DNA Structure, DNA Replication and Mitosis

DNA, chromosomes and gene structure revision

Structure of DNA DNA is made from Nucleotides that have 3 parts: –5 carbon Sugar (deoxyribose) –Phosphate group –Nitrogen base (Adenine, Thymine, Guanine or Cytosine)

Deoxyribonucleic acid (DNA) molecule has a double helix (twisted ladder) structure with 2 bases of 2 nucleotides forming the rungs and the sugar and phosphate groups forming the sides

Anti-parallel nature of DNA Sugar is a 5 carbon ring (joins ladder sides to rungs) Phosphate group attaches to the 3’ or the 5’ carbon (makes ladder sides) This gives direction to each half of the strand as each strand (ladder side) runs in the opposite direction Nitrogen bases join anti parallel strands as A-T and C- G only (make ladder rungs)

DNA replication The purpose of DNA replication is to make an exact copy of a DNA molecule prior to cell division (mitosis) so each daughter cell has the full set of genetic material. Creates a chromosome with two chromatids Ensures each resulting cell is able to receive a complete set of genes from the original cell Three main steps

Semi-conservative nature of DNA replication Means less opportunity for mistakes to occur during replication

Mitosis The purpose of mitosis is to make exact copies of existing cells for growth, repair in all living organisms and asexual reproduction in some Produces 2 daughter cells genetically identical to parent cell Continuous process but has 4 main stages

Where mitosis occurs in plants and animals Plants – root tips, shoot tips and inside the bark layer of trees. These are the active growing sites of plants and many larger perennial plants continue to grow throughout their life. Animals – in most tissues in immature animals and wherever cells need replacing or repair in mature animals.