DNA replication is semi conservative.
1. The DNA strands are separated by destroying
the H bonds.
2. Each strand serves as a partner of a new mode strand.
3. The resultant DNA molecule is composed of a new strand and an old strand.
4. This is repeated in both strands
DNA replication is bi-directional.
An enzyme unwinds the tight coils of DNA. Once the DNA is accessible another enzyme (DNA polymerase) attaches to a specific site on the DNA.
DNA polymerase add nucleotides to the 3 prime end of a free existing chain of nucleotides synthesis goes 5 prime - 3 prime. DNA polymerases cannot start DNA from scratch , but they require a primer, a short, nucleic acid chain that acts as an initiator. The primers are RNA molecules. The primers are removed later.
One of the new strands is synthesized, continuously (the leading strand in the 5 prime - 3 prime direction). The other strand cannot be made continuously, since no DNA polymerase is capable of synthesis in a 3 prime to 5 prime direction. This is called the lagging strand. The synthesis of the lagging strand is done in small interrupted fragments which are later united by another enzyme known as DNA ligase.
The DNA use a triplet code to direct the synthesis
of proteins. This triplet is called the codon.
Codons are read in a successive non-overlapping manner.
The central Dogma:
DNA-RNA-Protein
Information found in DNA can be accessed in 2 steps:
1. Transcription - formation of mRNA molecules from DNA molecule. mRNA carries the genetic information from the nucleus to the cytoplasm.
2. Translation- the information in the mRNA is translated
into an amino acid chain. Translation is carried out by a complex machinary
centering on the ribosomes. Another RNA molecule is involved (tRNA.)
Only one strand (the coding strand) of DNA has
the information necessary for protein synthesis. The coding strand serves
as a template for the synthesis of mRNA.
The mRNA is identical to that of the complementary non-coding strand of DNA, except that RNA has U instead of T.
An enzyme (RNA polymerase) are used to make the mRNA.
They bind to specific area in the DNA. The RNA polymerases separate the
strands of DNA and begins the synthesis of mRNA.
The ribosome is the site where the mRNA is translated
into protein. It has a large and a small subunit. The subunits are made
up of mRNA +82 proteins.
Transfer RNA (tRNA)- It is the molecule that translate the mRNA sequence into protein
he tRNA has a set of 3 nucleotides known as the anticodon.
This is complementary to the codon sequence in mRNA. There is a unique
tRNA for each amino acid.
1. Methionine is the first a.a. in every protein
sequence.
2. tRNA met is moved to the ribosome.
3. A 2nd tRNA is now brought into the ribosome.
4. An enzyme in the ribosome mediates the release of the 1st a.a. and the formation of a peptide bond with the 2nd a.a. -
5. The ribosome then come down the mRNA (translocation), this moves the 2 a.a. into the first position.
6. The empty tRNA exits the ribosome - and the process continues (new tRNA moves into the second site - until the 1st stop codon is encountered.
7. The ribosome releases the mRNA, the protein and the tRNA.