MEIOSIS AND SEXUAL MEIOSIS AND SEXUAL LIFE CYCLES LIFE CYCLES

Section B: The Role of Meiosis in Sexual Life CyclesSection B: The Role of Meiosis in Sexual Life CyclesSection B: The Role of Meiosis in Sexual Life CyclesSection B: The Role of Meiosis in Sexual Life Cycles

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A A life cyclelife cycle of an organism is the generation-to-generation of an organism is the generation-to-generation

sequence of stages in its reproductive history.sequence of stages in its reproductive history.

It starts at the conception of an organism until it produces its own It starts at the conception of an organism until it produces its own

offspring.offspring.

In humans, each In humans, each somatic cellsomatic cell ( (all cells other than sperm or ovum) has ) has

46 chromosomes.46 chromosomes.

These These homologous chromosomehomologous chromosome pairs carry genes that control the pairs carry genes that control the

same inherited characters.same inherited characters.

A A karyotype karyotype display of the 46 chromosomes shows 23 pairs of display of the 46 chromosomes shows 23 pairs of

chromosomes, each pair with the same length, centromere chromosomes, each pair with the same length, centromere

position, and staining pattern.position, and staining pattern.

Fertilization and Meiosis Fertilization and Meiosis alternate in sexual life cyclesalternate in sexual life cycles

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The KaryotypeThe Karyotype

It is a display of an individual’s chromosomes those are arranged It is a display of an individual’s chromosomes those are arranged according to size and shapesaccording to size and shapes

• An exception to the rule of homologous chromosomes is found in An exception to the rule of homologous chromosomes is found in

the the sex chromosomessex chromosomes, the , the XX and the and the YY..

• The pattern of inheritance of these chromosomes determine an The pattern of inheritance of these chromosomes determine an individual’s sex.individual’s sex.– Human females have a homologous pair of X chromosomes (Human females have a homologous pair of X chromosomes (XXXX).).– Human males have an X and a Y chromosome (Human males have an X and a Y chromosome (XYXY).).

• The other 22 pairs are called The other 22 pairs are called autosomesautosomes..

• We inherit one chromosome of each homologous pair from each We inherit one chromosome of each homologous pair from each parent.parent.– The 46 chromosomes in a somatic cell can be viewed as two sets of 23, The 46 chromosomes in a somatic cell can be viewed as two sets of 23,

a a maternal setmaternal set and a and a paternal setpaternal set..

• Sperm cells or ova (Sperm cells or ova (gametesgametes) have only one set of chromosomes - ) have only one set of chromosomes - 22 autosomes and an 22 autosomes and an XX or a or a YY..

Chromosomes (sex and autosomes)Chromosomes (sex and autosomes)Chromosomes (sex and autosomes)Chromosomes (sex and autosomes)

• A cell with a single chromosome set is called A cell with a single chromosome set is called haploidhaploid..– For humans, the haploid number of chromosomes is 23 (n = 23). For humans, the haploid number of chromosomes is 23 (n = 23).

• A haploid sperm reaches and fuses with a A haploid sperm reaches and fuses with a haploidhaploid ovum. ovum.

• These cells fuse (These cells fuse (syngamysyngamy) resulting in fertilization.) resulting in fertilization.

• The fertilized egg (The fertilized egg (zygotezygote) now has a ) now has a diploiddiploid set of chromosomes set of chromosomes from the maternal and paternal family lines.from the maternal and paternal family lines.

• The zygote and all cells with two sets of chromosomes are The zygote and all cells with two sets of chromosomes are diploid diploid cells cells 46 (2n = 46)46 (2n = 46). .

• As an organism develops from a zygote to a sexually mature adult, As an organism develops from a zygote to a sexually mature adult, the zygote’s genes are passes on to all somatic cells by the zygote’s genes are passes on to all somatic cells by mitosismitosis..

• Gametes, which develop in the gonads, are Gametes, which develop in the gonads, are notnot produced by mitosis. produced by mitosis.

• Instead, gametes undergo the process of meiosis in which the Instead, gametes undergo the process of meiosis in which the chromosome number is halved.chromosome number is halved.

Gametes, produced by meiosis, are

the only haploid cells.

Gametes undergo no divisions

themselves, but fuse to form a diploid

zygote that

divides by mitosis to

produce a

multicellular organism

• Fertilization restores the Fertilization restores the diploiddiploid condition by combining two condition by combining two haploidhaploid sets of sets of chromosomes.chromosomes.

• Fertilization and meiosis alternate in sexual life cycles. Fertilization and meiosis alternate in sexual life cycles.

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Many steps of meiosis resemble steps in mitosis.Many steps of meiosis resemble steps in mitosis.

Both are preceded by the replication of Both are preceded by the replication of

chromosomes.chromosomes.

However, in meiosis, chromosomes replicate However, in meiosis, chromosomes replicate

once followed by two consecutive cell divisions, once followed by two consecutive cell divisions,

meiosis meiosis II and and meiosis meiosis IIII, which results in , which results in four four

daughter cellsdaughter cells. .

Each final daughter cell has only half Each final daughter cell has only half

chromosomes number (chromosomes number (haploid)haploid)..

Meiosis reduces chromosome number by Meiosis reduces chromosome number by

copying the chromosomes once, but dividing copying the chromosomes once, but dividing

twicetwice.. The first division (The first division (meiosis meiosis II) separates homologous ) separates homologous

chromosomes.chromosomes.

The second (The second (meiosis meiosis IIII) separates sister ) separates sister

chromatids.chromatids.

Meiosis Meiosis ((Reduction DivisionReduction Division) ) RReduces chromosome number from diploid to haploideduces chromosome number from diploid to haploid : :

1)- interphase1)- interphase the chromosomes are the chromosomes are replicated to form sister chromatids.replicated to form sister chromatids.

2- Meiosis2- Meiosis

A)- Meiosis A)- Meiosis II::

2)- Prophase I, the chromosomes condense and homologous chromosomes pair up to form tetrads.

• Homologous chromosomes attached together (synapsis).– Chromatids of homologous chromosomes are

crossed (at chiasmata) and segments of the chromosomes are exchanged (Crossing OverCrossing Over).

3)- Metaphase 3)- Metaphase II,, the tetrads are all arranged at the metaphase plate. the tetrads are all arranged at the metaphase plate.– Microtubules from one pole are attached to the kinetochore of one Microtubules from one pole are attached to the kinetochore of one

chromosome of each tetrad, while those from the other pole are chromosome of each tetrad, while those from the other pole are attached to the other.attached to the other.

4)- Anaphase 4)- Anaphase II,, the homologous the homologous chromosomes separate chromosomes separate and are pulled toward and are pulled toward opposite poles. opposite poles.

5)- Telophase 5)- Telophase II,, movement of homologous movement of homologous chromosomes continues until there is a chromosomes continues until there is a haploid set at each pole.haploid set at each pole.– Each chromosome consists of linked sister Each chromosome consists of linked sister

chromatids.chromatids.

• Cytokinesis followsCytokinesis follows

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B)- Meiosis B)- Meiosis IIII 1)- Prophase 1)- Prophase IIII a spindle apparatus forms, attaches to a spindle apparatus forms, attaches to kinetochores of each sister chromatids, and moves them around.kinetochores of each sister chromatids, and moves them around.

2)- Metaphase 2)- Metaphase IIII,, the sister chromatids are arranged at the metaphase plate. the sister chromatids are arranged at the metaphase plate.

3)- Anaphase 3)- Anaphase IIII, the centromeres of sister chromatids separate and the , the centromeres of sister chromatids separate and the separate sisters chromatids travel toward opposite poles. separate sisters chromatids travel toward opposite poles.

4)- Telophase 4)- Telophase IIII, separated sister , separated sister chromatids arrive at opposite poles.chromatids arrive at opposite poles.– Nuclei are formed around the Nuclei are formed around the

chromatids.chromatids.

• CytokinesisCytokinesis separates the separates the cytoplasm.cytoplasm.

• At the end of meiosis, there are four At the end of meiosis, there are four haploid daughter cells.haploid daughter cells.

Meiosis DivisionMeiosis Division (Reduction Division)Reduction Division)

A)-A)- Meiosis Meiosis II B)-B)- Meiosis Meiosis IIII

- Separate homologous- Separate homologous chromosomes. chromosomes.

- - Results in 2 cells with replicated Results in 2 cells with replicated chromosomes.chromosomes.

- Separate homologous- Separate homologous chromosomes. chromosomes.

- - Results in 2 cells with replicated Results in 2 cells with replicated chromosomes.chromosomes.

- No further replication of- No further replication of chromosomes. chromosomes.

-Occurs in the newly resultingOccurs in the newly resulting cells from Meiosis cells from Meiosis I.I.

((4 haploid cells4 haploid cells))

- No further replication of- No further replication of chromosomes. chromosomes.

-Occurs in the newly resultingOccurs in the newly resulting cells from Meiosis cells from Meiosis I.I.

((4 haploid cells4 haploid cells))

It occurs mainly in sex gonads to form GametesGametes (sperms and ova)

Each of the resulting cells has half number of chromosomes of the original cell (23 in human). Thus, it is called Reduction DivisionReduction Division

Occurs in two steps

Crossing overCrossing over

Recombinant ChromosomesRecombinant Chromosomes

Chiasma

Crossing overCrossing over

-Occurs during prophase I.

-The two homologous chromosomes joint together very closely.

-Two non-sister chromatids of the homologous chromosomes are crossed over at a chiasma point and exchange corresponding segments.

-The resulting chromosomes are called “recombinant chromosomes”.

-It is important in genetic variation in sexual life cycle.

CrossingCrossing over over

• 3 steps3 steps– cross overcross over– breakage of DNA partsbreakage of DNA parts– re-fusing of DNA partsre-fusing of DNA parts

• New combinations of traitsNew combinations of traits

What are theadvantages of

crossing over insexual reproduction?

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• Three mechanisms contribute to genetic variationThree mechanisms contribute to genetic variation::

1)1) independent assortmentindependent assortment

2)2) crossing overcrossing over

3)3) random fertilizationrandom fertilization

Sexual life cycles produce genetic Sexual life cycles produce genetic variation among offspringvariation among offspring

1)- Independent assortment:1)- Independent assortment: of chromosomes contributes to of chromosomes contributes to genetic variability due to the random orientation of tetrads at the genetic variability due to the random orientation of tetrads at the metaphase plate.metaphase plate.– There is a There is a fifty-fiftyfifty-fifty chance chance

that a particular daughter that a particular daughter cell of meiosis I will get the cell of meiosis I will get the maternalmaternal chromosome of a chromosome of a certain homologous certain homologous pair and a pair and a fifty-fiftyfifty-fifty chance that it will chance that it will receive the receive the paternalpaternal chromosome. chromosome.

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• Independent assortment alone would Independent assortment alone would find each individual chromosome in a find each individual chromosome in a gamete that would be exclusively gamete that would be exclusively maternal or paternal in origin.maternal or paternal in origin.

3)- Crossing over:3)- Crossing over:

Homologous portions of two non-Homologous portions of two non-sister chromatids exchange places, sister chromatids exchange places, producing recombinant producing recombinant chromosomes which combine genes chromosomes which combine genes inherited from each parent.inherited from each parent.

2- The random fertilization:2- The random fertilization: it adds to it adds to the genetic variation arising from the genetic variation arising from meiosis.meiosis.

• Any sperm can fuse with any egg.Any sperm can fuse with any egg.

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Mitosis produces two identical daughter Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells, but meiosis produces 4 very different

cells.cells.

Comparison between Mitosis and Meiosis Comparison between Mitosis and Meiosis • The chromosome number is reduced by half in meiosis, but not The chromosome number is reduced by half in meiosis, but not

in mitosis.in mitosis.– Mitosis produces daughter cells that are genetically identical to the Mitosis produces daughter cells that are genetically identical to the

parent and to each other.parent and to each other.

– Meiosis produces cells that differ from the parent and each other.Meiosis produces cells that differ from the parent and each other.

• Three events, unique to meiosis, occur during the first division Three events, unique to meiosis, occur during the first division cycle.cycle.

1. During1. During prophase prophase II, homologous chromosomes pair up in a , homologous chromosomes pair up in a process called process called synapsissynapsis..– Later in prophase I, the joined homologous chromosomes are visible as Later in prophase I, the joined homologous chromosomes are visible as

a tetrad.a tetrad.

– At X-shaped regions called At X-shaped regions called chiasmatachiasmata, sections of nonsister chromatids , sections of nonsister chromatids are exchanged.are exchanged.

– Chiasmata is the physical manifestation of crossing over, a form of Chiasmata is the physical manifestation of crossing over, a form of genetic rearrangement.genetic rearrangement.

2. At2. At metaphase metaphase II homologous pairs of chromosomes, not homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase individual chromosomes are aligned along the metaphase plate.plate.

• In humans, you would see 23 tetrads.In humans, you would see 23 tetrads.

3. At 3. At anaphase anaphase II, it is homologous chromosomes, not , it is homologous chromosomes, not sister chromatids, that separate and are carried to sister chromatids, that separate and are carried to

opposite poles of the cell.opposite poles of the cell.– Sister chromatids remain attached at the centromere until Sister chromatids remain attached at the centromere until

anaphase II.anaphase II.

• The processes during the second meiotic division are The processes during the second meiotic division are virtually identical to those of mitosis.virtually identical to those of mitosis.

Comparison between Mitosis and meiosisComparison between Mitosis and meiosis