Meiosis & Sexual Reproduction CHAPTER 10 Meiosis & Sexual Reproduction

I. Meiosis Halves Chromosome #s A. Type of nuclear division that reduces # of chromosomes from diploid (2n) to haploid (1n) 1. In humans, parents are 2n (diploid) and gametes are haploid (1n) 2. Zygotes form from the fusion of gametes and thus have the diploid number of chromosomes  Meiosis reduces the chromosome # so gametes can fuse to form a zygote 4/4/2019

A. Somatic cells = body cells II. Homologous pairs A. Somatic cells = body cells 1. Human body cells have 46 chromosomes in 23 pairs 2. The paired chromosomes are called homologous chromosomes a. They carry genes for the same characteristics. Alternate forms of a gene are called alleles. b. Have same length c. Centromeres in same position d. When stained they show similar banding patterns 4/4/2019

Homologous chromosomes

3. Each gene occurs in duplicate: • Maternal copy from mother • Paternal copy from father 4. Homologous copies of a gene may be identical or contain differing genetic information 4/4/2019

B. 2 types of chromosomes 1. Autosomes a. 22 pairs found in both sexes 2. Sex Chromosomes a. Other pair that determines gender  Female = 2 X chromosomes  Male = X & Y chromosome b. Y chromosome is smaller; most genes are different than X 4/4/2019

III. Meiosis differs from mitosis: A. Produces haploid gametes B. Undergoes 2 divisions (I & II) C. 4 daughter cells result 1. Each new cell has half the # of chromosomes as mother cell 2. This occurs during Meiosis I D. Daughter cells inherit different combinations of chromosomes so they are not identical to each other or to the parent cell 4/4/2019

Overview of Meiosis

A. Interphase = same as mitosis IV. Stages of Meiosis A. Interphase = same as mitosis B. Prophase I (90% of meiosis) 1. Same as mitosis prophase except: a. Synapsis occurs: ♦ Homologous chromosomes come together & attach as a tetrad (4 chromatids). Held together by synaptonemal complex. ♦ Exchange segments in a process called crossing over 4/4/2019

1. Homologous pairs (tetrads) align at metaphase plate (equator) C. Metaphase I 1. Homologous pairs (tetrads) align at metaphase plate (equator) D. Anaphase I 1. Tetrads split up 2. Homologous chromosomes move away from each other a. These are still duplicated chromosomes. Sister chromatids still attached to each other 4/4/2019

1. Double-stranded chromosomes arrive at opposite poles of cell E. Telophase I 1. Double-stranded chromosomes arrive at opposite poles of cell F. Cytokinesis I 1. Cytokinesis forms 2 new cells 2. Each new cell will be haploid a. Has half the # of chromosomes it started out with. One chromosome of each type is present. G. Interkinesis 1. Similar to mitotic interphase 2. No replication of DNA occurs 4/4/2019

H. Meiosis II 1. Essentially the same as mitosis 2. Difference is: a. Each cell starts haploid, with half the original # of chromosomes 3. Ends with 4 cells each with the haploid # of single-stranded chromosomes 4/4/2019

Meiosis I & II in Plant Cells

1. Having 3 copies of 21st chromosome V. Meiosis Gone Wrong A. Trisomy 21 1. Having 3 copies of 21st chromosome 2. Creates characteristic symptoms which are known as Down’s Syndrome 3. Incidence of Down’s syndrome increases with the age of the mother 4/4/2019

B. Two key mechanisms of change: VI. Genetic Variation A. Events of meiosis ensure genetic variation occurs in each generation B. Two key mechanisms of change: 1. Crossing over between homologous chromosomes 2. Independent assortment of homologous chromosomes 4/4/2019

3. Creates genetic recombination C. Crossing Over 1. Exchange of genetic material between nonsister chromatids during meiosis I. a. Chiasma • Sites of crossing over • Appear X-shaped 2. Takes place during synapsis of Prophase I 3. Creates genetic recombination 4/4/2019

Crossing Over

D. Independent Assortment 1. Each pair of chromosomes aligns randomly at equator & then separates from each other 2. Causes random mixing of alleles into gametes 3. Number of combinations possible = 2n n = haploid number for species Humans = 223 = ~ 8.4 million 4/4/2019

Independent Assortment

However, it can happen several times in each chromosome! E. Fertilization 1. When gametes fuse at fertilization: a. Chromosomes of parents are combined b. Number of possible zygotes? 223 X 223 = ~ 70 trillion c. With one cross-over event: 423 X 423 = ~ 4.95 X 1027 However, it can happen several times in each chromosome! 4/4/2019

Genetic Recombination

F. Significance of Genetic Variation 1. Asexual reproduction produces genetically identical clones a. This might be advantageous when the environment is stable and doesn’t change for long periods of time 2. Sexual reproduction produces novel genetic recombinations a. If environment changes, variability may be an advantage 4/4/2019

Meiosis Mitosis Requires two nuclear divisions Chromosomes synapse and cross over Centromeres survive Anaphase I Halves chromosome number Produces four daughter nuclei Produces daughter cells genetically different from parent and each other Used only for sexual reproduction Mitosis Requires one nuclear division Chromosomes do not synapse nor cross over Centromeres dissolve in mitotic anaphase Preserves chromosome number Produces two daughter nuclei Produces daughter cells genetically identical to parent and to each other Used for asexual reproduction and growth 4/4/2019

Meiosis Compared to Mitosis

VII. Life Cycle Basics A. The term life cycle refers to all the reproductive events that occur from one generation to the next similar generation. 1. In humans, the individual is always diploid. Meiosis produces single- celled gametes, the only haploid phase. 2. Plants, however, alternate between haploid and diploid generations. Each phase can be an independent multi-cellular organism. 4/4/2019

The Human Life Cycle

B. Plant Life Cycles 1. The haploid generation is known as the gametophyte. These produce haploid gametes by mitosis. 2. The diploid generation is known as the sporophyte. They produce spores by meiosis. a. Moss plants are haploid and only produce a short-lived diploid generation. b. Most other plants are diploid and produce a short-lived haploid 4/4/2019

Typical Plant Life Cycle mitosis

C. Gametogenesis in Humans 1. Spermatogenesis a. Production of sperm b. Occurs in testes c. Diploid spermatocytes undergo meiosis d. Produce 4 haploid spermatids e. Spermatids differentiate into mature sperm (spermatozoa) 4/4/2019

2. Oogenesis a. Production of eggs in ovaries b. Diploid oocytes undergo meiosis c. Produces 2 haploid cells. ● One cell receives almost all of the cytoplasm (secondary oocyte). ● Other is a polar body which will not become an egg d. Meiosis II begins but stops at metaphase II. 4/4/2019

e. Secondary oocyte will leave ovary at e. Secondary oocyte will leave ovary at time of ovulation many years later. ● If no sperm are in oviduct, the oocyte disintegrates. ● If sperm enters an oocyte, it is activated to continue meiosis II. ♣ Produces one egg and another polar body. ♣ Polar bodies are a way to retain most of the cytoplasm in the egg. This is needed for the developing embryo. 4/4/2019

Gametogenesis