Cell division

Definition Of Cell division The process by which cells reproduce

themselves Two types: Somatic cell division ( Somatic cell is any

cell of the body other than the germ cell) Reproductive cell division ( Germ cell is a

gamete-sperm or oocyte) Mitosis & Cytokinesis: in somatic cell

division, a cell undergoes a nuclear division called mitosis and cytoplasmic division called Cytokinesis to produce 2 identical cell with same number of chromosomes as the original cell

Mitosis

Why Do Cells Divide?

For growth, repair, and reproduction

Organisms grow by the addition of cells

In multicellular organism some of these cells perform functions different from other cells.

Mitosis

The process of a cell becoming different is differentiation.

Cytokinesis: The actual division of the cell cytoplasm into two new cells.

Mitosis: The division of the nucleus of the cell into two new nuclei.

Cell Division vs. Nuclear Division

Chromatin - thin fibrous form of DNA and proteins

Sister chromatids- identical structures that result from chromosome replication, formed during S phase

Terminology

Anatomy of a Chromosome

Centromere - point where sister chromatids are joined together

P=short arm; upward

Q=long arm; downward

Telomere-tips of chromosome

p -arm

centromere

q-arm

telomerechromatids

Cell Cycle An orderly sequence of events by which a

somatic cell duplicates its contents and divides in two.

Human cells such as those in the brain, stomach and kidneys contain 23 pairs of chromosomes, for a total of 46.

One member of each pair is inherited from each parent

Consists of 2 major periods: Interphase – when a cell is not dividing Mitotic (M) phase – when a cell is dividing

Cell cycle - sequence of phases in the life cycle of the cell

How Do Cells Divide?

Interphase

Occurs between divisions

Longest part of cycle3 stages

State of high metabolic activity

Cell does most of its growing at this period

Interphase consists of 3 phases: S, G1, & G2

S or Synthesis stage: DNA replicates in S phase

Interphase

G1 or Gap 1Interval between mitotic and S phases

The cell just finished dividing so in Gap 1 the cell is recovering from mitosis

Organelles and cytosolic components replicates

Centromere replication begins, completed in G2

G 2 or Gap 2This is preparation for mitosisOrganelles are replicated.More growth occurs. Enzymes and other proteins are synthesized

Note: G0 cells e.g. Most Nerve cells: cells in G1 that may remain there and never divide again

MITOSISMitosis begins after G 2 and ends before G 1

Prophase

Chromosome condense

Microtubles formThe nuclear envelope breaks down

Metaphase

Chromosomes are pulled to center of cell

Line up along “metaphase plate”

AnaphaseCentromeres divide

Spindle fibers pull one set of chromosomes to each pole

Precise alignment is critical to division

Nuclear envelope form around chromosomes

Chromosomes uncoilCytokinesis- pinching of plasma membrane

Telophase

Meiosis

A 2-step division of the nucleus that reduces chromosome number by half.

Reproductive cell division that occurs in the gonads (ovaries & testes), produces gametes in which the number of chromosomes is reduced by half (n)

Fertilization restores the deploid (2n) of chromosomes

Involves combining the genetic information of one parent with that of the the other parent to produce a genetically distinct individual

meiosis

Diploid - two sets of chromosomes (2n), in humans 23 pairs or 46 total

Haploid - one set of chromosomes (n) - gametes or sex cells, in humans 23 chromosomes

Terminology

Homologous pair◦each chromosome in pair are identical to the other ( carry genes for same trait)

◦only one pair differs - sex chromosomes X or Y

Chromosome Pairing

A diploid cell replicates its chromosomes

Two stages of meiosis◦Meiosis I and Meiosis II◦Only 1 replication

Phases of Meiosis

◦Synapsis - pairing of homologous chromosomes forming a tetrad.

◦Crossing over - chromatids of tetrad exchange parts.

Meiosis I

Prophase IChromosomes condenseHomologous chromosomes pair w/ each other

Each pair contains four sister chromatids - tetrad

Metaphase I

Tetrads or homologous chromosomes move to center of cell

Anaphase I

Homologous chromosomes pulled to opposite poles

Telophase I

Daughter nuclei formed

These are haploid (1n)

Meiosis II

Daughter cells undergo a second division; much like mitosis

NO ADDITIONAL REPLICATION OCCURS

Prophase II

Spindle fibers form again

Metaphase II

Sister chromatids move to the center

Anaphase IICentromeres split

Individual chromosomes are pulled to poles

Telophase II & CytokinesisFour haploid daughter cells results from one original diploid cell

Review Mitosis & Meiosis

Both are forms of nuclear division

Both involve replicationBoth involve disappearance of the nucleus, and nucleolus, nuclear membrane

Both involve formation of spindle fibers

DIFFERENCESMeiosis produces daughter cells that have 1/2 the number of chromosomes as the parent. Go from 2n to 1n.

Daughter cells produced by meiosis are not genetically identical to one another.

In meiosis cell division takes place twice but replication occurs only once.

Value of VariationVariation - differences between members of a population.

Meiosis results in random separation of chromosomes in gametes.

Causes diverse populations that over time can be stronger for survival.