MEIOSIS Meiosis The form of cell division by

  • Slides: 38
Download presentation
MEIOSIS

MEIOSIS

Meiosis • The form of cell division by which gametes, with half the number

Meiosis • The form of cell division by which gametes, with half the number of chromosomes, are produced. • Diploid (2 n) haploid (n) • Meiosis is sexual reproduction. • Two divisions (meiosis I and meiosis II). II

Meiosis • Sex cells divide to produce gametes (sperm or egg) • Gametes have

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 chromosomal differences.

Spermatogenesis n=23 human sex cell sperm n=23 2 n=46 haploid (n) diploid (2 n)

Spermatogenesis n=23 human sex cell sperm n=23 2 n=46 haploid (n) diploid (2 n) n=23 meiosis II

Interphase I • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each

Interphase I • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres • Centriole pairs also replicate.

Interphase I • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

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.

Meiosis I (four phases) • Cell division that reduces the chromosome number by one-half. • four phases: phases a. prophase I b. metaphase I c. anaphase I d. telophase I

Prophase I • Longest and most complex phase (90%). • Chromosomes condense. • Synapsis

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 nonsister chromatids).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids

Homologous Chromosomes • Pair of chromosomes (maternal and paternal) paternal that are similar in

Homologous Chromosomes • Pair of chromosomes (maternal and paternal) 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. 01 pair of sex chromosomes

Homologous Chromosomes eye color locus hair color locus Paternal Maternal

Homologous Chromosomes eye color locus hair color locus Paternal Maternal

Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata

Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata • Crossing over: 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 chiasmata: site of crossing over Tetrad variation

Crossing Over - variation nonsister chromatids chiasmata: site of crossing over Tetrad variation

Sex Chromosomes XX chromosome - female XY chromosome - male

Sex Chromosomes XX chromosome - female XY chromosome - male

Prophase I spindle fiber aster fibers centrioles

Prophase I spindle fiber aster fibers centrioles

Metaphase I • Shortest phase • Tetrads align on the metaphase plate • INDEPENDENT

Metaphase I • Shortest phase • Tetrads align on the metaphase plate • INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2 n Example: 2 n = 4 then n = 2 thus 22 = 4 combinations

Metaphase I OR metaphase plate

Metaphase I OR metaphase plate

Question: • In terms of Independent Assortment how many different combinations of sperm could

Question: • In terms of Independent Assortment how many different combinations of sperm could a human male produce?

Answer • Formula: 2 n • Human chromosomes: 2 n = 46 n =

Answer • Formula: 2 n • Human chromosomes: 2 n = 46 n = 23 • 223 = ~8 million combinations

Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids

Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres

Anaphase I

Anaphase I

Telophase I • Each pole now has haploid set of chromosomes • Cytokinesis occurs

Telophase I • Each pole now has haploid set of chromosomes • Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Telophase I

Meiosis II • No interphase II (or very short - no more DNA replication)

Meiosis II • No interphase II (or very short - no more DNA replication) replication • Remember: Meiosis II is similar to mitosis

Prophase II • same as prophase in mitosis

Prophase II • same as prophase in mitosis

Metaphase II • same as metaphase in mitosis metaphase plate

Metaphase II • same as metaphase in mitosis metaphase plate

Anaphase II • same as anaphase in mitosis • sister chromatids separate

Anaphase II • same as anaphase in mitosis • sister chromatids separate

Telophase II • Same as telophase in mitosis • Nuclei form. • Cytokinesis occurs.

Telophase II • Same as telophase in mitosis • Nuclei form. • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg

Telophase II

Telophase II

Meiosis sex cell sperm n=46 n=23 2 n=46 (2 X in S Phase) diploid

Meiosis sex cell sperm n=46 n=23 2 n=46 (2 X in S Phase) diploid (2 n) n=23 haploid (n) n=23 n=46 n=23 meiosis II

Variation • Important to population as the raw material for natural selection. • Question:

Variation • Important to population as the raw material for natural selection. • Question: What are three sexual sources of genetic variation?

Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization

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 number before S phase) at the

Question: • A cell containing 20 chromosomes (diploid number before S phase) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes

Answer: • 10 chromosomes (haploid)

Answer: • 10 chromosomes (haploid)

Karyotype • A method of organizing the chromosomes of a cell in relation to

Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type.

Fertilization • The fusion of a sperm and egg to form a zygote •

Fertilization • The fusion of a sperm and egg to form a zygote • A zygote is a fertilized egg n=23 egg sperm n=23 2 n=46 zygote

Question: • A cell containing 40 chromatids at the beginning of meiosis would, at

Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes

Answer: • 10 chromosomes

Answer: • 10 chromosomes