D2.1 Meosis and non disjunction

Meiosis:

a reduction division to produce gametes

meosis is to make sure

after fertilization zygote form is diploid

prophase 1

chromatin condenses and becomes sister chromatids

The homologous chromosomes pair up forming bivalent

crossing over through exchanging genetic material (non sister chromatids form the chiasma)

• Nuclear membrane is broken down

• Centrosomes move to the opposite poles of the cell

• Spindle fibers begin to form

metaphase 1

Homologous chromosomes line up in the equator

random orientation

• Spindle fibers attach to the centromere of sister chromatids

anaphase 1

spindle fibers separate the pairs of homologous chromosomes

sister chromatids stay together.

Homologous chromosomes move to opposite poles of the cell

telophase 1

• Homologous chromosomes reach poles of the cell and decondense

• Nuclear membrane forms

• Cell prepares for cytokinesis

prophase 2

DNA recondenses

Nuclear membrane disintegrates

Spindle fibres form

anaphase 2

Spindle fibres shorten

Splits centromere

Sister chromatids pulled apart to each pole

metaphase 2

Spindle fibres attach to centromere

Sister chromatids line up in centre of cell

telophase 2

Chromatids now called chromosomes

Reach opposite poles of the cell

Decondense

Nuclear membrane forms

Cell prepares for cytokinesis

Bivalent

Pair of homologous chromosomes containing 4 chromatids all together

Form in prophase I

homologous chromosome

chromosome pairs in a diploid organism, where one chromosome is inherited from the mother and the other from the father

source of genetic variation

random orientation

crossing over

random orientation

Chromosomes randomly align on the equator during Metaphase I

Then pulled to different poles during Anaphase I

when does diploid becomes haplid

first round

Nondisjunction

genetic error in meosis fail to split chromosomes

nondisjunction when

Anaphase I: Pairs of homologous chromosomes fail to separate

Anaphase II: Sister chromatids fail to separate

result of nondisjunction in chromosome numbers

too many or too few chromosomes in the final gamete cell.

gametes have 22 or 24 chromosomes.

zygote will then have 47 or 45 chromosomes

Down Syndrome (Trisomy 21) symptoms

impairment in cognitive ability and physical growth, hearing loss,

oversized tongue, shorter limbs and social difficulties

if fail in anaphase 1 leading to

4gametes wrong

if fail in anaphase 2

50% wrong

trisomy 21 cause

maternal age higher risk higher

role of karyogram

diagram showing the descending length of homologous chromosomes

- function is to show gender and abnormal number of chromosomes

• unable to show genetic mutation