Chromosomes Chromosomes & Meiosis& Meiosis

Chromosome NumberChromosome Number

genesgenes are located on the chromosomes in the are located on the chromosomes in the cell nucleus cell nucleus

each organism must inherit a each organism must inherit a singlesingle copy of copy of every gene from each parent every gene from each parent

when an organism produces its own when an organism produces its own gametesgametes ((egg or spermegg or sperm), each set of genes must be ), each set of genes must be separated from each other so that each separated from each other so that each gamete contains just gamete contains just oneone set of genes set of genes

Genes on a ChromosomeGenes on a Chromosome

Chromosome NumberChromosome Number

homologoushomologous chromosomes– each set of chromosomes– each set of chromosomes that comes from the chromosomes that comes from the malemale parent has a corresponding (matching) parent has a corresponding (matching) chromosome from the chromosome from the femalefemale parent parent example – a human has example – a human has 4646 chromosomes ( chromosomes (2323

from the female and from the female and 2323 from the male) from the male)

the 23 from the the 23 from the malemale has a matching partner has a matching partner from the from the femalefemale

Homologous Homologous ChromosomesChromosomes

Chromosome Number Cont… Chromosome Number Cont…

We have We have twotwo terms when it comes to chromosome terms when it comes to chromosome numbers:numbers:

diploiddiploid cells contains both sets of chromosomes cells contains both sets of chromosomes (means “two-sets”) (means “two-sets”)

In humans, In humans, 2N2N = = 4646 chromosomes chromosomes the gametes of a the gametes of a sexuallysexually reproducing organism reproducing organism

contain a single set of chromosomescontain a single set of chromosomes haploidhaploid means “one set” or means “one set” or 2323 chromosomes. chromosomes.

Haploid vs. DiploidHaploid vs. Diploid

Haploid vs. DiploidHaploid vs. Diploid

MeiosisMeiosis

meiosismeiosis is a process of is a process of reductionreduction.. It cuts the number of chromosomes per cell It cuts the number of chromosomes per cell

in in halfhalf through the separation of homologous through the separation of homologous chromosomes.chromosomes.

Meiosis contains Meiosis contains twotwo distinct divisions called distinct divisions called meiosis Imeiosis I and and meiosis II meiosis II

at the end of meiosis II the diploid cell that at the end of meiosis II the diploid cell that started out has become started out has become 44 daughter cells daughter cells

MeiosisMeiosis

Introduction to Meiosis Introduction to Meiosis

each chromosome is each chromosome is replicatedreplicated prior to the prior to the start of meiosis I (interphase)start of meiosis I (interphase)

each chromosome pairs with its each chromosome pairs with its corresponding homologous chromosome to corresponding homologous chromosome to form a form a tetradtetrad

a tetrad is a group of a tetrad is a group of 44 chromosomeschromosomes chromosomes chromosomes exchangeexchange a portion of their a portion of their

chromatids in a process called chromatids in a process called crossing-overcrossing-over

A Tetrad & Crossing OverA Tetrad & Crossing Over

Crossing OverCrossing Over

Crossing OverCrossing Over Crossing-Over is a term that we as Crossing-Over is a term that we as

scientists use to explain how scientists use to explain how DNADNA from from twotwo parents comes together to form a parents comes together to form a newnew individual. individual.

Prophase IProphase I

In In Prophase 1Prophase 1, crossover occurs between the , crossover occurs between the tetrad (tetrad (4 chromatids4 chromatids).).

They begin to They begin to exchangeexchange DNA between the DNA between the chromosomes.chromosomes.

Crossing over results in the exchange of Crossing over results in the exchange of allelesalleles between homologous chromosomes between homologous chromosomes

Crossing over produces new Crossing over produces new combinationscombinations of allelesof alleles

Crossing OverCrossing Over

Metaphase IMetaphase I

Homologous chromosome pairs are spread Homologous chromosome pairs are spread across the across the metaphasemetaphase plateplate

Microtubules (spindle fibers) from each pole Microtubules (spindle fibers) from each pole are attached to the are attached to the centromerecentromere of one of one member of each homologous pairmember of each homologous pair

Anaphase IAnaphase I

Centromeres do Centromeres do NOTNOT split split One homologous One homologous pairpair of chromosomes is of chromosomes is

pulled to each polepulled to each pole

Telophase I Telophase I

homologous chromosomes separate and homologous chromosomes separate and twotwo new new cells are formed cells are formed

neither daughter cell has a neither daughter cell has a completecomplete set of set of chromosomeschromosomes

the chromosomes and alleles present in the new the chromosomes and alleles present in the new cells are cells are differentdifferent from each other and from the from each other and from the diploid cell that began the processdiploid cell that began the process

Cells may enter Cells may enter interphaseinterphase for a brief period of time for a brief period of time

Meiosis IMeiosis I

Meiosis I

Interphase I Prophase I Metaphase I Anaphase I

Cells undergo a round of DNA replication, forming duplicate Chromosomes.

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Spindle fibers attach to the chromosomes.

The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Meiosis IIMeiosis II

Meiosis is the second meiotic divisionMeiosis is the second meiotic division chromosome replication does chromosome replication does not not take place take place

Prophase II Prophase II

each chromosome has each chromosome has twotwo chromatids chromatids crossing over does crossing over does notnot take place take place

Metaphase IIMetaphase II

chromosomes line up as chromosomes line up as singlessingles in the center in the center of each cell of each cell

Anaphase IIAnaphase II

Chromosomes are pulled Chromosomes are pulled apartapart into two into two chromatids by the spindle fiberschromatids by the spindle fibers

The chromatids are now The chromatids are now chromosomeschromosomes Chromosomes migrate to the opposite polesChromosomes migrate to the opposite poles

Telophase IITelophase II

Each new daughter cell becomes a Each new daughter cell becomes a haploidhaploid cellcell

Cells contain only Cells contain only halfhalf the number of the number of chromosomeschromosomes

Chromosomes consist of only Chromosomes consist of only oneone chromatidchromatid When chromosomes are copied later in When chromosomes are copied later in

interphase, only interphase, only halfhalf the number is copiedthe number is copied

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis IIMeiosis II

Gamete Formation in MalesGamete Formation in Males

In males, the haploid gametes are called In males, the haploid gametes are called spermsperm

Meiosis is called spermatogenesisMeiosis is called spermatogenesis Meiosis begins in males at Meiosis begins in males at pubertypuberty It takes approximately It takes approximately 7474 hourshours Occurs Occurs dailydaily in males for the duration of their in males for the duration of their

lifelife Results in 4 genetically Results in 4 genetically differentdifferent sperm cellssperm cells

Gamete Formation in FemalesGamete Formation in Females

in females, the haploid gametes are called in females, the haploid gametes are called eggseggs Meiosis is called Meiosis is called oogenesisoogenesis cell divisions are cell divisions are unevenuneven resulting in one egg resulting in one egg

receiving most of the receiving most of the cytoplasmcytoplasm The larger egg becomes the egg that is involved in The larger egg becomes the egg that is involved in

reproduction reproduction the other three smaller cells become the other three smaller cells become polar bodiespolar bodies

and do not participate in reproductionand do not participate in reproduction Meiosis begins Meiosis begins before birth and finishes at before birth and finishes at

conceptionconception

Mitosis in GametesMitosis in Gametes

Comparing Mitosis and MeiosisComparing Mitosis and Meiosis

mitosis is the process by which mitosis is the process by which asexual asexual reproduction occurs reproduction occurs

mitosis results in the production of two genetically mitosis results in the production of two genetically identicalidentical diploid cells diploid cells

the diploid cells give rise to the diploid cells give rise to twotwo diploid daughter diploid daughter cells cells

the new sets of chromosomes are the new sets of chromosomes are identicalidentical to each to each other and to the parent other and to the parent

mitosis allows the body to mitosis allows the body to growgrow and replace cells and replace cells

Comparing Mitosis and MeiosisComparing Mitosis and Meiosis

meiosis results in four genetically meiosis results in four genetically differentdifferent haploid haploid cells cells

the diploid cell gives rise to the diploid cell gives rise to 44 different haploid cells different haploid cells the chromosomes are the chromosomes are notnot identicalidentical to each other or to each other or

the original diploid cell the original diploid cell meiosis is the process for meiosis is the process for sexuallysexually reproducing reproducing

organismsorganisms to produce gametes to produce gametes