DNA chromosomes cell cycle

chromatin

DNA + histone complex during

chromosomes

complex X shaped form of chromatin formed (and visible) during mitosis

chromatids

the 2 arms of an X shaped chromosome. The 2 chromatids are identical since they are formed by DNA replication

homologous chromosomes

Pair of the same type of chromosome of the same size and shape one originating from each parent. they contain the same genes but different may have different alleles

1. DNA double helix

2. histone proteins

3. chromatin

4. chromosome

5. centromere

6. chromatids

what is this

Karyotype

autosomes

these are non sex chromosomes

What is alleles

Alleles are different versions of the same gene that arise due to differences in the sequence of DNA bases.

what are the stages of the cell cycle

1. interphase = this is when the cell grows and the carries out its functions. cell typically spend 90 % of there time in this phase. This stage can be sub divided

   Gap phase/G1 = where the cell grows back to it original size. Genes are express into proteins by cells organelles are replicated

   synthesis S phase where DNA and histones are replicated this is separate from mitosis this can take a few hours

   Gap phase/G2 = where spindle proteins are synthesised for mitosis

2. mitosis = this is where the nuclei divides into 2 with identical copies of DNA. This can be divided into 4 more categories prophase anaphase and metaphase

3. cytokinesis = this is the division of the cytoplasm into 2 different cells

control cell cycle

The cell cycle is tightly controlled so that the cell only divides when needed

1. the G1 checkpoint is only reached if there is a need for more cells this is signalled by an external factor such as a growth hormone. This prevents excessive cell growth

2. the G2 checkpoint is only passed if DNA has been replicated correctly and there is the correct number of chromosomes

3. The m check point is only passed if mitosis has been completed successfully and there are 2 identical nuclei

what happens when this fails

cells divide uncontrollably causing cancer