Meiosis Reduction Division

Reduction-DivisionGenetic Recombination

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The form of cell divisioncell division by which GAMETESGAMETES, , with HALFHALF the number of CHROMOSOMESCHROMOSOMES, , are produced.

DIPLOIDDIPLOID (2n pair of (2n pair of chromosomes) chromosomes) →→ HAPLOIDHAPLOID (n half (n half a pair of chromosomes)a pair of chromosomes)

MeiosisMeiosis is SEXUALSEXUAL reproduction. reproduction. TWOTWO divisions divisions (MEIOSIS IMEIOSIS I and MEIOSIS IIMEIOSIS II).

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Sex cells divide to produce GAMETESGAMETES (sperm or egg)(sperm or egg).

Gametes have HALFHALF the # of chromosomes.

Occurs only in GONADSGONADS (testes or (testes or ovaries).ovaries).

Male: Male: SPERMATOGENESIS -SPERMATOGENESIS -spermsperm

Female: Female: OOGENESIS - OOGENESIS - egg or ovaegg or ova

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2n=46

humansex cell

diploid (2n)

n=23

n=23

Meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

Meiosis II4

2n=46

humansex cell

diploid (2n)

n=23

n=23

Meiosis I

n=23egg

Haploid (1n)

Meiosis II5

Polar Bodies (die)

Similar to mitosismitosis interphase. CHROMOSOMESCHROMOSOMES (DNA)(DNA) replicate in the S S phasephase

Each duplicated chromosomechromosome consist of twotwo identicalidentical SISTER CHROMATIDS SISTER CHROMATIDS attached at their CENTROMERESCENTROMERES.

CENTRIOLECENTRIOLE pairs also replicate.

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Nucleus and nucleolus become visiblevisible.

Nucleus

nucleolus

cell membrane

chromatin

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Cell division Cell division that reduces the chromosome number chromosome number by one-half.one-half.

Four phasesFour phases:a.a. Prophase IProphase Ib.b. Metaphase IMetaphase Ic.c. Anaphase IAnaphase Id.d. Telophase ITelophase I

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Prophase IProphase I

LongestLongest and and most complex most complex phase (90%).phase (90%).

ChromosomesChromosomes condense. SynapsisSynapsis occurs - Homologous Homologous chromosomeschromosomes come togetherto form a tetradtetrad.

TetradTetrad is two chromosomestwo chromosomesor four chromatids four chromatids (sister and non-sister chromatids). 9

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•Homologs contain DNA that codes for the same genes , but different versions of those genes

•Genes occur at the same loci

Homologous chromosomes

sister chromatids sister chromatidsTetrad 11

Pair of chromosomeschromosomes (maternal maternal and paternal paternal ) that are similar in shape and sizesimilar in shape and size..

Homologous pairs ((tetradstetrads) carry GENES ) carry GENES controlling the SAME inherited traitscontrolling the SAME inherited traits.

Each locuslocus (position of a gene) (position of a gene) is in the same position on homologues.

HumansHumans have 23 pairs 23 pairs of homologous homologous chromosomes:chromosomes:a. First 22 pairs 22 pairs of autosomesautosomes

b. Last pair of sex chromsex chromosomesosomes12

LOCILOCILOCILOCI

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Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

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Chromosomes exist in homologous pairs in Chromosomes exist in homologous pairs in diploiddiploid (2n) cells.(2n) cells.

Exception: Exception: Sex chromosomesSex chromosomes (X, Y). (X, Y).

Other chromosomes, known as Other chromosomes, known as autosomesautosomes, they , they have homologues.have homologues.

Crossing over Crossing over may occur between non-sister chromatidschromatids at sites called chiasmatachiasmata.

Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.

Chiasmata (chiasma) Chiasmata (chiasma) are where chromosomes touch each other and exchange genes exchange genes (crossing overcrossing over.)

Causes Genetic RecombinationGenetic Recombination

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nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

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XX chromosome - female XY chromosome - male18

Homologs Homologs separateseparate

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Meiosis IMeiosis I

• Nucleus & Nucleolus Nucleus & Nucleolus disappear• SpindleSpindle forms• Chromosomes coilcoil & SynapsisSynapsis (pairing)

occurs• TetradsTetrads form & Crossing over Crossing over Occurs

centriolesspindle fiber

asterfibers

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Shortest phase Tetrads align on the equator. Independent assortment occurs – chromosomes

separate randomly causing GENETIC RECOMBINATION

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Homologs line up at equator or metaphasemetaphase plate

OR

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Formula: 2n

Example:Example:2n = 42n = 4 then then 1n = 21n = 2 thusthus 222 2 = 4= 4 combinations combinations

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In terms of In terms of Independent Independent AssortmentAssortment -how many different -how many different combinations of sperm could a combinations of sperm could a humanhuman malemale produce? produce?

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Formula: 2Formula: 2nn

Human chromosomes:Human chromosomes: 2n = 2n = 4646

n = 23n = 23 222323 = ~8 million combinations = ~8 million combinations

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Homologous chromosomes Homologous chromosomes separate and move towards the poles.

Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

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HomologsHomologs separate separate

Each pole now has haploidhaploid (1n) set of chromosomeschromosomes.

CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

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cytokinesis

Sister Sister Chromatids Chromatids SeparateSeparate

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Meiosis IIMeiosis II

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

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Same as Same as ProphaseProphase in mitosis in mitosis Nucleus & nucleolus disappearNucleus & nucleolus disappear Chromosomes condenseChromosomes condense Spindle formsSpindle forms

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Same as Metaphase in mitosis

Chromosomes (not homologs) line up at equator33

Same as AnaphaseAnaphase in mitosismitosis SISTER CHROMATIDS SISTER CHROMATIDS separateseparate

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Same as Same as TelophaseTelophase in mitosis. in mitosis. Nuclei Nuclei and and Nucleoli Nucleoli reform, spindle reform, spindle disappearsdisappears

CYTOKINESISCYTOKINESIS occurs. occurs. Remember: Remember: FOUR HAPLOID FOUR HAPLOID DAUGHTER DAUGHTER cells are produced.cells are produced.

Called Called GAMETESGAMETES (eggs and sperm) (eggs and sperm)

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1n Sperm cell fertilizes 1n egg to form 2n zygote

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Also known as Also known as GENETIC GENETIC RECOMBINATIONRECOMBINATION

Important to population as the raw Important to population as the raw material for material for NATURAL SELECTION.NATURAL SELECTION.

All organisms are All organisms are NOTNOT alike alike StrongestStrongest “ “most fit” most fit” survive survive toto reproduce reproduce && pass on pass on traitstraits

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What are the What are the 33 sources of sources of genetic recombination genetic recombination or or variation? variation?

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1.1. CROSSING OVER CROSSING OVER (prophase I)(prophase I)2.2. INDEPENDENT ASSORTMENT INDEPENDENT ASSORTMENT

(metaphase I)(metaphase I)3.3. RANDOM FERTILIZATIONRANDOM FERTILIZATION

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A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

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1010 chromosomes (haploid or chromosomes (haploid or 1n)1n)

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AAn organized organized picture picture of the chromosomes of a human arranged in arranged in pairs by size pairs by size from largest to smallest.

PPairs 1-221-22 called AUTOSOMESAUTOSOMES

LLast pair are SEX SEX CHROMOSOMESCHROMOSOMES

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Male - XYMale - XY

Here is a human male karyotype Here is a human male karyotype showing showing duplicated homologous duplicated homologous

chromosomes.chromosomes.Here is a human male karyotype Here is a human male karyotype

of a human spermof a human sperm.

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45Female - XXFemale - XX

46Female - XXFemale - XXDown Syndrome – Trisomy 21

The fusion of a spermsperm and eggegg to form a zygotezygote.

A zygote is a FERTILIZED FERTILIZED EGGEGG

n=23egg

sperm n=23

2n=46zygote

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A cell containing 40 chromatids40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

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10 chromosomes10 chromosomes

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