D2.1 Meosis and non disjunction

Meiosis:

a reduction division to produce gametes

from mother cell (diploid) to daughter cell (haploid)

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

• Chromosomes uncoil to becomechromatin

• Spindle fibers break down

• New nuclear membrane reforms at opposite pole

• Chromosome number reduces from 2n (diploid) to n (haploid)

• However, each chromatid still has the replicated sister chromatid still attached (not homologous pairs anymore)

• Cytokinesis then occurs and splits the cell into two separate cells

prophase 2

• Now there are two daughter cells, each with 23 chromosomes (23 pairs of chromatids)

• DNA Supercoil: chromatin condenses again

• Nuclear membrane is broken down and disappeared

• Centrosomes move to the opposite poles of the cell

• Spindle fibers begin to form

metaphase 2

• Pair of sister chromatids line up in the equator

• Spindle fibers (microtubules) attach to the

centromere of sister chromatids

anaphase 2

• Contraction of the spindle fibers cause the

separation of the sister chromatids

• The chromatids are now considered as chromosomes

• Chromosomes move to opposite poles of the cell

telophase 2

• The chromosomes complete their moves to the opposite poles of the cell

• At each pole of the cell a full set of chromosomes gather together

• A membrane forms around each set of chromosomes to create two new cell nuclei

• This is the last phase of meiosis, however cell division is not complete without another round of cytokinesis

• Once cytokinesis is complete there are four granddaughter cells each with a half set of chromosomes

o In males, these four cells are all sperm cells

o In females, one of the cells is an egg cell while the other three are polar bodies (small cells that do

not develop into eggs

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 or II

Then pulled to different poles during Anaphase I or II

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 and cause

impairment in cognitive ability and physical growth, hearing loss,

oversized tongue, shorter limbs and social difficulties

chromosome 21 fails to split

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

meiosis 1 vs 2

replication prior: yes vs no

Prophase crossing over: yes vs no

rmetaphase randomly line up: homologous pair vs chromosome

seperate in anaphase: homologous to chromosome vs chromosome to chromatids

crossing over

Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I

The exchange of genetic material occurs between non-sister chromatids at points called chiasmata

As a consequence of this recombination, all four chromatids will be genetically different.

difference of mitosis and meiosis

mitosis: diploid to diploid, 1 division, two genetically identical daughter cells, growth and repair, asexual, somatic cells

Meiosis: diploid to haplod, 2 division, four genetically different daughter cells, formation of genetically unique, gametes