MEIOSIS Meiosis A reduction division of a diploid

MEIOSIS

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

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

Meiosis • Cells divide to produce gametes (sperm or egg) • Gametes have half the # of chromosomes. Gametes half chromosomes • Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis • Meiosis is similar to mitosis with some Meiosis mitosis chromosomal differences.

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

Meiosis – mouse testes Parent cell 1 st division 2 nd division 4 gametes

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

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

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

spindle fiber aster fibers centrioles

Prophase I • Longest and most complex phase (90%). • Chromosomes condense and pair up Chromosomes • Synapsis forms: homologous chromosomes Synapsis come together to form a tetrad or a bivalent tetrad • Tetrad = two chromosomes or four chromatids Tetrad chromosomes (sister and nonsister chromatids).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids

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

Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata. chromatids chiasmata • Crossing over: segments of nonsister over chromatids break and reattach to the other chromatids chromatid • Chiasmata (chiasma) are the sites of crossing over

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

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment

Sex Chromosomes XX chromosome - female XY chromosome - male

Prophase I spindle fiber aster fibers centrioles

Faze profaze I: 1. LEPTOTEN (A) - kromosomi(dvostruki) nalikuju tankim nitima u klupku - uočavaju se KROMOMERE –jače spiralizirana mjesta (stoga jače obojena) YO U DO NOT 2. ZIGOTEN(B) NEED TO - još se opaža jezgrica K NOW - SINAPTONEMSKI KOMPLEKS – struktura koja omogućuje sparivanje THIS homologa u bivalent - kromomere jednog kromosoma se “poslože” uz analogne kromomere drugog homologa i spoje se preko sinap. komp. 3. PAHITEN (C) - završeno je sparivanje homologa -zbiva se crossing over, vidljive hijazme http: //www. as-botanicalstudies. com/content/54/1/46/figure/F 3

YOU D O NOT NEED KNOW TO THIS 4. DIPLOTEN (D) - zbog razgradnje sinaptonemskog kompleksa popušta veza među homolozima - homolozi povezani preko hijazmi - kromosomi se skraćuju(kondenziraju), deblji su 5. DIJAKINEZA (E) - daljnja kondenzacija, sada su vidljive sve četiri kromatide bivalenta http: //www. as-botanicalstudies. com/content/54/1/46/figure/F 3

Pollard: Cell Biology, 3 rd edition

• • Aceto-carmine stained normal (A-M) & abnormal (N-T) meiotic stages of Aloe vera. (A) Leptotene stage. (B) Zygotene stage showing chromatin threads (C) Pachytene stage showing beaded chromosomes. (D) Diplotene stage showing ‘X’ shaped chiasma. (E) Diakinesis showing 7 bivalents with 4 long and 3 short chromosome pairs. (F) Side view of metaphase-I. (G) Side view of anaphase-I. (H) Side view of telophase-I. (I) Side view of prophase-II (J) Side view of metaphase-II. (K) Side view of anaphase-II. (L) Side view of telophase-II. (M) Tetrad showing 4 pollen grains. (N) Side view of anaphase-I with single chromosomal bridge and lagged chromosome. (O) Side view of anaphase-I with double chromosomal bridge. (P) Side view of anaphase-I with single chromosomal bridge (Q) Telophase-I with lagged chromosome. (R) Metaphase-II with lagged chromosome. (S) Telophase-II with single chromosomal bridge. (T) Telophase-II with univalent laggard chromosome. Haque and Ghosh Botanical Studies 2013 54: 46 doi: 10. 1186/1999 -3110 -54 -46 http: //www. as-botanicalstudies. com/content/54/1/46/figure/F 3

http: //www. as-botanicalstudies. com/content/54/1/46/figure/F 3

Metaphase I • Shortest phase • Tetrads align at the eqator of the cell (chiasmata are Tetrads eqator in the equatorial plane = metaphase plate) o Moved through the cell by spindle microtubules • 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

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 • 223 = ~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 haploid chromosomes • Chromosomes uncoil. • Cytokinesis occurs and two haploid daughter Cytokinesis cells are formed.

Telophase I

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

Prophase II • same as prophase in prophase mitosis

Metaphase II • same as metaphase in metaphase mitosis metaphase plate

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

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

Telophase II

Meiosis sex cell n=2 sperm n=2 2 n=4 haploid (n) diploid (2 n) n=2 n=2 meiosis II

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 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? chromosomes

Answer: • 10 chromosomes (haploid)

Fertilization • The fusion of a sperm and egg to form a zygote. sperm egg 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 chromatids beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes

Answer: • 10 chromosomes

Did u know? Humans =46 Great apes = 48 Human chromosome 2 was formed by a merger of ancestral chromosomes, reducing their number.

Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues. Homologue

KARYOTYPE KARYOGRAM IDIOGRAM

KARYOTYPE – is the number and appearance of chromosomes in the nucleus of a eukaryotic cell

KARYOGRAM – a graphic representation of the complete set of human chromosomes

IDIOGRAM – the diagrammatic representation of a karyotype - it may be based on measurements of the chromosomes

http: //learn. genetics. utah. edu/content/chromosomes/karyotype/

Autosomes (The Autosomes code for most of the offspring’s traits) In Humans the “Autosomes” are sets 1 - 22

Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. If the offspring has two “X” chromosomes it will be a female. If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23 rd set XX chromosome - female XY chromosome - male

Boy or Girl? The Y Chromosome “Decides” Y chromosome X chromosome

21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?

Cross section through a testis

Sources • www. ursulinehs. org/powerpoint/meiosis. ppt • www. biology 4 teachers. com/Cell%20 Division/MEIOSIS. p pt • http: //www. iteachbio. com/Life%20 Science/Life. Functions and. The. Cell/Meiosis. ppt