Genetics

MEIOSIS

GRADE 12

LIFE SIENCES

WHAT IS GENETICS?

• Genetics is a study in biology that was established by a famous

scientist named Gregor Mendel

• This is a study about how traits are inherited though the

interaction of genes.

CHROMOSOMES

• In humans somatic cell (body cells) have 23 pairs of chromosomes

• 22 pairs of these have the same size and genetic composition and are known as Autosomes

• and the last pair are the sex chromosomes and are known as Gonosomes

HOMOLOGOUS CHROMOSOMES

• The gonosomes (X and Y) are different both in size and genetic composition.

• Autosomes are arranged in homologous pairs, which means they have the same length, centromere position and gene location(locus).

• Different versions of genes may be found at the same locus on both maternal and partenal chromosomes

HOMOLOGOUS PAIR

MEIOSIS

Meiosis is a type of cell division that produces haploid gametes

(sperm and egg cells) from diploid cells, during fertilization the two

haploid gametes fuse together forming a diploid (2n) zygote.

MEIOSIS 1

INTERPHASE 1

• The cell accumulates energy

• DNA is replicated for duplication of

chromosomes

• Structure of the cell remains the same.

MEIOSIS1

PROPHASE 1

• Chromatin network coils forming chromosomes

• Chromosomes lay next to one another by synapsis

forming a tetrad to allow crossing over to occur

to ensure genetic variation

• Nuclear membrane and nucleolus disappear

• Centrioli move to opposite poles forming spindle fiblres in between them.

MEIOSIS1

METAPHASE 1

• Centrioli have reached the poles

• Homologous pairs of chromosomes align on the

equatorial region of the cell

• Each homologous pair is attached to one spindle

fiber

• Chromosomes attache to spindle fibre by means of a kinetochore of centomere

MEIOSIS1

ANAPHASE 1

• Centrioli on poles contract

• Homologous chromosomes are pulled to

opposite poles

MEIOSIS1

TELOPHASE 1

• Nuclear membrane and nucleolus reappear.

• Number of chromosomes is now haploid(n)

on each chromosome

• Cell invagination occurs forming a cleavage

furrow to form two n cells

MEIOSIS II

PROPHASE II

• Now the two cells from telophase 1 have a

haipliod (n) number of chromosomes.

• Nuclear membrane and nucleolus re-disappear

• Cetrioli begin moving to poles forming spindle

fibres in both cells.

MEIOSIS II

• Individual duplicated chromosomes align at

the equator

• One individual deplicated chromosome is

attached on one spindle fiber

MEIOSISII

ANAPHASE II

• Centrioli on the poles contract

• Duplicated chromosomes split in half as well

as the centromere

• Daughter chromosomes are pulled to the poles

MEIOSIS II

TELOPHASE II

• Nuclear membrane and nucleolus are formed

• Centrosome is formed

• Daughter chromosomes have reached the poles

• They uncoil forming chromatin

• Cell invagination occurs in each cell

• We end up with four cells

REFERENCE LIST

Reece, J.B, Urry, L.A. ,Wasserman, S.A. , Minorsky, P.V. , Jackson, R.B. (2008).

Cambell Biology. Pearson Education: San Francisco

Anonymous. (2013). Zygote pictures. [Online], Available:

www.canstockphoto.com Accessed 06 March 2014