MeiosisMeiosis

ReproductionReproduction AsexualAsexual

Results from cell Results from cell division (division (mitosismitosis).).

One “One “parentparent” cell ” cell divides to form two divides to form two identical “daughter” identical “daughter” cells.cells.

Ex. Ex. BuddingBudding & & binary fissionbinary fission

SexualSexual Results from Results from fusionfusion

of 2 cells to form of 2 cells to form offspring.offspring.

Offspring are Offspring are genetically genetically differentdifferent from parents.from parents.

Meiosis : An OverviewMeiosis : An Overview Meiosis is the process by which an Meiosis is the process by which an

organism creates organism creates gametesgametes.. Gametes are known as Gametes are known as sexsex cells. cells. The sex cells are The sex cells are spermsperm and egg. and egg.

Meiosis reduces the number of Meiosis reduces the number of chromosomes in these cells by chromosomes in these cells by halfhalf..

This has to happen so that when This has to happen so that when fertilization occurs, the child has the fertilization occurs, the child has the correctcorrect number. number.

Chromosome NumberChromosome Number Body cells (called Body cells (called somaticsomatic cells) are cells) are

diploiddiploid (2 (2nn) in chromosomes.) in chromosomes. This means that there are This means that there are twotwo sets of sets of

chromosomes – one received maternally chromosomes – one received maternally and the other paternally.and the other paternally.

In humans this number is In humans this number is 4646 or or 2323 pairs of pairs of chromosomeschromosomes

Sex cells (called Sex cells (called gametesgametes) are haploid ) are haploid ((nn).). Meiosis halves chromosome number so that Meiosis halves chromosome number so that

each cell has each cell has 11 of each chromosome. of each chromosome. In humans this is In humans this is 2323 chromosomes. chromosomes.

Diploid vs. HaploidDiploid vs. Haploid Diploid Diploid is the total is the total

number of chromosomes number of chromosomes in body cells.in body cells. The diploid number can The diploid number can

also be represented by also be represented by pairspairs..

HaploidHaploid is the total is the total number of chromosomes number of chromosomes in in spermsperm and and eggegg cells cells only.only. The haploid number is The haploid number is ½½

of the diploid number.of the diploid number.

Organism

Diploid # pairs Haploid

Human 46 23 pairs 23

Frog 60 30 pairs 30

Dog 78 39 pairs 39

Carrot 14 7 pairs 7

Tulip 32 16 pairs 16

MeiosisMeiosis

The process of meiosis consists of two The process of meiosis consists of two

consecutiveconsecutive divsions of the nucleus. divsions of the nucleus. Meiosis I – creates 2 Meiosis I – creates 2 diploiddiploid nuclei nuclei

Meiosis II – creates 4 Meiosis II – creates 4 haploidhaploid nuclei nuclei

DNA is not DNA is not duplicatedduplicated between divisions between divisions

Prophase IProphase I Matching Matching

chromosomes pair chromosomes pair together.together. A pair of the same A pair of the same

chromosomes is called chromosomes is called a a homologoushomologous pairpair..

A homologous pair is A homologous pair is also called a also called a tetradtetrad because it consists of 4 because it consists of 4 chromatids.chromatids.

Crossing OverCrossing Over occurs. occurs.

Chromosome StructureChromosome Structure

pair of homologous chromosomes = tetrad

Centromere

Crossing Over

Chromosome from mother

Chromosome from father

Chromatids

Crossing OverCrossing Over•When chromosomes are close together in a homologous pair, parts of chromatids can become tangled.•These segments of the chromosomes are exchanged to create more variability in chromosomes.•Creates more variety in offspring.

FertilizationFertilization Male and female gametes Male and female gametes uniteunite during during

fertilization and nuclei fuse (combine).fertilization and nuclei fuse (combine).

Fusion of two haploid nuclei produces Fusion of two haploid nuclei produces

diploiddiploid nucleus in the nucleus in the zygotezygote – fertilized – fertilized

eggegg

The genetic combination of gametes which The genetic combination of gametes which

unite is unite is randomrandom Adds to Adds to variationvariation (differences) among offspring (differences) among offspring

Mistakes in MeiosisMistakes in Meiosis If an accident occurs If an accident occurs

during meiosis, during meiosis, gametesgametes will have the will have the wrong number of wrong number of chromosomes.chromosomes.

These “These “mistakesmistakes” are ” are called nondisjunction.called nondisjunction.

If it occurs in Meiosis I, If it occurs in Meiosis I, a pair of a pair of homologoushomologous chromosomes doesn’t chromosomes doesn’t separate.separate.

Mistakes in MeiosisMistakes in Meiosis

If nondisjunction If nondisjunction occurs in Meiosis II, occurs in Meiosis II, chromatidschromatids fail to fail to separate on at least separate on at least one chromosome. one chromosome.

What does this mean?What does this mean? An extra An extra chromosomechromosome is carried along is carried along

with the normal two copies of a with the normal two copies of a chromosome and ends up in the chromosome and ends up in the fertilized egg (zygote) and then in the fertilized egg (zygote) and then in the embryo. embryo.

Nature's way of dealing with this Nature's way of dealing with this abnormalabnormal development is called development is called spontaneous abortion (spontaneous abortion (miscarriagemiscarriage). ).

It is thought that well over It is thought that well over halfhalf of all of all pregnancies end in miscarriage pregnancies end in miscarriage (although it is usually at such an early (although it is usually at such an early stage that the pregnancy isn't even stage that the pregnancy isn't even noticed). noticed).

Some Cases of SurvivalSome Cases of SurvivalDownDown Syndrome – Syndrome – TrisomyTrisomy 21 21 Cause: Cause: threethree copies of chromosome 21. copies of chromosome 21. Effect: Some impairment of mental Effect: Some impairment of mental

ability, physical growth and differences ability, physical growth and differences in facial appearance. in facial appearance.

Diagnosis: Down syndrome can be Diagnosis: Down syndrome can be identified during pregnancy or at birth. identified during pregnancy or at birth.

Occurrence: 1 per 800 to 1,000 births, Occurrence: 1 per 800 to 1,000 births, rate increases with age of mother. rate increases with age of mother.

Some Cases of SurvivalSome Cases of Survival

Some Cases of SurvivalSome Cases of SurvivalTurnerTurner Syndrome Syndrome Cause: Female with only 1 X chromosome.Cause: Female with only 1 X chromosome. Effect: Characteristic physical Effect: Characteristic physical

abnormalitiesabnormalities short height, broad chest, low hairline, low-set short height, broad chest, low hairline, low-set

ears, and webbed neck. ears, and webbed neck. Effect: Usually experience lack of Effect: Usually experience lack of

menstruation and infertility. menstruation and infertility. Effect: Often difficulties in spatial, Effect: Often difficulties in spatial,

mathematic, and memory areas. mathematic, and memory areas. Occurrence: 1 out of every 2500 girls.Occurrence: 1 out of every 2500 girls.

Some Cases of SurvivalSome Cases of SurvivalKlinefelter’sKlinefelter’s Syndrome Syndrome Cause: Male with XXY chromosomes Cause: Male with XXY chromosomes

instead of normal XY arrangement.instead of normal XY arrangement. Effect: Development of small testicles and Effect: Development of small testicles and

reduced fertility – almost always sterile. reduced fertility – almost always sterile. Effect: Possible characteristics vary widely Effect: Possible characteristics vary widely

and most often include a lanky, youthful and most often include a lanky, youthful build and facial appearance, or a rounded build and facial appearance, or a rounded body type with some more breast tissue).body type with some more breast tissue).

Effect: Language problems are also Effect: Language problems are also common.common.