cell cycle docx - D.sura

Cell Cycle & Cell division

Human biology
Chapter 18
18.2 The Cell Cycle
Lecture -10-
Wednesday 20/1/2016
Objectives
By the end, the student should be able to:-
Define the following terms:-
Cell cycle , Mitosis , Meiosis
Identify the stages of mitosis & meiosis
Compare between mitosis & meiosis
The Cell Cycle
The cell cycle is an orderly process that has two parts: interphase and cell division.

Interphase

Interphase is divided into three stages:
G1 stage occurs before DNA synthesis
S stage includes DNA synthesis
G2 stage occurs after DNA synthesis

Mitosis
Mitosis is: duplication division. The nuclei of the two new cells have the same
Number and types of chromosomes as the cell that divides.
The cell that divides is called the parent cell, and the new cells are
called the daughter cells.
The parent cell and daughter cells have the same number and types of chromosomes, so they are genetically identical.

Phases of Mitosis

Prophase
Metaphase
Anaphase
Telophase

1- Prophase

The cell is preparing to divide. The centrosomes outside the nucleus have duplicated, and they begin moving away from one another toward opposite ends of the nucleus.
Spindle fibers appear between the separating centrosomes.
The nuclear envelope begins to fragment.
The chromosomes are now visible.
Each is composed of two sister chromatids held together at a
centromere.
Spindle fibers attach to the centromeres as the chromosomes continue to shorten and to thicken.
2- Metaphase
The nuclear envelope is fragmented and the spindle occupies the region formerly occupied by the nucleus.
The chromosomes are now at the equator (center) of the spindle.
Metaphase is characterized by a fully formed spindle.
The chromosomes, each with two sister chromatids, are aligned at the equator.

3-Anaphase

The centromeres uniting the sister chromatids divide.
Then the sister chromatids separate, becoming chromosomes that move toward opposite poles of the spindle.
Separation of the sister chromatids ensures that each cell receives a copy of each type of chromosome and thereby has a full complement of genes.
Anaphase is characterized by the 2n (diploid) number of -chromosomes moving toward each pole.
The number of centromeres indicates the number of chromosomes.
Each pole receives four chromosomes: two are red and two are blue.
4- Telophase
During telophase, the chromosomes become indistinct chromatin again.
The spindle disappears as the nuclear envelope components reassemble in each cell.
Each nucleus has a nucleolus because each has a region of the DNA where ribosomal subunits are produced.
Telophase is characterized by the presence of two daughter nuclei.

Cytokinesis

Cytokinesis is the division of the cytoplasm and organelles.
In human cells, a slight indentation called a cleavage furrow passes around the circumference of the cell.
Actin filaments form a contractile ring; as the ring becomes smaller, the cleavage furrow pinches the cell in half.
As a result, each cell becomes enclosed by its own plasma membrane.
Cytokinesis

Meiosis

Meiosis is reduction division. Meiosis involves two divisions,,
So there are four daughter cells. Each daughter cell has one of each type of chromosome and, therefore, half as many chromosomes as the parent cell.
The parent cell has the diploid (2n) number of chromosomes;
The daughter cells have half this number, called the haploid (n) number of chromosomes.
The daughter cells that result from meiosis go on to become the gametes.
At the start of meiosis, the parent cell is 2n, or diploid, and the chromosomes occur in pairs.
The short chromosomes are one pair, and the long chromosomes are another.
The members of a pair are called homologous chromosomes or homologues because they look alike and carry genes for the same traits, such as type of hair or color of eyes.
The parent cell has the 2n number of chromosomes; the daughter cells have the haploid (n) number of chromosomes, equal to two chromosomes.
Phases Of Meiosis
Meiosis I and Meiosis II.
Meiosis I
Prophase I, Metaphase I, Anaphase I and Telophase I.
Meiosis II
Prophase II, Metaphase II, Anaphase II and Telophase II.
Meiosis is a part of sexual reproduction.
The process of meiosis ensures that the next generation of individuals will have the diploid number of chromosomes and a combination of characteristics different from that of either parent.
Special events occur during these phases... Prophase I and metaphase I
Meiosis I
DNA replication has occurred and the chromosomes are duplicated.
Each chromosome consists of two chromatids held together at a centromere.
During meiosis I, the homologous chromosomes come together and line up side by side.
This so-called synapsis
Results in an association of four chromatids that stay in close proximity during the first two phases of meiosis I.
Synapsis is significant because its occurrence leads to a reduction of the chromosome number.
There are pairs of homologous chromosomes at the equator during meiosis I because of synapsis.
Prophase I
The nuclear envelope fragments and the nucleolus disappears.
An exchange of genetic material may occur between the nonsister chromatids of the homologous pair.
This exchange is called crossing-over. Crossing-over means that , the chromatids held together by a centromere are no longer identical.
When the chromatids separate during meiosis II, the daughter cells receive chromosomes with recombined genetic material.

Metaphase I

During metaphase I, the homologous pairs align independently at the equator.
This means that the maternal or paternal member may be oriented toward either pole.
The first four orientations will result in gametes that have different combinations of maternal and paternal chromosomes.
The next four will result in the same types of gametes as the first four.