Related Subjects:
|DNA and RNA short notes
|DNA replication
|DNA structure in Nucleus
|Mitosis and Meiosis
|Telomeres
|Autosomal Recessive
|X Linked Recessive
|Autosomal Dominant
|Li Fraumeni syndrome
|Genetic Linkage
|Cell Cycle
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. The total amount of purines equals that of pyrimidines. These rules of base pairing were discovered before the structure of DNA was known.
Building Blocks
- 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.
Nucleic Acids
- DNA: Adenine pairs with Thymine, and Guanine pairs with Cytosine.
- RNA: Adenine pairs with Uracil, and Guanine pairs with Cytosine.
- DNA is typically double-stranded.
- RNA is usually single-stranded.
- DNA contains 2' deoxyribose, while RNA contains ribose.
- RNA is less stable than DNA because it is more prone to hydrolysis.
- The double strands of DNA 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 and Their Functions
- Messenger RNA (mRNA): Template for polypeptide synthesis with a cap at the 5' end and a poly-A tail at the 3' end.
- Transfer RNA (tRNA): Cloverleaf structure that carries amino acids into position by mRNA template.
- Ribosomal RNA (rRNA): Component of ribosomes where polypeptide synthesis occurs.
Eukaryotic Cell Cycle
- Consists of the following phases: G0, G1, S, G2, and M.
- G1 Phase: Gap or pre-synthesis phase - increased cell mass and centrosome duplication. Can move to G0 phase. G1 restriction point checks for DNA damage.
- S Phase: Synthesis phase where DNA replication occurs and chromosome duplication happens.
- G2 Phase: Gap or pre-mitosis phase. G2/M checkpoint checks for damaged or unduplicated DNA.
- M Phase: Mitosis.
Replication
- The DNA strands are held together by hydrogen bonds, which can be disrupted by heat or pH changes. 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, which can result in cancer.
- During each mitosis and DNA replication, a human diploid cell must replicate 6 billion base pairs. Each base pair has A=T (2 hydrogen bonds) and G=C (3 hydrogen bonds). DNA replication has an error rate of 1 in 10^10 nucleotides.
- DNA molecules are split lengthwise, and each strand acts as a template for replication. The process is semiconservative, meaning one copy is made, and the original is kept.
- DNA polymerase synthesizes DNA; the alpha form is the most common. DNA polymerase requires a primer (usually RNA). One strand is synthesized continuously (leading strand), and the other in discontinuous fragments (lagging strand).
- Okazaki fragments are produced by the lagging strand. DNA grows at the 3' end.
Quality Control
- There are several quality control checkpoints in the cell cycle, governed by proteins called cyclins and enzymes called cyclin-dependent kinases.
- The main checkpoint is at the transition from G1 to Synthesis phase to ensure there are no faults before proceeding.
Telomeres
- Telomeres are sequences of DNA that protect the ends of chromosomes from degradation by nucleases.
- They consist of repeating sequences of TxGy (x,y <4) base-paired to a complementary chain of A's and C's.
- Telomeres shorten with the aging process. When they are gone, the DNA is degraded.
- Cells that do not age, such as germline cells and cancer cells, contain telomerase that replaces these lost ends.
Energy for Replication
- The energy driving DNA polymerase comes from the elimination of pyrophosphate with every additional nucleoside triphosphate, which are then hydrolyzed. The new strand forms in a 5' to 3' direction.
- See here
