligned midway between spindle poles
Anaphase I
The homologous chromosomes separate and begin heading toward the spindle poles
Telophase I
Two clusters of chromosomes reach the spindle poles, a new nuclear envelope forms around each cluster, so two haploid (n) nuclei form

12. Meiosis II Prophase II Metaphase II Anaphase II Telophase II
The chromosomes condense, nuclear envelope breaks up, spindle microtubules attach to each sister chromatid
Metaphase II
The (still duplicated) chromosomes are aligned midway between poles of the spindle
Anaphase II
All sister chromatids separate, the (now unduplicated) chromosomes head to the spindle poles
Telophase II
A complete set of chromosomes clusters at each spindle pole, new nuclear envelope encloses each cluster, four haploid (n) nuclei form

14. Variation Two events in meiosis introduce new combinations of alleles
Crossing Over: chromosome segments are exchanged between homologous chromosomes during prophase I, increases genetic diversity

15. Variation
The second event is the segregation of chromosomes into gametes
When homologous chromosomes separate in meiosis I, one of each chromosome pair goes to each of the two new nuclei
For each pair, the maternal or paternal version is equally likely to end up in either nucleus
Each time a human germ cell undergoes meiosis the four gametes that form end up with one of 8,388,608 (or 223) possible combinations of homologous chromosomes

17. Mitosis and Meiosis
There are striking parallels between mitosis and meiosis II
Meiosis may have evolved by remodeling the existing mechanisms of mitosis, or for repairing damaged DNA to maintain the integrity of the chromosome