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) (2n) HAPLOIDHAPLOID (n) (n) MeiosisMeiosis is SEXUALSEXUAL

reproduction.reproduction. TWOTWO divisions divisions (MEIOSIS IMEIOSIS I and

MEIOSIS IIMEIOSIS II).2

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

Gametes have HALFHALF the # of chromosomes.

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

Male: Male: SPERMATOGENESIS -SPERMATOGENESIS -spermsperm

Female: Female: OOGENESIS - OOGENESIS - egg or egg or

ovaova3

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 SISTER CHROMATIDS CHROMATIDS attached at their CENTROMERESCENTROMERES.

CENTRIOLECENTRIOLE pairs also replicate.

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Nucleus and nucleolus 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 chromosomesHomologous chromosomes come togetherto form a tetradtetrad.

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

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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

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Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are similar in shape similar in shape and sizeand size..

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

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

LOCLOCII

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

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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 - male

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Homologs Homologs separateseparate

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Meiosis Meiosis II

• 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 metaphase 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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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 or very shortshort

No DNA ReplicationNo DNA Replication Remember: Meiosis II is Remember: Meiosis II is

similar to mitosissimilar to mitosis

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Same as Same as ProphaseProphase in in mitosismitosis

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 equator

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Same as AnaphaseAnaphase in mitosismitosis

SISTER CHROMATIDS SISTER CHROMATIDS separateseparate

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

disappearsdisappears CYTOKINESISCYTOKINESIS occurs. occurs. Remember: Remember: FOUR HAPLOID FOUR HAPLOID

DAUGHTER cells DAUGHTER cells are produced.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 to survive to

reproduce & pass on traitsreproduce & pass on traits

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AAn organized organized picture picture of the chromosomes of a human arranged in pairs arranged in pairs by size 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

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Female - XXFemale - XX

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Female - XXFemale - XXDown Syndrome – Trisomy 21

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

A zygote is a FERTILIZED EGGFERTILIZED EGG

n=23egg

sperm n=23

2n=46zygote

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