MEIOSIS
You have body cells and gametes Body cells are known as somatic cells. Germ cells develop into gametes or sex cells. Germ cells are located in the ovaries and testes. Gametes are sex cells: egg and sperm. Gametes have DNA that can be passed to offspring
Every cell has autosomes and sex chromosomes. Your body cells have 23 pairs of chromosomes. Homologous pairs of chromosomes have the same structure. For each homologous pair, one comes from each parent. Chromosome pairs 1-22 are autosomes. Sex chromosomes (pair 23), X and Y, determine sex in mammals (XX=female, XY=male).
Sex Chromosomes XX chromosome - female XY chromosome - male
Karyotype A method of organizing the chromosomes of a cell in relation to number, size, and type. Creates an image that is easy to understand
Meiosis The form of cell division which produces gametes, with half the number of chromosomes. Human gametes= 22 autosomes and 1 sex chromosome Diploid (2n) haploid (n) Meiosis is part of sexual reproduction. Two divisions (meiosis I and meiosis II).
Meiosis Sex cells divide to produce gametes: sperm or egg Gametes have half the # of chromosomes. Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis Meiosis is similar to mitosis with some differences in the chromosomes.
Spermatogenesis human sex cell n=23 n=23 n=23 sperm 2n=46 diploid (2n) n=23 n=23 haploid (n) n=23 meiosis I meiosis II
Interphase I Similar to mitosis interphase. Chromosomes replicate (S phase). Each duplicated chromosome consists of two identical sister chromatids attached at their centromeres.
Interphase I Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus
Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I
Prophase I Longest and most complex phase (90%). Chromosomes condense. Synapsis occurs: homologous chromosomes come together to form a tetrad. Tetrad is two chromosomes or four chromatids (sister and non-sister).
Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids
Homologous Chromosomes Pair of chromosomes (maternal and paternal) that are similar in shape and size. Homologous pairs (tetrads) carry genes controlling the same inherited traits. Each locus (position of a gene) is in the same position on homologues. Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes b. 1 pair of sex chromosomes
Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal
Crossing Over Crossing over (variation) may occur between nonsister chromatids at the chiasmata. Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. Chiasmata (chiasma) are the sites of crossing over.
Crossing Over - variation nonsister chromatids Tetrad chiasmata: site of crossing over variation
Prophase I spindle fiber centrioles
Metaphase I Shortest phase Tetrads align at the equator. INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. This is a source of genetic variation 3. Formula: 2 n Example: 2n = 4 then n = 2 thus 2 2 = 4 combinations
Metaphase I OR
Question: In terms of Independent Assortment -how many different combinations of sperm could a human male produce?
Answer Formula: 2 n Human chromosomes: 2n = 46 n = 23 2 23 = ~8 million combinations
Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres.
Anaphase I
Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.
Telophase I
Meiosis II No interphase II (no more DNA replication) Remember: Meiosis II is similar to mitosis
Prophase II same as prophase in mitosis
Metaphase II same as metaphase in mitosis Centromeres line up randomly at the equator
Anaphase II same as anaphase in mitosis sister chromatids separate
Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Remember: four haploid daughter cells produced. gametes = sperm or egg
Telophase II
Meiosis n=2 sex cell n=2 n=2 sperm 2n=4 diploid (2n) n=2 n=2 haploid (n) n=2 meiosis I meiosis II
Fertilization The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote
Variation Important to population as the raw material for natural selection. Question: What are the three sexual sources of genetic variation?
Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!
Question: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?
Answer: 10 chromosomes (haploid)
Figure 13.7 The stages of meiotic cell division: Meiosis I 41
Figure 13.7 The stages of meiotic cell division: Meiosis II 42
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The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis 44
Mitosis Meiosis How many divisions? 1 division 2 divisions When does DNA replication occur? S phase of interphase ONLY S phase of interphase before meiosis 1 What is the final product? 2 daughter cells (somatic cells) 4 daughter cells (gametes) How do the resulting cells compare? What types of cells divide like this? Why does this process occur? What lines up during metaphase (I, II)? What separates during anaphase (I, II)? What is the chromosome number of the daughter Identical to each other and the parent cell Somatic cells Growth and repair Centromeres line up along equator Sister chromatids 2n Each is unique Germ cells To produce gametes and provide genetic variation through sexual reproduction I: tetrads line up along equator II: centromeres line up along equator I: homologous chromosomes II: sister chromatids n
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