Meiosis , crossing over and linked genes

Dr.Aida Fadhel Biawi 2013

The Mitotic Process

Figure :Cell division cycle. Adividing cell with 46 chromosomes passesthrough the G1, DNA synthesis (S), andG2 phases before entering mitosis, whichconsists, in succession, of prophase (P),metaphase (M), anaphase (A), andtelophase (T). The end result of thecell division cycle is the formation oftwo daughter cells, each with 46chromosomes.

Figure : Essential features of thephases of the cell division cycle of a cellwith two pairs of chromosomes.

Figure : Metaphase chromosome.The centromere (primary constriction) ismarked by the downward diagonalpattern. The kinetochore is the region ofattachment of spindle fibers that forms onthe outward portion of the centromere. Atmetaphase the chromosome is duplicated,and each complete chromosome is achromatid.

The Meiotic Process

• Figure : Meiotic process. Meiosis I• consists of G1, DNA synthesis (S), and G2• phases; prophase I (PI), which includes• leptotene (L), zygotene (Z), pachytene• (Pa), diplotene (D), and diakinesis (Di);• metaphase I (MI), anaphase I (AI), and• telophase I (TI). The products of meiosis• I carry 23 duplicated chromosomes (23d)• from cells that entered meiosis I with 46• chromosomes. Meiosis II consists of a• combined G1/G2 phase followed by• prophase II (PII), metaphase II (MII),• and telophase II (TII). The final products• of the meiotic process are cells that• contain 23 chromosomes that comprise• one of each of the original set of 23 pairs• of chromosomes. In human females, each• meiotic process produces one functional• gamete and three nonfunctional cells• called polar bodies. In human males, all• meiotic products are functional.

Figure : Essential features of chromosomedistribution during the phases ofthe meiotic process of a cell with twopairs of chromosomes.

Figure : Overview of meiosis. Meiosis is a form of cell division in which certain cells are set aside and give rise to haploid gametes.This simplified illustration follows the fate of two chromosome pairs rather than the true 23 pairs. In actuality, the first meiotic divisionreduces the number of chromosomes to 23, all in the replicated form. In the second meiotic division, the cells essentially undergo mitosis.The result of the two meiotic divisions (in this illustration and in reality) is four haploid cells. In this illustration, homologous pairs ofchromosomes are indicated by size, and parental origin of chromosomes by color.

Meiosis – A Source of DistinctionMeiosis – A Source of Distinction

Why do you share some but not all characters of each parent?

What are the rules of this sharing game?

At one level, the answers lie in meiosis.

Meiosis does two things -Meiosis does two things -

1) Meiosis takes a cell with 1) Meiosis takes a cell with two copiestwo copies of every of every chromosome (diploid) and makes cells with a chromosome (diploid) and makes cells with a single copysingle copy of every chromosome (haploid). of every chromosome (haploid).

This is a good idea if you’re going to combine This is a good idea if you’re going to combine two cells to make a new organism. This trick two cells to make a new organism. This trick is accomplished by is accomplished by halvinghalving chromosome chromosome number. number.

In meiosis, In meiosis, one diploid cells produces four one diploid cells produces four haploid cells.haploid cells.

Why do we need meiosisWhy do we need meiosis??

• Meiosis is necessary to halve the Meiosis is necessary to halve the number of chromosomes going into number of chromosomes going into the sex cellsthe sex cells

Why halve the chromosomes in Why halve the chromosomes in gametes?gametes?

• At fertilization the male and female sex At fertilization the male and female sex cells will provide cells will provide ½ of the ½ of the chromosomeschromosomes each – so the offspring each – so the offspring has genes from bothhas genes from both parents parents

This makes for a lot of This makes for a lot of genetic diversitygenetic diversity. This . This trick is accomplished through trick is accomplished through independent independent assortmentassortment and and crossing-overcrossing-over..

Genetic diversity is important for the Genetic diversity is important for the evolution evolution of populations and speciesof populations and species..

MeiosisMeiosis

Parent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis I : Separates Meiosis I : Separates Homologous Homologous

ChromosomesChromosomes• InterphaseInterphase

– Each of the chromosomes Each of the chromosomes replicatereplicate

– The result is two genetically The result is two genetically identical sister chromatids which identical sister chromatids which remain attached at their remain attached at their centromerescentromeres

Prophase IProphase I• This is a crucial phase for mitosis.This is a crucial phase for mitosis.• During this phase each pair of chromatids During this phase each pair of chromatids

don’t move to the equator alone, they match don’t move to the equator alone, they match up with their homologous pair and fasten up with their homologous pair and fasten together together (synapsis) in a group of four called (synapsis) in a group of four called a tetrad.a tetrad.

• Extremely IMPORTANT!!! It is during this Extremely IMPORTANT!!! It is during this phase that phase that crossing over can occurcrossing over can occur. .

• Crossing Over is the Crossing Over is the exchange of segments exchange of segments

during synapsisduring synapsis..

Metaphase IMetaphase I

• The chromosomes line up at The chromosomes line up at the the equator equator attached by their attached by their centromeres to spindle fibers centromeres to spindle fibers from centrioles.from centrioles.– Still in homologous pairsStill in homologous pairs

Anaphase IAnaphase I

• The spindle guides the movement The spindle guides the movement of the chromosomes toward the of the chromosomes toward the polespoles– Sister chromatids remain attachedSister chromatids remain attached– Move as a unit towards the same poleMove as a unit towards the same pole

• The homologous chromosome The homologous chromosome moves toward the opposite polemoves toward the opposite pole– Contrasts mitosis – chromosomes Contrasts mitosis – chromosomes

appear as individuals instead of pairs appear as individuals instead of pairs (meiosis)(meiosis)

Telophase ITelophase I

• This is the end of the first meiotic cell This is the end of the first meiotic cell division. division.

• The cytoplasm divides, forming two new The cytoplasm divides, forming two new daughter cells. daughter cells.

• Each of the newly formed cells has half Each of the newly formed cells has half the number of the parent cell’s the number of the parent cell’s chromosomes, but each chromosome is chromosomes, but each chromosome is already replicated ready for the second already replicated ready for the second meiotic cell divisionmeiotic cell division

CytokinesisCytokinesis

• Occurs simultaneously with Occurs simultaneously with telophase Itelophase I– Forms 2 daughter cellsForms 2 daughter cells

• Plant cells – cell platePlant cells – cell plate• Animal cells – cleavage Animal cells – cleavage

furrowsfurrows• NO FURTHER REPLICATION NO FURTHER REPLICATION

OF GENETIC MATERIAL OF GENETIC MATERIAL PRIOR TO THE SECOND PRIOR TO THE SECOND DIVISION OF MEIOSISDIVISION OF MEIOSIS

Figure :The stages of meiotic cell division: Meiosis I

Meiosis II : Meiosis II : Separates sister Separates sister

chromatidschromatids

• Proceeds similar to mitosisProceeds similar to mitosis

• THERE IS NO INTERPHASE II !THERE IS NO INTERPHASE II !

Prophase IIProphase II

• Each of the daughter cells Each of the daughter cells forms a spindle, and the forms a spindle, and the double stranded double stranded chromosomes move toward chromosomes move toward the equatorthe equator

Metaphase IIMetaphase II

• The chromosomes are The chromosomes are positioned on the metaphase positioned on the metaphase plate in a mitosis-like fashionplate in a mitosis-like fashion

Anaphase IIAnaphase II

• The centromeres of sister The centromeres of sister chromatids finally separatechromatids finally separate

• The sister chromatids of each The sister chromatids of each pair move toward opposite pair move toward opposite polespoles– Now individual chromosomesNow individual chromosomes

Telophase II and Telophase II and CytokinesisCytokinesis

• Nuclei form at opposite poles Nuclei form at opposite poles of the cell and cytokinesis of the cell and cytokinesis occursoccurs

• After completion of After completion of cytokinesis there are four cytokinesis there are four daughter cells daughter cells – All are haploid (n)All are haploid (n)

Figure 13.7 The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Different Sex Cells (Gametes) – Independent AssortmentIndependent Assortment

Independent assortment produces 2Independent assortment produces 2nn distinct gametes, where n = the number distinct gametes, where n = the number of unique chromosomes. of unique chromosomes.

That’s a lot of diversity by this That’s a lot of diversity by this mechanism alone.mechanism alone.

In humans, n = 23 and 2In humans, n = 23 and 22323 = ???! = ???!

Figure : Overview of meiosis. Meiosis is a form of cell division in which certain cells are set aside and give rise to haploid gametes.This simplified illustration follows the fate of two chromosome pairs rather than the true 23 pairs. In actuality, the first meiotic divisionreduces the number of chromosomes to 23, all in the replicated form. In the second meiotic division, the cells essentially undergo mitosis.The result of the two meiotic divisions (in this illustration and in reality) is four haploid cells. In this illustration, homologous pairs ofchromosomes are indicated by size, and parental origin of chromosomes by color.

Figure :Crossing over recombines genes. Crossing overgenerates genetic diversity by recombining genes and therebymixing parental traits. The capital and lowercase forms of the sameletter represent different variants (alleles) of the same gene. Achromosome actually has hundreds to thousands of genes.

Another Way Meiosis Makes Lots of Different Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-OverSex Cells – Crossing-Over

Crossing-over multiplies the already huge number of different gamete Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.types produced by independent assortment.

- Genes close on a chromosome are packaged into the same gametes and are said to be “linked”

- Linkage has this very precise meaning in genetics..- Linkage refers to the transmission of genes on the same chromosome. Linked genes do not assort independently .

Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”

X chromosomeX chromosomeY chromosomeY chromosome

Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”

Meiosis – division errorMeiosis – division error

Chromosome pair

Meiosis error - Meiosis error - fertilizationfertilization

Should the gamete with the Should the gamete with the chromosome pair be fertilized chromosome pair be fertilized then the offspring will not be then the offspring will not be ‘normal’.‘normal’.

In humans this often occurs In humans this often occurs with the 21with the 21stst pair – producing pair – producing a child with Downs Syndromea child with Downs Syndrome

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