OCR AS F211 Cell division 1.1.3

a) state that mitosis occupies only a small percentage of the cell cycle ans that the remaining percentage includes the copying and checking of genetic information

the cell cycle incolves 3 main stages:

• interphase
• mitosis
• cytokinesis

interphase occupies the largest percentage of the cell cycle and is subdivided into 3 additional stages:

• G1 - protein synthesis and replication of organelles
• S - DNA replication, proof reading enzymes check this has been done accurately
• G2 - growth of the cell

b) describe, with the aid of diagrams and photographs, the main stages of mitosis (behaviour of the chromosomes, nuclear envelope, cell membrane and centrioles)

Prophase:

• chromosomes shorten and thicken so become visible
• nuclear envelope disintegrates
• centriole move to opposite poles of the cell ans start to make spindle fibres

Metaphase:

• chromosomes attach to spindle fibres by their centromeres and move towards the cell equator
• chromosomes align on the euqator of the cell

Anaphase:

• centromeres split
• spindle fibres contract and shorten
• chromatids are pulled apart
• chromatids move to opposite poles of the cell. centromeres first

Telophase:

• chromosomes relax at the cell poles so become indistinct
• nuclear envelopes form around each set of chromosomes
• two nuclei form

The cell then goes into cytokinesis which is the division of the cytoplasm to make two new cells

c) explain the meaning of the term homologous pair of chromosomes

homologous chromosomes are the same size, they carry the same alleles and carry the same genes

d) explain the significance of mitosis for growth, repair and asexual reproduction

mitosis produces two genetically identical cells which are also genetically identical to the parent cell which produced them. This is important for:

• tissue growth
• tissue repair
• asexual reproduction in some organisms

e) outline with the aid of diagrams and photographs the process of cell division by budding in yeast

• the nucleus divides by mitosis
• a swelling appears on the cell surface
• the replicated nucleus moves into the bud
• the bud pinches off
• leaving a birth scar on the parent cell

f) state that cells produced as a result of meiosis are not genetically identical

meiosis creates 4 gametes, each containing one set of chromosomes (they are haploid) and each is genetically different

g) define the term stem cell

stem cell = an unspecialised cell which can differentiate to become a cell with a specialised function

h) define the term differentiation, with reference to the production of erythrocytes (red blood cells) and neutrophils derived from stem cells in bone marrow, and the production of xylem vessels and phloem sieve tubes from…