Chargaff's rules state that in any sample of double-stranded DNA the amount of adenine equals that of thymine and the amount of guanine equals that of cytosine and the total amount of purines equals that of pyrimidines - this is Chargaff's rules of base pairing and were discovered before the structure of DNA was known.
- Purines: 2 rings: Adenine and Guanine. Purine synthesis requires Amino acids Glycine, aspartate and glutamine
- Pyrimidines 1 ring: Cytosine, thymine (DNA), Uracil (RNA)
- Cross link 3 rings Purine + Pyrimidine
- DNA Adenine: Thymine and Guanine: Cytosine
- RNA Adenine: Uracil and Guanine: Cytosine
- DNA which is double-stranded
- RNA is usually a single-stranded molecule
- DNA contains 2' deoxyribose, RNA contains ribose
- RNA less stable than DNA because it is more prone to hydrolysis
- The double strands are held together by hydrogen bonds A=T(2 bonds) and G=C(3 bonds)
- Each diploid cell contains 23 pairs of chromosomes
- DNA is found within the nucleus but 90% of RNA is found in the cytoplasm
Types of RNA function
- Messenger RNA - Template for polypeptide synthesis with a cap at the 5' end and a poly-A tail at the 3' end
- Transfer RNA - Cloverleaf structure carries amino acids into position by mRNA template.
- Ribosomal RNA - Component of ribosomes where there is polypeptide synthesis.
Eukaryotic cell cycle [you are a eukaryote]
- Consists of the following phases G0, G1, S, G2 and M
- 1. G1 (gap or pre synthesis phase - increased cell mass and centrosome duplicated) phase ← can move to → G0 phase [G1 restriction point - check for DNA damage]
- 2. Synthesis phase DNA replication occurs and chromosome duplication
- 3. G2 (gap or pre mitosis phase) [G2/M checkpoint - check for damaged or unduplicated DNA]
- 4. Mitosis
- The DNA strands are held together by many hydrogen bonds which can be disrupted by heat or by changing the pH. The DNA is read from its 5' to its 3' end. Eukaryotes have a piece of circular DNA within the mitochondria mtDNA. Proto-oncogenes code for proteins involved in the control of the cell cycle. Mutations form oncogenes and can result in cancer.
- With each mitosis and DNA replication a human diploid cell must replicate 6 billion base pairs. Each base pair A=T (2 hydrogen bonds) and G=C (3 hydrogen bonds). DNA replication has an error rate of 1 in 10^10 nucleotides. The DNA molecules are split lengthwise and each acts as a template for replication. The process is semiconservative as one copy is made and the original is kept. It involves DNA polymerase of which there are 4 types but the alpha form is the commonest. DNA polymerase requires a primer (usually RNA) (RNA polymerase doesn't). Although it proceeds 5' to 3' and the strands run opposite the flow is in one direction. One strand is synthesized continuously(leading strand) and the other in discontinuous fragments (lagging strand) at the replication fork. There is a leading strand and a lagging strand. DNA Polymerase alpha synthesizes the leading strand and DNA Polymerase Beta synthesizes the lagging strand which are then joined by a DNA ligase. Okazaki fragments are produced by the lagging strand. DNA grows at the 3' end
- There are several quality control checkpoints in the cycle governed by proteins called cyclins and enzymes called cyclin-dependent kinases.
- The main checkpoint is at the transition from G1 to Synthesis phase - it makes sense not to invest energy if there are faults and to stop the cycle early
- These are sequences of DNA that protect the ends of chromosomes from attack by nucleases.
- They consist of repeating sequences of TxGy (x,y <4) base-paired to a complementary chain of A's and C's.
- They get shorter with the ageing process. When the telomeres are gone the DNA is degraded.
- Cells that do not age such as germline cells and cancer cells contain telomerase that replace these lost ends.
- The energy driving DNA polymerase is from the elimination of pyrophosphate with every additional nucleoside triphosphate which are then hydrolysed. The new strand forms in a 5' to 3' direction