MEIOSIS MEIOSIS In sexual reproduction 2 sex cells

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MEIOSIS

MEIOSIS

MEIOSIS In sexual reproduction, 2 sex cells (GAMETES) combine their DNA to create offspring

MEIOSIS In sexual reproduction, 2 sex cells (GAMETES) combine their DNA to create offspring that is genetically different from the parents. Organisms that reproduce sexually get half their chromosomes from their father and half from their mother. Normal human body cells, SOMATIC CELLS, have 46 chromosomes. We call this the diploid number (2 n) Normal human sex cells/gametes have 23 chromosomes. This is called the haploid number (n). Species 2 n / Diploid # n / haploid # banana 22 11 chicken 78 39 cow 40 20

MEIOSIS Sex cells divide to produce gametes (sperm or egg) Gametes have half the

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.

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. However, they are not exactly the same. One chromosome may code for blue eyes, and the other chromosome may code for brown eyes. But, they both code for eye colour! 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. b. 22 pairs of autosomes 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

SEX CHROMOSOMES XX chromosome - female XY chromosome - male

SEX CHROMOSOMES XX chromosome - female XY chromosome - male

SEX CHROMOSOMES Early in the 20 th Century scientists identified the chromosome specific to

SEX CHROMOSOMES Early in the 20 th Century scientists identified the chromosome specific to males and females Females have two rod shaped chromosomes of identical length ( XX) XX Males have one rod shaped chromosome and a smaller hook shaped chromosome ( XY) XY These chromosomes are involved in gender determination in many organisms

SEX CHROMOSOMES There are 22 autosomes. The 23 rd pair is known as the

SEX CHROMOSOMES There are 22 autosomes. The 23 rd pair is known as the sex chromosomes and they determine the sex of the individual. Male sperm contain either an X chromosome or a Y chromosome. If an X chromosome fertilizes the egg, the baby will be a girl. If a Y chromosome fertilizes the egg, a boy will be born.

MEIOSIS

MEIOSIS

INTERPHASE Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids

INTERPHASE Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres Centriole pairs also replicate.

INTERPHASE Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

INTERPHASE Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

MEIOSIS I Cell division that reduces the chromosome number by one-half. four phases: phases

MEIOSIS I 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 spindle fiber aster fibers centrioles

PROPHASE I spindle fiber aster fibers centrioles

PROPHASE I Longest and most complex phase Chromosomes condensed. N uclear membrane dissolves Centrioles

PROPHASE I Longest and most complex phase Chromosomes condensed. N uclear membrane dissolves Centrioles move into opposite poles of the cell S pindle fibres attach to chromosomes Synapsis occurs: homologous chromosomes cross over to form a tetrad A tetrad/bivalent is two pairs of chromosomes or four chromatids (sister and non-sister chromatids). Crossing over can occur.

CROSSING OVER (PROPHASE I) Crossing over (variation) may occur between nonsister chromatids at the

CROSSING OVER (PROPHASE I) 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 Creates genetic variation. Chiasmata (chiasma) are the sites of crossing over http: //highered. mcgraw-hill. com/sites/dl/free/0072835125/126997/animation 5. html

PROPHASE I - SYNAPSIS Homologous chromosomes sister chromatids Tetrad sister chromatids

PROPHASE I - SYNAPSIS Homologous chromosomes sister chromatids Tetrad sister chromatids

CROSSING OVER - VARIATION Tetrad nonsister chromatids chiasmata: site of crossing over variation

CROSSING OVER - VARIATION Tetrad nonsister chromatids chiasmata: site of crossing over variation

METAPHASE I Shortest phase Tetrads align on the metaphase plate INDEPENDENT ASSORTMENT OCCURS: 1.

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

INDEPENDENT ASSORTMENT During metaphase I, when the chromosomes line up in their pairs in

INDEPENDENT ASSORTMENT During metaphase I, when the chromosomes line up in their pairs in middle of the cell, they line up randomly. (It’s not like all the maternal chromosomes line up on one side, and the paternal chromosomes on the other) This allows for different combinations of chromosomes, producing different gametes. In fact, since humans have 23 pairs of chromosomes, that means that you can form 2 2 3 = 8 388 608 different gametes! This is why you and your sibling are not exactly the same. Crossing over also adds to variation.

METAPHASE I OR metaphase plate

METAPHASE I OR metaphase plate

QUESTION: In terms of Independent Assortment -how many different combinations of sperm could a

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 = 23 2

ANSWER Formula: 2 n Human chromosomes: 2 n = 46 n = 23 2 23 = ~8 million combinations

ANAPHASE I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached

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 and two

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 (no more DNA replication) replication Remember: Meiosis II is

MEIOSIS II No interphase II (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. Remember: four haploid

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

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

OOGENESIS n=23 human sex cell egg Haploid (1 n) n=23 2 n=46 diploid (2

OOGENESIS n=23 human sex cell egg Haploid (1 n) n=23 2 n=46 diploid (2 n) Meiosis I Polar Bodies (dissolve) n=23 Meiosis II 34

FERTILIZATION The fusion of a sperm and egg to form a zygote A zygote

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

MITOSIS VS. MEIOSIS Mitosis Meiosis Parent diploid daughter cells Parent diploid haploid daughter cells

MITOSIS VS. MEIOSIS Mitosis Meiosis Parent diploid daughter cells Parent diploid haploid daughter cells 1 division 2 divisions Genetically Identical daughter cells Genetically different daughter cells Anaphase separates sister chromatids Anaphase I separates homologous chromosomes. Anaphase II separates sister chromatids No crossing over Crossing over 2 daughter cells 4 gametes/daughter cells

VARIATION Important to population as the raw material for natural selection. Question: What are

VARIATION Important to population as the raw material for natural selection. Question: What are three sources of genetic variation as a result of meiosis?

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) at the beginning of meiosis would, at

QUESTION: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes

ANSWER: 10 chromosomes (haploid)

ANSWER: 10 chromosomes (haploid)

QUESTION: A cell containing 40 chromatids at the beginning of meiosis would, at its

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

KARYOGRAM A method of organizing the chromosomes of a cell in relation to number,

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

MALE OR FEMALE?

MALE OR FEMALE?

MALE OR FEMALE?

MALE OR FEMALE?