sexual reproduction: meiosis. what is sexual reproduction? combines genetic material from two...
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Sexual Sexual Reproduction:Reproduction:
MeiosiMeiosiss
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What is Sexual What is Sexual Reproduction?Reproduction?
Combines genetic material from two Combines genetic material from two parents to produce a new individualparents to produce a new individual
Creates an increase in variation among Creates an increase in variation among the offspringthe offspring
Populations can adapt to changing Populations can adapt to changing environmentsenvironments
The world is more enjoyable with The world is more enjoyable with variationvariation
Costly – it takes twice as many Costly – it takes twice as many individuals to produce an offspringindividuals to produce an offspring
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Genetic material (DNA) is found in chromosomes
Each chromosome has a “twin” that:• contains the same genes • codes for the same traits • has the same shape • is the same size• Has the same location of the centromere • Has the same banding pattern when stained
One chromosome came from the father, the other came from the mother
These twins are called homologous.
Homologous Chromosomes
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Homologous Chromosomes
Homologous Chromosomes
Cells that have only one of each chromosome are called Haploid (n).
This is half of the usual amount of DNA.
Cells that have two of each chromosome are called Diploid (2n).
This is the normal condition for most cells.
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N =
1 – 2
9
3
6
7
10
631
13
23
24
39
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Homologous Homologous ChromosomesChromosomesHumans have 23 pairs of chromosomes –
22 pairs of autosomes and one pair of sex chromosomes
In females they are all homologous
In males, the autosomes
are homologous, but the sex
chromosomes are not Karyotype – arranges chromosomes in homologous
pairs
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Special Special Considerations:Considerations:
1.1. If a sperm containing 46 chromosomes If a sperm containing 46 chromosomes fused with an egg containing 46 fused with an egg containing 46 chromosomes, how many chromosomes would chromosomes, how many chromosomes would the resulting fertilized egg contain? the resulting fertilized egg contain?
2.2. In order to produce a fertilized egg with In order to produce a fertilized egg with the appropriate number of chromosomes (46), the appropriate number of chromosomes (46), how many chromosomes should each sperm how many chromosomes should each sperm and egg have?and egg have?
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MeiosisMeiosis
Purpose: Purpose: Divide the nucleus to reduce the Divide the nucleus to reduce the
number of chromosomes in halfnumber of chromosomes in half Ensure that each new cell has a Ensure that each new cell has a
complete set of chromosomescomplete set of chromosomes Create gametesCreate gametes Also called reduction divisionAlso called reduction division
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Human Human Life Life
CycleCycleMost of our cells are diploid (2n)
Reproductive cells called gametes are haploid (n)
They are produced in gonads (ovary and testes) from diploid gametocytes
Gametes
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Meiosis I and Meiosis II
Two Stages of Meiosis
In meiosis I the homologous
chromosomes are separated
In meiosis II the sister chromatids
are separated
Four cells are produced, each with one set of
chromosomes (n).
A cell normally has two sets of chromosomes
(2n)
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Meiosis IThe phases of meiosis I are similar to mitosis:
Prophase I, Metaphase I, Anaphase I, Telophase I
The formation of tetrads is called synapsis
Dyads are sister chromatids attached by the centromere
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Prophase 1Prophase 1•chromosomes condense•homologs pair up forming tetrads (four chromatids are lined up)•Nuclear envelope fragments •Crossing-over can occur in this stage.
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What is Crossing Over?
During synapsis of prophase I:
Homologous chromosomes tangle and trade parts of their DNA
Variation increases
Genes that are usually linked (on the same
chromosome) can become separated
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Homologous chromosomes line up at the equatorial plate.
Meiosis 1
Metaphase 1Metaphase 1
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Homologous pairs separate Sister chromatids remain together.
Meiosis 1
Anaphase 1Anaphase 1
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Meiosis 1
Telophase 1Telophase 1
Two daughter cells are formed Each daughter contains only one chromosome of the homologous pair (haploid).
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Meiosis IIAfter a phase called interkinesis (similar to G2), both cells formed in
meiosis I go through phases again
Prophase II, Metaphase II, Anaphase II, Telophase II
Inte
rkin
esis
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chromosomes condense, nuclear envelope fragments
Meiosis 2
Prophase IIProphase II
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Chromosomes align at the equatorial plate.
Meiosis 2
Metaphase IIMetaphase II
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Sister chromatids separate
Meiosis 2
Anaphase IIAnaphase II
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Cell division is complete. Four haploid daughter cells are produced.
Meiosis 2
Telophase IITelophase II
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Meiosis I in an Animal Cell
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Homologous pairsalign at the metaphaseplate.
Metaphase I
DNA Replication
Homologous chromosomes separate, pulled to opposite poles by centromeric spindle fibers.
Anaphase I
Daughter cells haveone chromosome fromeach homologous pair.
Telophase I
Homologous chromosomespair during synapsis.
Prophase I
Meiosis I2n = 4
Chromosomes stillconsist oftwo chromatids.
Interkinesis
n = 2
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Meiosis II in an animal
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Metaphase IIChromosomes alignat the metaphaseplate.
Anaphase IIDaughter chromosomesmove toward the poles.
Telophase IISpindle disappears,nuclei form, and cytokinesis takesplace.
Meosis II
Prophase IICells have onechromosomefrom each homologous pair.
n = 2 n = 2
Daughter CellsMeiosis resultsin four haploiddaughter cells.
n = 2 n = 2
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Nondisjunction
Disjunction is the separation of tetrads or sister chromatids
Nondisjunction- no separation occurs
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Turner Syndrome
XO
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Klinefelter Syndrome
XXY
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Down Syndrome
(trisomy 21)
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Four cells not identical to parent
Two cells genetically identical to parent cell
2n 2n
n
2nn
Meiosis vs.
Mitosis
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Mitosis Meiosis
One division Two divisions
Two daughter cells per cycle
Four daughter cells per cycle
Daughter cells genetically identical
Daughter cells genetically different
Same chromosome number as parents
Chromosome number half that of parents
Occurs in somatic cells Occurs in gametocytes
Throughout life cycleCompleted after sexual
maturityUsed in growth, repair, asexual reproduction
Sexual reproduction, new gene combinations