MEIOSISMEIOSIS, , Chromosomes, & Chromosomes, & FertilizationFertilization

Objective: D4 - Recognize the significance of meiosis to sexual reproduction

Homologous Homologous ChromosomesChromosomesTerm used to refer to chromosomes that

each have a corresponding chromosome from the opposite-sex parent.

In other words: A pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are similar in shape and size.

Humans have 23 pairs of homologous homologous chromosomes.chromosomes.

22 pairs of autosomesautosomes 1 pair of sex sex

chromosomeschromosomes

Organisms that reproduce Organisms that reproduce Sexually are made up of two Sexually are made up of two different types of cells.different types of cells.1. Somatic Cells are “body” cells and

contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc.

2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova (eggs) are gametes.

Meiosis is describe as reduction division because is describe as reduction division because the the number of number of chromosomes per cell chromosomes per cell are cut in are cut in halfhalf through the separation of homologous through the separation of homologous chromosomes.chromosomes.

During Meiosis diploid cells are reduced to haploid cells

Diploid (2n) Diploid (2n) Haploid (n) Haploid (n)

If Meiosis did not occur the If Meiosis did not occur the chromosome number in each new chromosome number in each new generation would double…. The generation would double…. The

offspring would die.offspring would die.

MeiosisMeiosisHas 2 cell divisionsHas 2 cell divisions

(called Meiosis I Meiosis I and Meiosis Meiosis IIII)

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

So, 1 diploid cell becomes 4 haploid cells.

In prophase of meiosis I, In prophase of meiosis I, each chromosome each chromosome pairs with its corresponding homologous pairs with its corresponding homologous

chromosome chromosome to form a to form a Tetrad Tetrad (shown below)

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Crossing OverCrossing Over--Process in which homologous chromosomes Process in which homologous chromosomes exchange portions of their chromatids during exchange portions of their chromatids during Prophase IProphase I of meiosis of meiosis- - Creates variation (diversity) in the offspring’s Creates variation (diversity) in the offspring’s traitstraits

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Phases of Phases of MeiosisMeiosisInterphase IInterphase ISimilar 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.

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 I

b.b. metaphase Imetaphase I

c.c. anaphase Ianaphase I

d.d. telophase Itelophase I

Prophase IProphase I

ChromosomesChromosomes condense.

Homologous chromosomesHomologous chromosomes come together to form a tetradtetrad.

Reminder: Reminder: 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 Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing overcrossing over.

Crossing Over is one of the Two major occurrences of Meiosis

(The other is Non-disjunction)

Prophase IProphase I

centriolesspindle fiber

asterfibers

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

TetradsTetrads align on the metaphase metaphase plateplate.

INDEPENDENT ASSORTMENT INDEPENDENT ASSORTMENT OCCURS:OCCURS:1. Orientation of homologous pair to poles is random.2. Variation

Anaphase IAnaphase IHomologous chromosomesHomologous chromosomes

separate and move towards the poles.Sister chromatids Sister chromatids remain attached

at their centromerescentromeres.

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 IINo interphase II No interphase II

(or very short - no more DNA DNA replicationreplication)

Meiosis IIMeiosis II is similar to mitosismitosis

Prophase IIProphase IIsame as prophaseprophase in mitosismitosis

Metaphase IIMetaphase IIsame as metaphasemetaphase in mitosismitosis

metaphase platemetaphase plate

Anaphase IIAnaphase IIsame 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

Compare Mitosis & Compare Mitosis & MeiosisMeiosisMitosis results in the production of two

genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells

Meiosis in Males = Meiosis in Males = 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 hapliod gametes or sperm cells are produced

from each primary spermatocyte.

Primary Spermatocyte

Secondary Spermatocyte

Secondary Spermatocyte

Meiosis in Females = Meiosis in Females = OogenesisOogenesis

*** The polar bodies (do not participate in reproduction) die… only one haploid gamete or

ovum (egg) is produced from each primary oocyte.

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

FertilizationFertilization

Fertilization Fertilization (continued)

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

A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

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)

EtymologyEtymologyHaploid: haplo- (Gk. HAPLOS,

single) + -oid (shape or form)Diploid: Diplo- (Gk. DIPLOOS,

double) + -oid (shape or form)