Week 9 theory lesson1 April 2014
Cell division
The beginning of life• Multi cellular organisms
begin as just one cell, which keeps dividing until the organism grows into an adult
• Cells are also replaced as part of body maintenance (eg skin/red blood cells)
Cell division• Mitosis- cells divide to produce an identical
cell with same DNA• Meiosis- cells divide to produce gametes (sex
cells) with ½ the DNA of parent cell
Sites of mitosis• Any area where rapid
growth occurs or repair is needed (eg skin/wounds)
• Somatic (body) cells originate from pre existing cells via mitosis
• All somatic cells originate from an original stem cell in an organism formed via sexual reproduction
Mitosis in Plants-eg roots tips
Mitosis in animals- eg skin
Process of mitosis
• Occurs in stages– Interphase– Prophase– Metaphase– Anaphase– Telophase
Interphase• Chromosomes are replicated• Chromosomes double (eg humans have 46
92 are made)• Organelles replicate, centrioles start to
separate
Prophase• Chromosomes become visible• Chromosomes held together in pairs (by
centromere)• Centrioles move to poles and spindle fibres
form• Nuclear membrane
breaks down
Metaphase• Chromosomes move through the cytoplasm to
the spindle• The centromere attaches to spindle and takes
chromosomes to middle of the cell
Anaphase• Centromeres divide (chromosome pairs split)• Chromosomes move to opposite poles of cell
along spindle• Cell starts to shrink in the middle
Telophase• Nuclear membranes form around
chromosomes• Chromosomes uncoil• Cytoplasm divides to form two cells
Slide of a root tip
Mitosis summary• Mitosis refers to the changes involved in the
chromosomes during cell division• Cytokinesis is the division of the cytoplasm and
replication of organelles within the cytoplasm• Mitosis is involved in cell division where cells are
replicated as part of growth and development