Cell Division

TERMINOLOGIES• DNA - linear sequence of nucleotides

• Chromosome - A full chain length double stranded, supercoiled DNA molecule. Usually becomes visible when the DNA condenses during prophase.

• Chromatid - After the DNA molecule replicates, each DNA molecule of the identical pair is called a chromatin. The pair of chromosomes is called sister chromatids.

Terminologies Cont’d

Centromere - The region where the two sister chromatids are joined

Microtubule - These are made from globular protein molecules called tubulin that are a part of the cytoskeleton structure of the cell.

Spindle Fibre - These are structures derived from microtubules that are used to pull apart the genetic material in a cell. They are attached to the centrosome and the centromere during moving about of chromosomes

Terminologies Cont’d

Centriole - Each centriole is a ring of nine groups of fused microtubules. There are three microtubules in each group, made from protein called tubulin.

During cell division, microtubules form spindle, and are responsible for moving the chromosomes around.

Kinetochore-This is a protein located in the centromere which serves as an attachment point for the

spindle fibre. This attachment allows the spindle fibre to move the chromosomes around.

Cell CycleCells go through a process called the cell cycle in to ensure their

continued existence, that is to replicate themselves.

Somatic cells (all cells except sex cells) have two phases in the cell cycle.

• Interphase• Mitosis

Interphase is the longest duration of the cell cycle. It is the time when the cell prepares itself for division, by increasing in size, the amount of genetic materials as well as the number of each cell organelle

Mitosis is the process by which the cell divides into two identical daughter cells.

Interphase

Interphase consists of three stages:• G-1 stage- During this stage the cell begins to increase

in size to accommodate the increase number of organelles.

• S stage- Also called the synthesis stage. During this period, the cell synthesize organelles and

DNA replication occurs in the nucleus.

• G-2 stage- During the stage the cell completes its growth, thus attaining its maximum size before cell division begins.

Mitosis

Mitosis consists of four stages:• Prophase• Metaphase (prometaphase and metaphase)• Anaphase• Telophase

At the end of mitosis, a final action called cytokinesis occurs to split the cytoplasm in two, thus forming two daughter cells.

Prophase

The following occurs during prophase

• Condensation (supercoil and thickening) of the DNA into defined chromosomes.

• Nucleolus disappears

• The nuclear membrane begins to breakdown

• Centrioles move away from each other

Prometaphase• The centrioles position themselves at opposite ends

of the cell

• Spindle fibres formed and attaches to the centromere of the chromosomes by kinetechores

• The nuclear membrane fully disappears

Metaphase

• With the nuclear membrane gone, the chromosomes are able to move about in a wider space. The spindle fibres are now attached to all the chromosomes in the cell.

• The microtubules pull in opposite direction on the centromeres, bringing the chromosomes to an aligned position along the equator (middle of the cell).

Anaphase

The microtubules’ continued pull on the centromeres cause the chromatids to pull apart and move to either end of the cell.

Telophase• The two groups of chromatids are at the poles• At this stage the chromatids are now called chromosomes.• The microtubules making up the spindle fibres, break

down, causing the spindle to disappear.• New nuclear envelopes form around each group of

chromosomes.• The chromosomes then uncoiled, become thinner, and

once again they are difficult to see when stained.

Cytokinesis

• The cell is compressed by a contractile ring that divides the cell into two equal halves.

• The other cell organelles begin to appear.• Each new cell has its own centrioles• The new cells are genetically identical to each other and

the parent cell.

Mitosis Cont’d

Importance of mitosis

• Replication of somatic cells• Growth in plants – occurs in the meristem region

of roots and shoots• Facilitate asexual reproduction in unicellular

organisms.• Important for development in simple

multicellular organisms

Mitosis Cont’d

Importance of Mitosis Cont’d

• Preservation of diploid number of chromosomes to ensure the preservation of genetic traits.

• It is involved in immune response - When the lymphocytes come in contact with bacteria, which has an antigen that binds with a cell receptor, it is stimulated to divide repeatedly by mitosis.

MeiosisMeiosis is a type of cell division that only occurs in gametes.It divides a diploid cell to produces four haploid cells that

are not identical to each other.

Each new cell becomes a gamete.When the nuclei of a male and a female gamete fuse

together they form a zygote which now has a diploid number of chromosome.

In a diploid cell, there are two matching copies of each chromosomes. These are known as homologous chromosomes.

Homologous Chromosomes

Homologous chromosomes consists of a pair of chromosome, one of each donated by either of the two parents.

Homologous chromosomes carry the same gene at the same positions (loci).

Genes are base sequences that carry genetic information in a section of a DNA chain.

Activities Unique to Meiosis

• During Prophase, homologous chromosomes join into pairs called bivalents.

• In a bivalent, the chromatid of one chromosome cross a chromatid of another forming a chiasma (crossing over point). Pural is chiasmata.

• In metaphase, bivalents are pulled to the equator by microtubule spindle attachment.

• During anaphase, homologous chromosomes are pulled together to opposite poles.

Prophase - 1

DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses.

The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent.

Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent.

Prometaphase - 1

The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move.

Metaphase - 1

Bivalents, each composed of two chromosomes, align at the metaphase plate. The orientation is random, with either parental homologue on a side.

This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

Anaphase - 1

Chiasmata separate. Chromosomes move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids

Independent Assortment takes place.

Telophase - 1

Nuclear envelopes may reform, or the cell may quickly start meiosis II.

Cytokinesis - 1

Analogous to mitosis where two complete daughter cells form when the cell divides.

Meiosis - II

In meiosis-II, each daughter cells from meiosis-I now undergoes cell division, identical to mitosis.

Features of Chromosomes

humans with 23 pairs of chromosomes, a gamete (egg or sperm) could have 223 or 8,388,604 possible combinations of chromosomes from that parent. Any couple could have 223 × 223 or 70,368,744,177,644 (70 trillion) different possible children, based just on the number of chromosomes, not considering the actual genes on those chromosomes.

Thus, the chance of two siblings being exactly identical would be 1 in 70 trillion.

In addition, something called crossing over, in which the two homologous chromosomes of a pair exchange equal segments during synapse in Meiosis I, can add further variation to an individual’s genetic make-up.

Features of Chromosomes Cont’d

AllelesAn allele is an alternative form of a gene (one member of a

pair) that is located at a specific position on a specific chromosome.

These DNA codes determine distinct traits that can be passed on from parents to offspring. The process by which alleles are transmitted was discovered by Gregory Mendel and formulated in what is known as Mendel's law of segregation.

Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive.

Features of Chromosomes Cont’d