DNA Structure and Replication


Before a cell divides by mitosis, each DNA molecule is copied so that one copy can be distributed to each daughter cells.

Each new molecule of DNA constains one old polypeptide strand and one new one. This method of replication is said to be semi-conservative.

DNA Replication:

  • DNA Helicase unwinds and unzips the DNA strands. (Step 1)
  • DNA polymerase synthesises new, complementary, DNA strands. This is done in the direction 5′ to 3′. This means that DNA complementary to the ‘lagging’ strand is synthesised discontinously and other enzymes are require to complete this process.
  • Primase synthesises RNA primers (about 10 nucleotides long RNA fragments) which are necessary for the beginning of the synthesis of Okazaki Fragments.
  • RNA Primes are later cleaved by the enzyme Nuclease, and replaced with DNA.
  • DNA Ligase joins synthesised DNA fragments into continous strands.

Some History:

  • The structure of DNA was worked out by James Watson and Francis Crick in 1953.
  • Mathew Melson & Fanklin Sthal proved in 1958 that DNA is copied semi-conservatively.

DNA Primary structure:

There are two Hydrogen bonds between the A=T and A=U bases, but three Hydrogen bonds between C≡G bases.

Higher Stucture of DNA:

  • DNA molecules wrap around ‘histones,’ highly alkaline proteins, and form nucleosomes. This is to reduce the length of the DNA molecules and pack them nicely – this is useful in gene regulation.
  • In euchromatin, the DNA molecule of a chromosome is arranged loosely, but the DNA is still wrapped around the histones.
  • In heterochromatin, the DNA molecule of a chromosome is a compact spiral, with the DNA wrapped around histone molecules.

telomere = A telomere is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. (A telomere is necessary because chromosomes are shortened with each cell division).

centromere = The centromere is the part of a chromosome that links sister chromatids. During mitosis, spindle fibers attach to the centromere via the kinetochore. Centromeres were first defined as genetic loci that direct the behavior of chromosomes.

Notes Compiled by Andrei Cociug.

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