MEIOSISMEIOSIS

The majority of all human cells have 46 chromosomes.

Question: What would happen if the sperm and egg each contained 46 chromosomes?

FertilizationFertilization

The fertilized egg would have 92 chromosomes, and DIE!

n=46egg

sperm n=46

2n=92zygote

Organisms that reproduce Sexually are Organisms that reproduce Sexually are made up of made up of two different types two different types of cells.of cells.

1. Somatic Cells are “body” cells and contain the normal number of chromosomes

2. Somatic cells are called the “Diploid” (the symbol is 2n). Examples would be … skin cells, brain cells, etc.

Gametes are the “sex” cells and contain only ½ the normal number of chromosomes

Gametes are called the “Haploid” (the symbol is n)….. Sperm cells and ova are gametes.

n = number of chromosomes in the set… so….2n means 2 chromosomes in the set…. Polyploid cells have more than two chromosomes per set… example: 3n (3 chromosomes per set)

Polyploid is very common in goldfish, salmon Polyploid is very common in goldfish, salmon and salamandersand salamandersAlso banana’s and watermelon’sAlso banana’s and watermelon’s

Examples

GametesGametes

The Male Gamete is the Sperm and is produced in the male gonad the Testes.

The Female Gamete is the Ovum (ova = pl.) and is produced in the female

gonad the Ovaries.

During During OvulationOvulation the ovum is released the ovum is released from the ovary and transported to an from the ovary and transported to an area where area where fertilizationfertilization, the joining of , the joining of the sperm and ovum, can occur…… the sperm and ovum, can occur…… fertilization, in Humans, occurs in the fertilization, in Humans, occurs in the Fallopian tube. Fertilization results in Fallopian tube. Fertilization results in the formation of the the formation of the ZygoteZygote. (fertilized . (fertilized egg)egg)

Sperm + Ovum (egg) Zygotefertilization

FertilizationFertilization

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

A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

ChromosomesChromosomes

If an organism has the Diploid number (2n) it has two matching homologues per set. One of the homologues comes from

the mother (and has the mother’s DNA).… the other homologue comes from the father (and has the father’s DNA).

Most organisms are diploid. Humans have 23 sets of chromosomes… therefore

humans have 46 total chromosomes….. The diploid number for humans is 46 (46

chromosomes per cell).

Homologous ChromosomesHomologous ChromosomesPair of chromosomeschromosomes (maternalmaternal and paternalpaternal)

that are similar in shape and size.Homologous pairs (tetrads) (tetrads) carry genes

controlling the same inherited traits.Each locuslocus (position of a gene) (position of a gene) is in the same

position on homologues.Humans have 23 pairs of homologous homologous

chromosomes.chromosomes.

22 pairs of autosomesautosomes 1 pair of sex chromosomessex chromosomes

Homologous ChromosomesHomologous Chromosomes((because a homologous pair consists of 4 chromatids it is called a “because a homologous pair consists of 4 chromatids it is called a “TetradTetrad”)”)

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Humans have 23 Sets of Homologous ChromosomesHumans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues.Each Homologous set is made up of 2 Homologues.

Homologue

Homologue

AutosomesAutosomes(The Autosomes code for most of the offspring’s traits)(The Autosomes code for most of the offspring’s traits)

In Humans the “Autosomes” are sets 1 - 22

Sex ChromosomesSex ChromosomesThe Sex Chromosomes code for the sex of the offspring.The Sex Chromosomes code for the sex of the offspring.** If the offspring has two “X” chromosomes it will be a ** If the offspring has two “X” chromosomes it will be a femalefemale.. ** If the offspring has one “X” chromosome and one “Y” ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a chromosome it will be a malemale..

XX chromosome - female XY chromosome - male

In Humans the “Sex Chromosomes” are

the 23rd set

Sex ChromosomesSex Chromosomes

“Sex Chromosomes” …….the 23rd set

23

This person has 2 “X” chromosomes…

and is a female.

Meiosis is the process by which is the process by which ””gametesgametes” (sex cells) , ” (sex cells) , with with halfhalf the number of the number of chromosomes,chromosomes, are produced.are produced.

During Meiosis diploid cells are reduced to haploid cells

Diploid (Diploid (2n2n) ) Haploid (Haploid (nn))

If Meiosis did not occur the chromosome If Meiosis did not occur the chromosome number in each new generation would number in each new generation would

double…. The offspring would die.double…. The offspring would die.

MeiosisMeiosis

Meiosis is Meiosis is Two cell divisionsTwo cell divisions (called meiosis Imeiosis I and meiosis IImeiosis II)

with only one duplication of with only one duplication of chromosomes.chromosomes.

Meiosis in males is called Meiosis in males is called spermatogenesisspermatogenesis and produces and produces

sperm.sperm.

Meiosis in females is called Meiosis in females is called oogenesisoogenesis and produces ova. and produces ova.

SpermatogenesisSpermatogenesis

2n=46

humansex cell

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

4 sperm cells are produced from each

primary spermatocyte.

Primary Spermatocyte

Secondary Spermatocyte

Secondary Spermatocyte

OogenesisOogenesis

*** The polar bodies die… only one ovum (egg) is produced from each primary oocyte.

Interphase IInterphase I

Similar to mitosismitosis interphase.

ChromosomesChromosomes replicate (S phase).(S phase).

Each duplicated chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.

CentrioleCentriole pairs also replicate.

Interphase IInterphase I

NucleusNucleus and nucleolusnucleolus visible.

nuclear membrane

nucleolus

cell membrane

chromatin

Meiosis I (four phases)Meiosis I (four phases)

Cell division Cell division that reduces the chromosomechromosome 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

Prophase IProphase I

Longest and most complex phase.Longest and most complex phase.90%90% of the meiotic process is spent in of the meiotic process is spent in

Prophase IProphase IChromosomesChromosomes condense.

SynapsisSynapsis occurs: homologoushomologous chromosomes chromosomes come together to form a tetradtetrad.

TetradTetrad is two chromosomeschromosomes or four chromatidschromatids (sister and nonsister chromatids).

Prophase IProphase I - - SynapsisSynapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

During Prophase I During Prophase I “Crossing Over” occurs.“Crossing Over” occurs.

During Crossing overDuring Crossing over segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid. The ChiasmataChiasmata

(chiasma) (chiasma) are the sites of crossing overcrossing over.

Crossing Over is one of the Two major occurrences of Meiosis

(The other is Non-disjunction)

Crossing OverCrossing Over creates variation (diversity) in the offspring’s traits.creates variation (diversity) in the offspring’s traits.

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Question:Question:

A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

Answer:Answer:

10 chromosomes (haploid)10 chromosomes (haploid)

Question:Question:

A cell containing 40 chromatids40 chromatids at the beginning of meiosis would, at its

completion, produce cells containing how many chromosomeschromosomes?

Answer:Answer:

10 chromosomes10 chromosomes

Prophase IProphase I

centriolesspindle fiber

asterfibers

Metaphase IMetaphase IShortest phaseShortest phaseTetradsTetrads align on the metaphase platemetaphase plate.INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:

1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n

Example:Example: 2n = 42n = 4then then n = 2 n = 2

thusthus 2 22 2 = 4 combinations= 4 combinations

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Anaphase IAnaphase I

HomologousHomologous chromosomes chromosomes separate and move towards the poles.

Sister chromatids Sister chromatids remain attached at their centromerecentromeress.

Anaphase IAnaphase I

Telophase ITelophase I

Each pole now has haploidhaploid set of chromosomeschromosomes.

CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

Telophase ITelophase I

Meiosis IIMeiosis II

No interphase II No interphase II (or very short - no more DNA replicationDNA replication)

Remember:Remember: Meiosis IIMeiosis II is similar to mitosismitosis

Prophase IIProphase II

same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II

same as metaphasemetaphase in mitosismitosis

metaphase platemetaphase plate

Anaphase IIAnaphase II

same as anaphaseanaphase in mitosismitosissister chromatids separatesister chromatids separate

Telophase IITelophase II

Same as telophasetelophase in mitosismitosis.

Nuclei form.

CytokinesisCytokinesis occurs.

Remember:Remember: four haploid daughter four haploid daughter cells cells produced.produced.

gametes = sperm or egggametes = sperm or egg

Telophase IITelophase II

Meiosis KM 46

AnimationAnimation

Meiosis KM 47

Meiosis creates genetic Meiosis creates genetic variationvariationDuring normal cell growth, mitosis produces

daughter cells identical to parent cell (2n to 2n)

Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over.

No daughter cells formed during meiosis are genetically identical to either mother or father

During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

Meiosis KM 48

Sex is costly!Sex is costly!Large amounts of energy required to find a

mate and do the mating: specialized structures and behavior required

Intimate contact provides route for infection

by parasites (AIDS, syphillis, etc.) Genetic costs: in sex, we pass on only half of

genes to offspring.

Males are an expensive luxury - in most species they contribute little to rearing offspring.

Meiosis KM 49

But … The BenefitsBut … The Benefits

More genetic diversity: more potential for survival of species when environmental conditions change. ◦ Shuffling of genes in meiosis ◦ Crossing-over in meiosis ◦ Fertilization: combines genes from 2 separate individuals

DNA back-up and repair. ◦Asexual organisms don't have back-up copies of

genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged.

◦Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene.

Non-disjunctionNon-disjunction

Non-disjunction is the failure of homologous chromosomes, or sister

chromatids, to separate during meiosis. Non-disjunction results with the

production of zygotes with abnormal chromosome numbers…… remember….

An abnormal chromosome number (abnormal amount of DNA) is damaging

to the offspring.

Non-disjunction is one of the Two major occurrences of Meiosis

(The other is Crossing Over)

Non-disjunctions usually occur Non-disjunctions usually occur in one of two fashions.in one of two fashions.

The first is called Monosomy, the second is called Trisomy. If an organism has

Trisomy 18 it has three chromosomes in the 18th set, Trisomy 21…. Three chromosomes in the 21st set. If an

organism has Monosomy 23 it has only one chromosome in the 23rd set.

Common Non-disjunction DisordersCommon Non-disjunction Disorders

Down’s Syndrome – Trisomy 21Turner’s Syndrome – Monosomy 23 (X)Kleinfelter’s Syndrome – Trisomy 23

(XXY)Edward’s Syndrome – Trisomy 18

AmniocentesisAmniocentesis

An Amniocentesis is a procedure a pregnant woman can have in order to

detect some genetics disorders…..such as non-disjunction.

AmniocentesisAmniocentesis

Amniotic fluid withdrawn

KaryotypeKaryotype(picture of an individual’s chromosomes)(picture of an individual’s chromosomes)

One of the ways to analyze the amniocentesis is to make a Karyotype

What genetic disorder does this karyotype

show?Trisomy

21….Down’s Syndrome