Cell division

Cell replication

Is a form of cell division in which a parent cell divides to produce two genetically identical daughter cells

Why eukaryotic cells replicate

• Restoring the nucleus-to-cytoplasm ratio

• Growth and development

• Maintenance and repair

Unicellular replication

Organisms do not need to replicate because they remain a singular cell their entire life cycle and instead is a simple form of reproduction to create a new, genetically identical individual

Nucleus-to-cytoplasm

A consequence of a cell being large is a very low nucleus-to-cytoplasm ratio. Therefore they are unable to balance nucleus acid and protein synthesis, which results in a decrease in cell efficiency due to relatively large diffusion distances

Mitosis

How cells replicate by the division of the nucleus into two daughter nuclei in eukaryotes

Cytokinesis

Where at the end of mitosis the cytoplasm divides, separating the two nuclei and other organelles into two complete daughter cells

Replication

Produced two identical cells from one parent cell

Reproduction

Produces a new organism from one or two parent organisms

Origin

The point at where a singular circular DNA chromosome in prokaryotes is attached

Nucleoid

The irregularly shaped region where the circular DNA chromosome of prokaryotes is contained

Binary fission step 1

The DNA molecule is duplicated within the nucleoid, resulting in 2 identical DNA molecules

Replication of chromosome begins at origin

Binary fission step 2

The two DNA molecules are pulled to separate poles as the cell increases in size

The cell grows until it has almost doubled in size

Binary fission step 3

A new cell wall and plasma membrane form between the separating chromosomes, dividing the cell into 2 relatively equal halves

These halves eventually separated forming 2 daughter cells from the single parent cell

Cell Cycle

The series of stages that a cell passes through, from its formation by cell division through its growth and function until it divides again

Interphase

The first stage of the cell cycle, and begins immediately after the end of cell division.

A cell that is about to divide grows larger, and copies its chromosomes in preparation for cell division.

Always lasts much longer than mitosis.

G1 Phase (Gap 1)

• The cell gains energy and undergoes metabolic processes such as protein and membrane synthesis, and almost doubles in size

• Growth includes various structures within the cytoplasm, including a large increase in the number of organelles

• Longest part of interphase

G0 Phase

• Carry out the normal functions of the cell but do not change their internal structure or size

• It is temporary in most cells and ends when the cells reenter G1, however some cells remain permanently and do not replicate (e.g. nerve and red blood cells)'

• Cells enter at the start of G1

S Phase (Synthesis)

• Where chromosomes are replicated in the nucleus

G2 Phase (Gap 2)

• Cell undergoes a secondary stage of growth, metabolism and energy acquisition

• Prepares for mitosis by synthesising the materials needed for division, such as proteins

Mitosis

The division of the nucleus in the cell cycle

Prophase

• Chromosomes begin to condense

• Chromosome can be seen as 2 chromatids held together at the centrioles

• The nuclear membrane breaks down and the centrioles begin to break down into the mitotic spindle which extends between the 2 poles of the cell

Metaphase

• The centromere of each individual chromosome attaches to spindle fibres so that the chromosomes are aligned in the middle of the cell

Anaphase

• The spindle fibres contract, pulling the 2 centromeres in opposite directions and separating the 2 chromatids by splitting the centromere

Telophase

• A nuclear membrane forms around the chromosomes at each pole and the mitotic spindle disappears

• When complete, each daughter nucleus moves into G1 of interphase

Cytokinesis

The cytoplasm divides and the new nuclei separate

Cytokinesis in animal cells

• The plasma membrane moves inwards, pinching the 2 daughter cells apart

Cytokinesis in plant + fungi cells

• Lay down a new plasma membrane and cell wall between the 2 daughter nuclei to separate the cells

Coenocyte

A large cell containing many nuclei which forms when mitosis is not followed by cytokinesis

DNA replication

The cell mechanism for accurately copying (replicating) and synthesising new DNA .

1. The parent DNA molecule starts to ‘unzip’ at one end

2. Complementary bases attach to both strands of the DNA

3. Replication results in 2 identical strands of DNA