Two types 1.Mitosis - Cell division by which body grows or replaces dead\injured tissue. - Occurs in somatic cells. - Each cell division gives rise to 2 identical daughter cells with chromosomes of the parent cell.(46chr.2n) CELL DIVISION 2.Meiosis – Cell division for reproduction. - Occurs in reproductive cells only. - Each cell division gives rise to 4 nonidentical reproductive cells or gametes with haploid chromosomes oocyte spermatocyte
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Two types 1.Mitosis - Cell division by which body grows or replaces dead\injured tissue.
- Occurs in somatic cells. - Each cell division gives rise to 2 identical daughter cells with chromosomes of the parent cell.(46chr.2n)
CELL DIVISION
2.Meiosis – Cell division for reproduction. - Occurs in reproductive cells only.
- Each cell division gives rise to 4 nonidentical reproductive cells or gametes with haploid chromosomes (23chr.n)
oocyte
spermatocyte
MITOTIC CELL CYCLE
Human studies show that the complete cycle lasts for 12 to 24 hrs of which 1hr involves mitosis
46chr. 4N
46chr. 2N
ITOTIC
(1 hr.)
Interphase (90% of cycle)• G1 phase~ growth • S phase~ synthesis of DNA • G2 phase~ preparation for cell division
•The reproductive cell – Spermatocyte \ Oocyte undergoes 2 divisions - Meiosis I and Meiosis II
•Each division has the stages of prophase, metaphase, anaphase & telophase
•In meiosis I the prophase is prolonged and the chromosomes are reduced from 46 4n to 23 2n (reduction division)
•In meiosis II the division occurs as in normal mitotic division
MEIOSIS I – PROPHASE STAGES
Leptotene “thin threads”- become visible due to condensation of chromatin
Zygotene “paired threads”- homologous chr. pair called bivalent chr. Process is called synapsis.
Pachytene “thick threads”- thickening of chr. & separation of chromatids to form tetrads.
Diplotene “2 threads”- homologous pairs separate but remain bound at the chiasmata (cross over) where exchange of genetic material & chromatid segments takes place
Diakinesis “moving through”- separation of chr. Nuclear membrane disappears & spindle forms.
MEIOSIS I
•Bivalent chromosomes move to equatorial plane
•One member of each bivalent pair moves to either pole – disjunction – no splitting of chrs. leading to the fomation of 2 unidentical cells
•Each cell has haploid (23) chr. & 2n DNA. Hence called reduction division.
METAPHASE
ANAPHASE
TELOPHASE
46 4n
23 2n
23 2n
•No DNA replication in interphase
METAPHASE
ANAPHASE
TELOPHASE
MEIOSIS II•Resembles mitotic division.
•Splitting of chrs. leads to formation of 2 cells of 23 n chrs.
•Thus 4 unidentical cells of 23 n chrs. formed from 2 unidentical cells of 1st meiotic division
Abnormal disjunction of chromosomes leads to abnormalities like trisomy or monosomy.
Common examples –
Trisomy 21 – Down’s syndrome
Trisomy 18 – Edward’s syndrome
Trisomy 13 – Patau’s syndrome
Monosomy – 45X – Turner’s syndrome - the only monosomy compatible with life
MITOSIS MEIOSISMitosis requires one division
Meiosis requires two divisions
Two diploid daughter cells result from mitosis
Four haploid daughter cells result from meiosis
Daughter cells are genetically identical to parental cells
Daughter cells are not genetically identical to parental cells
Occurs in all somatic cells for growth and repair.
Occurs only in the reproductive organs for the production of gametes.
In the mitotic cell cycle the interphase consists of all the following phases excepta.G1 b.Mitotic c.S (DNA synthesis)d.G2
Prophase of mitosis consists of all of the following execpta.Visibilty of chromsomesb.Degenration of nuclear membranec.Duplication of centrioled.Formation of mitotic spindle
All are true for metaphase of mitosis excepta.Chromosomes have reached their maximum contractionb.Spindle of microtubules is formedc.Chromsomes reach the equatorial planed.Disjunction (splitting) of chromsomes takes place
Which of the following is a major characteristic of meiosis Ia.Splitting of centromereb.Reducing the amount of DNA to 1Nc.Achieving the haploid number of chromosomesd.Producing primordial germ cells
During meiosis large segments of DNA are exchanged. What is the process called?