CELL DIVISION: MEIOSISGrade 12 Life Sciences

Homologous Chromosomes

In human being, somatic cells (autosomes) have 23 pairs of Homologous chromosomes

One from each parent.

Humans have 1pair of sex chromosomes called Gonosomes, (X and Y) which differ in Genetic information and size

The other 22 pairs have the same size and genetic information

Homologuous

Homologous chromosomes are paired according to:

Length

Gene Location (Singular: Locus Plural: Loci)

Centromere position

Locus: Is the position in which a gene can be found on a chromosome.

Different versions (alleles) of a gene may be found at the same locus on the maternal and paternal chromosomes.

Homologous Chromosome Pair

GametesMost animals and plants have a diploid number of body (somatic)

cells.

This means they have two sets of chromosomes (homologous pair), one from each parent

A diploid number is written as 2n

It refers to the total number of chromosomes a cell can have.

• Meiosis, is a cell division process which converts the diploid nuclei into a haploid nuclei.

• Diploid cells have 2 sets of chromosomes

• Haploid cells have 1 set of chromosomes

• Meiosis only occurs in sex organs and results in gametes- sperm and an egg cell

• Fertilization is fusion of a sperm and egg cell.

• The zygote has a diploid chromosomes number, one set from each parent

Fertilization between gametes

Meiosis

o Meiosis is a special type of cell division necessary for sexual reproduction in eukaryotes, such as animals, plants and fungi.

oMeiosis results in the formation of haploid gametes which combine during fertilization to restore the diploid state in a zygote.

Summary of Meiosis

Meiosis is divided into 2 stages

• Meiosis I

• This process consist of 5 phrases:

• Interphase I,Prophase I, Metaphase I, Anaphase I, Telophase I.

• Meiosis II

• This process consist of 4 phrases:

• Prophase II, Metaphase II, Anaphase II; Telophase II

Meiosis I: Interphase I

• DNA replication: Duplication of Chromosomes

• Cell builds up energy for the process

• Events occurring in the nucleus:

• Chromosomes coil and become individual chromosomes, nucleolus and nuclear envelope disappear.

• Homologous chromosomes come together as pairs by synapsis forming a tetrad (Each pair, with four chromatids)

• Non-sister chromatids from homologous chromosomes exchange genetic material through the process of crossing over to ensure genetic variation.

• Centrioli move to opposite poles with spindle fibers between them.

Crossing Over

• Genetic recombination is the production of new combinations of genes due to crossing over.

• Crossing over is an exchange of genes between separate chromatids on homologous chromosomes.

• Non-sister chromatids join at a chiasma (plural, chiasmata), the site of attachment.

• Genetic material are exchanged between maternal and paternal chromatids

Meiosis I: Metaphase I

• Centriole has reached the poles.

• Homologous pairs align at the cell equator.

• The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere.

Meiosis I: Anaphase I

• Spindle fibers contract.

• Duplicated chromosomes move to opposite poles.

Meiosis: Telophase I

• Duplicated chromosomes reach the poles.

• A nuclear envelope and nucleolus re-forms around chromosomes.

• Each nucleus now has the haploid number of chromosomes.

• Cell invaginates forming a cleavage furrow, which extends to for 2 separate haploid cells.

Meiosis II

• This stage follows Meiosis I however, no chromosome duplication will occur

• All the haploid products enters meiosis II

Meiosis II: Prophase II

• Chromosomes coil and become compact (if uncoiled after telophase I).

• Nuclear envelope and nucleolus, if re-formed, dissappears again.

• Centrioli move to opposite poles, forming spindle fibers between them

Meiosis II: Metaphase II

• Individual duplicated chromosomes align on the equator.

• One chromosome per spindle fibre attached by means of kinetochore of centromere.

• Centrioli has reached the poles.

Meiosis II: Anaphase II

• Spindle fibers contract.

• Duplicated chromosomes split in half (centromere dividing in 2)

• Daughter chromosomes move to opposite poles.

Meiosis II: Telophase II

• Daughter chromosomes has reached the poles.

• Two cells invaginate and form 4 daughter haploid cells (gametes)

• They uncoil and form chromatin.

• Nuclear envelope and nucleolus for around chromatin again.

• Centrioli for centrosome.

Differences and Similarities between Mitosis and Meiosis

Genetic variation in gametes

• Independent orientation at metaphase I

• Random fertilization.

• Crossing over of genes during prophase I

Consequences of Abnormal Meiosis

• An extra copy of chromosome 21 causes Down Syndrome which is also known as Trisomy 21

• This results in many debilitating characteristics eg. mental retardation

• Other diseases include: Klinefelter Syndrome and Turner Syndrome

Sources for my Presentation

• www.cybercambridge.com 2013

• Ms J Williamsons : Meiosis slides for Life Sciences 3A FET 2014

• http://www.waukeshasouth.com/ 2011

• http://en.wikipedia.org/wiki/Meiosis 2010

The End