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