LEC7 Biology Cell Division PDF

(Cell division) Mitosis division disteducation.mashreq.edu.sd/ The DNA:- -The genetic material is Deoxyribonucleic acid (DNA) ▶ Eukaryotes DNA consists of multiple molecules of double-stranded linear DNA, while that of prokaryotes is double-stranded and circular What is the Function...

(Cell division) Mitosis division disteducation.mashreq.edu.sd/ The DNA:- -The genetic material is Deoxyribonucleic acid (DNA) ▶ Eukaryotes DNA consists of multiple molecules of double-stranded linear DNA, while that of prokaryotes is double-stranded and circular What is the Function role Of DNA?  Carries and save the genetic information in the chromosomes from generation to generation. The structure of DNA:- ▶ The DNA consists of a large number of nucleotides arranged to form a single – strand. ▶ The building block of a DNA molecule is called The nucleotides consist of : sugar (S) (5 carbon) ,phosphate group (P) and nitrogen bases (purine or pyrimidine) Nitrogen bases: 1- purine ▶ Is adenine (A) & guanine (G) with a double cycle structure. 2-The pyrimidine: ▶ Is thymine (T) & cytosine (c) with a cycle structure. ▶ Usually, two strands are coiled around each other to form a double helix.  The two strands held together by hydrogen bonds between the base pairs.  The A linking to T by double bond A═T  The G linking to C by triplet bond G≡C. Chromosome ▶ The number of Chromosomes exist as pairs except in the germ cells ▶ Multicellular organisms are composed of two kinds of cells:  Somatic cells (body cells): contain two sets or diploid number of chromosomes written as (2n)(46 in human).  Reproductive cells (gametes; sperms & ova): contain a single set or haploid number of chromosomes written as (n) (23). ▶ A fiber of chromatin consists of a single, long DNA double helix to which is added two kinds of proteins- histones and nonhistone =chromosomal proteins. ▶ The chromatin fibers are scattered within the nucleus. The cell divided to: 1. Cell growth. 2. Reproduction. 3. Repair of damaged tissue.  Mitosis The nucleus divides one time so the parent cell division into two daughter cells. The phases: ▶ Interphase ▶ Early prophase ▶ Late prophase ▶ Metaphase ▶ Anaphase ▶ Telophase and cytokinesis 1. Interphase: -period (stage) between divisions, -Variable time length -Divided into G1, S (DNA Synthesis and replication) and G2 sub phases. 2. Prophase: Replicated chromosomes condense (each chromatid is connected to its duplicate at the centromere). Nuclear envelope breaks down near end of prophase The nucleolus disappears The mitotic apparatus (mitotic spindle) forms. 3. Metaphase: Replicated chromosomes range on the “equator” of the mitotic apparatus. The chromatids are attached to the microtubules of the mitotic spindle at the centromere. 4. Anaphase: Duplicate chromatids separate at the centromere and move toward opposite poles of the mitotic spindle. 5. Telophase: Chromosomes reach the poles of the mitotic spindle. Nuclear envelope,… reforms. Cell completes the division. Cytokinesis: comes after Telophase Means division of the cytoplasm. The parent cell forms two new cells(2 diploid cells ). A ring of microfilaments made up to forms around the cells equator, these filaments constrict the cell to form a cleavage furrow.  In animal cell the Cytokinesis begins during early anaphase.  Meiosis (meiotic) division  Occurs in sex organ (sexually cells, gametes: oocyte and spermatocyte)  In his type of division the number of chromosome is reduced from diploid (2n) to haploid (n)  At the end of that division parent cell gives four cells (haploid-n) Meiosis division: has two steps : 1. Reductional division (І) 2. Equational division (ІІ) Prophase І  Longest stage(one day)  The nucleolus and nuclear membrane disappear  Replicated of the chromatin material to form chromosome  Synapsis:  Tetrad: homologous pairs of chromosome come together, this tetrad may be exchanged the portion of chromatids called crossing over. Metaphase І  All the tetrad chromosome lie up at the spindle equator of the cell.  Spindale fibers  Kinetochore fibers anaphase І  The shape of the cell change to oval shape.  The homologues chromosome travel toward the opposite pole.  Each chromosome move to toward a pole of the spindle.  In anaphase І the chromosome do not separate. Telophase І  During this face The chromosome have formed two groups, then organized into a new nucleus to form two daughter cells  So the new daughter cells contains haploid (n) number of chromosome.  Meiosis І Parent cell(2n) ↓ Interphase 1 prophase 1 metaphase 1 anaphase 1  Homologous chromosomes separate from their partners  Meiosis ІІ: these stages are similar of mitosis division Prophase ІІ ▶ DNA no replication. ▶ A new spindle forms in two new cells. Metaphase ІІ: ▶ Each chromosome moves along spindle fibers to the equator plate. Anaphase ІІ: ▶ In each cell the chromosome separate (by spindle fiber) to chromatid and move to toward opposite poles of the cell. ▶ Now each chromatid called chromosome. Telophase ІІ: ▶ Nuclear envelopes form and cytokinesis occurs. ▶ Formed four haploid (n=23chromosome) nuclei. ▶ The chromatin material reappear.