Cell division Prof. Abdulameer Al-Nuaimi E-mail: a.al-nuaimi@sheffield.ac.uk E. mail: abdulameerh@yahoo.com

Chromosomes Chromosomes are structures located in nucleus of the cell. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. Chromosomes are long thin threads inside the nucleus, they are called chromatin until cell division occurs, then they become visible as rod-like chromosomes. Chromosomes are composed of genes. All the genes on chromosomes make up the cell’s genome, Genes contain genetic information vital for proper cell function. Genes control the physical characteristics of a species. All organisms of the same species contain the same number of chromosomes in their nuclei.

Diploid cells contain 2 sets of chromosomes in their nuclei. Diploid and Haploid CELLS Diploid cells contain 2 sets of chromosomes in their nuclei. The human species contain 46 chromosomes in their nuclei i.e. 23 sets of two copies of autosomal chromosomes, it is a diploid (2n) number. Haploid cells contain 1 set of chromosomes in their nuclei. Human sex cells contain 23 chromosomes in their nuclei. This is the haploid (n) number. When fertilisation takes place the 23 chromosomes (n) from the father (called Paternal chromosomes) and the 23 chromosomes (n) from the mother (called the Maternal chromosomes) combine to form the diploid (2n=46) number of chromosomes in the fertilised egg cell.

(two identical parts of a chromosome after S phase) Scheme of a replicated Chromosome. (two identical parts of a chromosome after S phase) Chromatid Centromere Chromatid: One of the two identical parts of the chromosome after S phase (synthesis phase) . (2) Centromere: The point where the two chromatids touch, and where the microtubules attach. (3) Short arm (4) Long arm. In accordance with the display rules in Cytogenetics, the short arm is on top

Centrosome the centrosome is an organelle that serves as the main microtubule organizing centre Centrosomes are associated with the nuclear membrane  In mitosis, when the nuclear membrane breaks down, the centrosome nucleated Microtubules can interact with the chromosomes to build the mitotic spindle. The centrosome is copied during cell division cycle so that each daughter cell inherits one centrosome. The centrosome replicates during the S phase of interphase the cell cycle.   The centrosomes migrate to opposite poles of the cell. The mitotic spindle then forms between the two centrosomes. After division, each daughter cell receives one centrosome.

Centrosome replication S Phase Resting cell centrosome replicates Centrosome replication

Mitosis Mitosis is a type of cell division through which they reproduce themselves.   Mitosis  is signaled by cyclins and cyclin dependent kinase. The primary result of mitosis is the transfer of a parent cell's genome into two daughter cells. 1-The parent cell must make a copy of each chromosome before mitosis. This process is the first step in mitosis and occurs during the S phase (synthesis phase) of interphase. Interphase is a resting phase. When a cell prepares it self for division the interphase then called S phase of Interphase. In this phase chromosomes and centrosomes are replicated. Chromosome replication results in two identical sister chromatids bound together by cohesin proteins at the centromere

2-When mitosis begins, the chromosomes condense and become visible 2-When mitosis begins, the chromosomes condense and become visible. The nucleolus disappears 3-Nuclear envelope disintegrates into small vesicles.   4-Microtubules project from opposite ends of the cell, attach to the centromeres, and align the chromosomes centrally within the cell. 5-The microtubules then contract to pull the sister chromatids of each chromosome apart. Sister chromatids at this point are called daughter chromosomes. 6-As the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends of the cell and condense maximally in late anaphase.

7-Nucleolus is formed an a new nuclear envelope develops around the separated daughter chromosomes 8-After anaphase onset, the cell undergoes cytokinesis (the cytoplasmic division of a cell at the end of mitosis or meiosis, bringing about the separation into two daughter cells) The cell membrane pinches inward between the two developing nuclei to produce two new cells. 9- Chromosomes decondensed to form interphase nuclei.

centrosome replicates chromosome Condensation , nucleolus disappears Resting cell Nucleolus is formed

Meiosis   Meiosis is the program used by sexually reproducing organisms to produce haploids from diploid precursors It is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, In Meiosis  DNA replication is followed by two rounds of cell division to produce four potential daughter cells, each with half the number of chromosomes as the original parent cell Phases of meiosis 1-Premeiotic S-phase: the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through cohesion.

2-Meiotic prophase: homologous chromosomes can swap different parts of their DNA by “crossing over” to create new genetic combinations. In this stage, DNA is cut and then repaired, which allows them to exchange some of their genetic information 3-Meiosis I: Homologous chromosomes segregate away 4-Meiosis II: the cohesion between sister chromatids is released and they segregate from one another

Meiosis (1) (2) (3) premeiotic S-phase meiotic prophase (4)Meiosis II

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