Gametogenesis at the Chromosomal level Mitosis and Meiosis

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Gametogenesis at the Chromosomal level: Mitosis and Meiosis Animal Science 434 John J. Parrish

Gametogenesis at the Chromosomal level: Mitosis and Meiosis Animal Science 434 John J. Parrish

Definitions Chromosome - linear DNA molecule Centromere - special region of spindle attachment Homologous

Definitions Chromosome - linear DNA molecule Centromere - special region of spindle attachment Homologous Chromosomes • Have the same kind of genes in the same order • 1 from father, 1 from mother Sister chromosomes (X, Y)

Definitions Homologous Chromosomes DNA Replication Chromatids A chromatid pair has an identical sequence of

Definitions Homologous Chromosomes DNA Replication Chromatids A chromatid pair has an identical sequence of DNA

Cell Cycle M Go MPF MPF G 2 MPF MPF Interphase S G 1

Cell Cycle M Go MPF MPF G 2 MPF MPF Interphase S G 1

M - Phase l l l Prophase Metaphase Anaphase Telophase Cytokinesis

M - Phase l l l Prophase Metaphase Anaphase Telophase Cytokinesis

Grand Scheme Mitosis Meiosis 2 N 2 N Replicate Division 2 N 2 N

Grand Scheme Mitosis Meiosis 2 N 2 N Replicate Division 2 N 2 N Division N N

Go Mitosis G 1 M - phase Prophase S G 2 Chromatids Metaphase Homologous

Go Mitosis G 1 M - phase Prophase S G 2 Chromatids Metaphase Homologous chromosomes pair along side of each other Centromere Spindles (microtubules) MPF high

Mitosis Anaphase MPF decreases Chromatids pull apart

Mitosis Anaphase MPF decreases Chromatids pull apart

Mitosis Anaphase MPF decreases Chromatids pull apart

Mitosis Anaphase MPF decreases Chromatids pull apart

Mitosis Telophase Chromosomes pull apart further Microtubules Disappears Nuclear envelopes reform

Mitosis Telophase Chromosomes pull apart further Microtubules Disappears Nuclear envelopes reform

Mitosis Telophase Chromosomes pull apart further Microtubules Disappears Nuclear envelopes reform

Mitosis Telophase Chromosomes pull apart further Microtubules Disappears Nuclear envelopes reform

Mitosis Cytokinesis

Mitosis Cytokinesis

Mitosis Cytokinesis 2 N 2 N

Mitosis Cytokinesis 2 N 2 N

Meiosis 2 N Go G 1 Primary Spermatocyte Primary Oocyte 4 N S G

Meiosis 2 N Go G 1 Primary Spermatocyte Primary Oocyte 4 N S G 2 Prophase

The Five Phases of Meiotic Prophase 4 N Leptotene • Chromosomes condense Zygotene 4

The Five Phases of Meiotic Prophase 4 N Leptotene • Chromosomes condense Zygotene 4 N • Homologous chromosomes begin to pair • Bivalents

The Five Phases of Meiotic Prophase Pachytene 4 N • Pairing is completed •

The Five Phases of Meiotic Prophase Pachytene 4 N • Pairing is completed • Crossing over of homologues occur Diplotene/Dictyatene • Oocytes stop here before puberty 4 N • Homologous chromosomes pull apart but remain attached at crossover points (chiasma) • RNA synthesis is possible

The Five Phases of Meiotic Prophase Diakinesis • RNA synthesis stops 4 N Homologous

The Five Phases of Meiotic Prophase Diakinesis • RNA synthesis stops 4 N Homologous chromosomes linked at chiasma Chromatids linked at centromere • The chromatids become visible Crossover (chiasma) Centromere

Metaphase I of Meiosis 4 N Homologous chromosomes move toward opposite poles and pair

Metaphase I of Meiosis 4 N Homologous chromosomes move toward opposite poles and pair across from one another Homologous chromosomes held together via chiasma

Anaphase I of Meiosis 4 N Recombination as a result of crossing over

Anaphase I of Meiosis 4 N Recombination as a result of crossing over

Telophase I 4 N

Telophase I 4 N

Telophase I 4 N Cytokinesis 2 N 2 N Secondary Spermatocyte or Secondary Oocyte

Telophase I 4 N Cytokinesis 2 N 2 N Secondary Spermatocyte or Secondary Oocyte

Meiosis Metaphase II 2 N 2 N 2 N No Replication Rest Same As

Meiosis Metaphase II 2 N 2 N 2 N No Replication Rest Same As Mitosis 2 N Oocytes stop here until fertilized

Meiosis Anaphase II This does not exist for oocyte Telophase II Cytokinesis N N

Meiosis Anaphase II This does not exist for oocyte Telophase II Cytokinesis N N 2 N Spermatid N 2 N N

Abnormalities l Anueploidy – Not an exact multiple – Hyper or hypo – Example:

Abnormalities l Anueploidy – Not an exact multiple – Hyper or hypo – Example: 2 N - 1 or 2 N + 1 l Euploidy – Exact multiple of haploid – Example: 3 N, 4 N, etc. l Non-disjunction – Causes aneuploidy and occurs in meiosis – Example: N - 1 or N + 1

Abnormalities Non-disjunction XXYY XY XXYY X XXYY O XX YY XY XY O O

Abnormalities Non-disjunction XXYY XY XXYY X XXYY O XX YY XY XY O O XX O Y Y XXY XO XO XXX XO XY XY Kleinfelter’s Syndrome Turner’s Syndrome

Down’s Syndrome l Non-disjunction l Extra copy of chromosome 21 l Probability increases with

Down’s Syndrome l Non-disjunction l Extra copy of chromosome 21 l Probability increases with age – Age 35, 2 per 1000 births – Age 40, 6 per 1000 births – Age 45, 16 per 1000 births