Cellular Reproductionand the Cell Cycle

Mitosis

Why do cells divide?

GrowthDevelopmentRepair.

Cell division

All complex organisms originated from a single fertilised egg

Every cell in your body started here, through cell division the numbers are increased

Cells then specialise and change into their various roles.

Essential Features of Cell Division

Transmit a complete copy of genetic information (DNA) Transmit materials necessary for

cell to survive and use genetic information.

Two Fundamental Types of Cells(Organisms)

Prokaryotes Eukaryotes.

Prokaryotic Cell

No nucleus – genetic material (DNA) in cytoplasm

No membrane-bound organelles

Cell division is called binary fission

Example: bacteria.

Prokaryotic Cell

Rod-Shaped Bacterium, E. coli, dividing by binary fission

Prokaryotic Cell

Rod-Shaped Bacterium, hemorrhagic E. coli, strain 0157:H7

Prokaryotic Cell

Eukaryotic Cell

Membrane-bound organelles, including a nucleus

Genetic material (DNA) contained within the nucleus

Cell division of somatic cells called mitotic cell division

Examples: fungi, protists, plants, animals.

Eukaryotic Cell

What is Mitotic Cell Division?

Division of somatic cells (non reproductive cells) in eukaryotic organisms

A single cell divides into two identical daughter cells◦ Maintains chromosome ploidy of cell.

Ploidy Level

Ploidy – refers to the number of pairs of chromosomes in cells

haploid – one copy of each chromosome– designated as “n”

diploid – two copies (=pair) of each chromosome

– designated as “2n”

triploid – three copies of each chromosome – designated as “3n”

Number of Chromosomes

Each species has a characteristic number of chromosomes:

◦ Prokaryotes = one chromosome

◦ Crayfish (2n) = 200 chromosomes

◦ Fruit fly (2n) = 8 chromosomes

◦ Human (2n) = 46 chromosomes

◦ Wheat (6n) = 42 chromosomes

◦ Potato (4n) = 48 chromosomes.

Number of Chromosomes

Diploid organisms receive

◦ one set of chromosomes from female parent (= maternal)

◦ one set of chromosomes from male parent (= paternal)

A “matched” pair of maternal and paternal chromosomes are called homologues

gamete (n)gamete (n)

zygote (2n)

fertilization

unreplicated chromosome

arm arm

centromere

Structure of a eukaryotic chromosome

Structure of a eukaryotic chromosome Prior to cell division:

chromosomes (DNA) are replicated (duplicated)

duplicated chromosome

◦ attached at their centromeres

◦ as long as attached, known as sister chromatids

duplicated

chromosome

sister

chromatids

Structure of a eukaryotic chromosome

metacentric submetacentric acrocentric telocentric

Structure of a eukaryotic chromosome

daughter

chromosomes

sister

chromatids

Structure of a eukaryotic chromosome

The Cell Lifecycle

The cell lifecycle is well defined and can be divided into four stages:◦ Gap 1 (G1) - The growth phase in which

most cells are found most of the time◦ Synthesis (S) - During which new DNA

is synthesized◦ Gap 2 (G2) - The period during which

no transcription or translation occurs and final preparations for division are made

◦ Mitosis - Cell division.

G1

M

G2

S

The Cell LifecycleGap 1 - Doubling

of cell size.

Regular cellular

activities.

transcription and

translation etc.

Synthesis of DNA -

Regular cell

activities cease

and a copy of all

nuclear DNA is

made

Gap 2 - Final

preparation for

division

Mitosis - Cell

division

MitosisCell Division

Mitosis

Mitosis is the process by which new body cell are produced for:◦ Growth◦ Replacing damaged or old cells

This is a complex process requiring different stages.

The Cell Lifecycle

G1 S

G2M

Stages of Mitosis

During mitosis an exact copy of the genetic material in the “mother” cell must be distributed to each “daughter” cell

Each stage of mitosis is designed to achieve equal and exact distribution of the genetic material which has been copied during the S phase of the cell cycle.

Stages of Mitosis

Interphase - The ‘in between’ stage - this is the stage most cells spend their time in doing the things that cells do and, if they are preparing to divide, growing and replicating their DNA

G1

M

G2

S

Interphase

Mitosis

Prophase

Metaphase

Anaphase

Telophase.

Prophase

The chromosomes condense

The nuclear envelope and nucleolus disappear

The centrosomes move to opposite poles

The spindle starts to form, growing out of the centrosomes towards the chromosomes.

Metaphase

Metaphase is a short resting period

the chromosomes are lined up on the equator of the cell

with the centrosomes at opposite ends and the spindle fibers attached to the centromeres

Everything is aligned for the rest of the division process to occur.

Anaphase

In anaphase, the centromeres divide at this point, each individual chromosome goes from:◦ 1 chromosome with 2

chromatids

◦ to:

◦ 2 chromosomes with one chromatid each

Then the spindle fibers contract, and the chromosomes are pulled to opposite poles, towards the centrosomes.

Telophase

In telophase the cell actually divides

The chromosomes are at the poles of the spindle

The spindle disintegrates

The nuclear envelope re-forms around the two sets of chromosomes

The cytoplasm is divided into 2 separate cells, the process of cytokinesis.

Cytokinesis

The organelles get divided up into the 2 daughter cells passively: they go with whichever cell they find themselves in

In plant cells, a new cell wall made of cellulose forms between the 2 new nuclei, about where the chromosomes lined up in metaphase◦ Cell membranes form along the surfaces of this

wall

In animal cells, a ring of actin fibers forms around the cell equator and contacts, pinching the cell in half.

Cytokinesis

Summary of Mitosis Prophase:

◦ Chromosomes condense◦ Nuclear envelope disappears◦ centrosomes move to opposite sides of the cell◦ Spindle forms and attaches to centromeres on the chromosomes

Metaphase◦ Chromosomes lined up on equator of spindle◦ centrosomes at opposite ends of cell

Anaphase◦ Centromeres divide: each 2-chromatid chromosome

becomes two 1-chromatid chromosomes◦ Chromosomes pulled to opposite poles by the spindle

Telophase◦ Chromosomes de-condense◦ Nuclear envelope reappears

Cytokinesis: the cytoplasm is divided into 2 cells.