cell division

what does meiosis produce?

four genetically different haploid daughter cells (gametes)

what happens during interphase?

• preparation phase - cells get ready for division

• the cells DNA unravels and replicates to produce double-armed chromosomes, called sister chromatids.

• in a human diploid cell - there would be 23 homologous pairs at this point

what happens during prophase 1?

• homologous chromosomes pair up (crossing over occurs) and condense

• centrioles start moving to opposite ends of cell, forming spindle fibres

• nucleolus disappears

• the nuclear envelope breaks down.

what happens during metaphase 1?

the homologous pairs line up across the equator of the cell and attach to the spindle fibres by their centromere.

what happens during anaphase 1?

the spindles contract and shorten, pulling the homologous chromosome pairs apart, one chromosome goes to each end of the cell.

what happens during telophase 1?

• chromosomes start to uncoil

• new nuclear envelope forms around each set of chromosomes

• new nucleoli start to appear

what happens during cytokinesis (end of meiosis 1)

• two haploid daughter cells are produced (have half the number of original chromosomes)

• however each chromosome still contains two chromatids

what happens during prophase 2?

• chromosomes condense

• centrioles start moving to opposite ends of cell, forming spindle fibres

• the nuclear envelope breaks down.

what happens during metaphase 2?

the homologous pairs line up across the equator of the cell and attach to the spindle fibres by their centromere.

what happens during anaphase 2?

sister chromatids are separated by spindle fibres and are pulled to opposite ends of the cell.

what happens during telophase 2?

as sister chromatids are separated, the cell divides into two cells (nuclear envelope reforms).

what happens during cytokinesis (end of meiosis 2)

production of four genetically different haploid cells.

What are the three stages of the cell cycle?

• Interphase

• Mitosis

• Cytokinesis

What three growth stages make up interphase ?

• Gap phase 1 (G1)

• Synthesis (S)

• Gap phase 2 (G2)

What happens during Gap phase 1?

The cells grow and new organelles and proteins are made.

What happens during Synthesis?

The cell replicates it’s DNA, ready to divide by mitosis.

What happens during Gap Phase 2?

The cell keeps growing and checks for errors in replicated DNA & makes repairs if needed

Why do multicellular organisms need mitosis?

To grow and repair damaged tissues.

What are the four stages of mitosis?

• Prophase

• Metaphase

• Anaphase

• Telophase

What happens during prophase?

• The chromosomes condense, getting shorter and fatter (making them visible under a microscope)

• Tiny bundles of protein (called centrioles) start moving to opposite ends of the cell, forming a network of protein fibres across it called the spindle.

• nucleolus disappears

• The nuclear envelope breaks down and the chromosomes lie free in the cytoplasm.

What happens during Metaphase?

• The replicated chromosomes line up along the equator and become attached to the spindle by the centromere.

• metaphase checkpoint occurs

What happens during Anaphase?

• The centromeres divide, separating each pair of sister chromatids.

• The spindles contract, pulling chromatids to opposite poles of the cell, centromere first.

What happens during telophase?

• The chromatids reach the opposite poles on the spindle. They uncoil and become long and thin again.

• They are now called chromosomes again.

• A nuclear envelope forms around each group of chromosomes, so there are now two nuclei.

What happens during cytokinesis?

• The cytoplasm divides.

• Cleavage furrow forms to divide the cell membrane (cell membrane pinches inwards)

• There are now two daughter cells that are genetically identical to the original cell and to each other.

What are the three checkpoints that occur during mitosis?

• G1 checkpoint

• G2 checkpoint

• metaphase checkpoint

What happens during the G1 checkpoint?

The cell checks that any chemicals needed for replication are present and for any damage to the DNA before entering S-phase.

What happens during the G2 checkpoint?

The cell checks whether all the DNA has been replicated without any errors. If it has, the cell can enter mitosis.

What happens during the Metaphase checkpoint?

The cell checks that all the chromosomes are attached to the spindle fibres before mitosis can continue.

what is the cell cycle

series of stages cells go through to divide and produce new cells (role in growth and tissue repair)

why is mitosis important

• allows organisms to grow

• helps us replace old or damaged cells

• asexual reproduction

• critical for producing stem cells

how can uncontrolled cell division lead to cancer

• if there is a mutation in genes, they can no longer regulate the cell cycle

• this can lead to uncontrolled cell division

• which can result in the mass formation of abnormal cells (tumour)

• these can spread into other tissues to become invasive (cancerous)