Reduction-Division Genetic Recombination MEIOSIS Reduction-Division Genetic Recombination

Meiosis The form of cell division by which GAMETES, with HALF the number of CHROMOSOMES, are produced. DIPLOID (2n)  HAPLOID (n) Meiosis is 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 -sperm Female: OOGENESIS - egg or ova

Involves 2 cell divisions Meiosis Similar in many ways to mitosis Several differences however Involves 2 cell divisions Results in 4 cells with 1/2 the normal genetic information. Vocabulary: Diploid (2N) - Normal amount of genetic material Haploid (N) - 1/2 the genetic material. 4 4

In Humans, these are the Ova (egg) and sperm. Meiosis Meiosis results in the formation of haploid cells. In Humans, these are the Ova (egg) and sperm. Ova are produced in the ovaries in females Process is called oogenesis Sperm are produced in the testes of males. Process is called spermatogenesis Meiosis occurs in 2 phases: Meiosis I & Meiosis II 5 5

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

Oogenesis n=23 n=23 2n=46 Haploid (1n) human egg sex cell Meiosis II 2n=46 human sex cell diploid (2n) n=23 Meiosis I Polar Bodies (die)

Spermatogenesis & Oogenesis Sperm formation Egg formation 8 8

Interphase I Similar to mitosis interphase. CHROMOSOMES (DNA) replicate in the S phase Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES. CENTRIOLE pairs also replicate.

Meiosis I Prior to division (S phase), amount of DNA doubles 10 10

Interphase I Nucleus and nucleolus visible. Nucleus chromatin 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

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 chromatids).

Non-Sister Chromatids-HOMOLOGS Homologs contain DNA that codes for the same genes, but different versions of those genes Genes occur at the same loci

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

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. First 22 pairs of autosomes b. Last pair of sex chromosomes LOCI

Homologous Chromosomes eye color locus hair color Paternal Maternal

Crossing Over Crossing over may occur between non-sister chromatids at sites called chiasmata. Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.) Causes Genetic Recombination

Genetic Recombination Tetrad nonsister chromatids chiasmata: site of crossing over variation

Sex Chromosomes XX chromosome - female XY chromosome - male

MEIOSIS I Meiosis I Homologs separate

Prophase I Nucleus & Nucleolus disappear Spindle forms Chromosomes coil & Synapsis (pairing) occurs Tetrads form & Crossing over Occurs centrioles spindle fiber aster fibers TETRAD

Metaphase I Shortest phase Tetrads align on the equator. Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION

Homologs line up at equator or metaphase plate Metaphase I Homologs line up at equator or metaphase plate OR

Formula: 2n Example: 2n = 4 then 1n = 2 thus 22 = 4 combinations

Metaphase I Genes are exchanged at these connections During Metaphase I homologous chromosomes line- up along the metaphase plate or EQUATOR Areas of homologous chromosomes connect at areas called CHIASMATA Genes are exchanged at these connections 27 27

Crossing Over Segments of homologous chromosomes break and reattach at similar locations. Results in new genetic combinations of offspring. This is the main advantage of sexual reproduction 28 28

Chiasmata 29 29

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

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

Anaphase I During Anaphase I, each HOMOLOGOUS CHROMOSOME is pulled to opposite sides of the cell. Unlike mitosis, the CENTROMERES DO NOT BREAK. Nuclei MAY OR MAY NOT reform following division. CYTOKENESIS may or may not occur. 32 32

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

Anaphase I Homologs separate

Telophase I Each pole now has haploid (1n) set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I cytokinesis

Sister Chromatids Separate MEIOSIS II Sister Chromatids Separate Meiosis II

Meiosis II No Interphase II or very short No DNA Replication Remember: Meiosis II is similar to mitosis

Meiosis II DNA DOES NOT double Chromosomes randomly line-up along metaphase plate like regular mitosis. During Anaphase II, CENTROMERES BREAK and each SISTER CHROMATID is pulled to opposite sides of the cell. Nuclei reform and cytokinesis usually occurs (although it is often unequal). Sister chromatids 39 39

Prophase II Same as Prophase in mitosis Nucleus & nucleolus disappear Chromosomes condense Spindle forms

Metaphase II Same as Metaphase in mitosis Chromosomes (not homologs) line up at equator

Anaphase II Same as Anaphase in mitosis SISTER CHROMATIDS separate

1n Sperm cell fertilizes 1n egg to form 2n zygote Telophase II Same as Telophase in mitosis. Nuclei and Nucleoli reform, spindle disappears CYTOKINESIS occurs. Remember: FOUR HAPLOID DAUGHTER cells are produced. Called GAMETES (eggs and sperm) 1n Sperm cell fertilizes 1n egg to form 2n zygote

Telophase II

Variation Also known as GENETIC RECOMBINATION Important to population as the raw material for NATURAL SELECTION. All organisms are NOT alike Strongest “most fit” survive to reproduce & pass on traits

Question: What are the 3 sources of genetic recombination or variation?

Answer: CROSSING OVER (prophase I) INDEPENDENT ASSORTMENT (metaphase I) RANDOM FERTILIZATION

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 or 1n)

Karyotype An organized picture of the chromosomes of a human arranged in pairs by size from largest to smallest. Pairs 1-22 called AUTOSOMES Last pair are SEX CHROMOSOMES Male - XY

Karyotype Female - XX

Karyotype Down Syndrome – Trisomy 21 Female - XX

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

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

Answer: 10 chromosomes

Meiosis Gamete Production 57 57

Overview of Meiosis 58 58

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