General Review
DNA stands for deoxyribonucleic acid, it is the genetic blueprint for life and a hereditary material. Every living organisms has its own unique DNA. In the human case we have 46 DNA or as it known today 46 chromosomes. Chromosomes are thread like strands of DNA and they are highly condensed being coiled and then supercoiled. It is researched that they whole genome of the human body accounts for about 20 000 to 25 000 genes which specifies your individual characteristics such hair and eye colour and height. DNA is the genetic code which is passed on from generation to generation. Chromosomes are found in the nucleus of a cell and are so called nuclear DNA. There are two more types of DNA and these are chloroplast DNA and mitochondrial DNA. The discovery of DNA is credited to James Watson, Francis Crick and Maurice Wilkins who all recieived the Nobel Prize for Chemistry in 1962.
DNA Structure
DNA is put together by a series of condensation reactions. A DNA molecule consists of a phosphate group, a deoxyribose sugar group and one of the four bases; adenine (A), cytosine (C), guanine (G) and thymine (T). A and G belong to the organic compound purine (double ringed) and are called purine bases whereas C and T belong to organic compound pyrimidine (single ringed) and are called pyrimidine bases. The bases follow base pairing rules that is A goes with T and G goes with C. In each case a purine base goes with a pyrimidine base. These form two strands which runs anti-parallel (one goes from 3' prime to 5' prime and the other goes from 5' to 3') and also forms a structure know as the Double Helix
Protein Synthesis
For a protein to be synthesized, the code for it is stored in the nucleus in the form of DNA, since DNA cannot leave from the nucleus, other process occurs to successfully transcribe (copy) and translate it into a protein.
Transcription
What transcription really means is that creating a complementary RNA sequence from DNA. RNA is a portable molecule which can travel across the cell. Both DNA and RNA are nucleic acids and use the base pairing rule expect that in RNA, thymine (T) is replaced with Uracil (U). The process is carried out via enzymes such as RNA polymerase which starts at promotor region and then moves along the template strand, creating a strand known as pre mRNA (messenger RNA) which runs antiparallel to the DNA.
Transcription can be explained easily in 5 steps:
Transcription can be explained easily in 5 steps:
- The hydrogen bonding are weak and are broken by the enzyme Helicase, creating a coding and template strand.
- The RNA Polymerase runs along the template strands and adds matching RNA nucleotides (A, U, G, C) which are complementary to that of the DNA
- With this a sugar-phosphate backbone forms which is due to the enzyme RNA plymerase.
- When the RNA polymerase runs along the template strand it is forming another strand which is bonded by hydrogen bonding to the template strand, so this step involves breaking this hydrogen bond to release the newly synthesized RNA strand known now as pre mRNA.
- The pre mRNA undergoes slicing to remove introns (non-codeing regions of the DNA) and a poly-A tail (AAA) is added to 3' and a cap is added to 5' end. This is so to make the mRNA unreactive in the cytoplasm because there will be free roaming nucleotides which might bind the to the mRNA and cause a mutation. Now it can leave the nucleus through the nuclear pore complex.
Translation
This process is performed by the membrane bound organelle called a ribosome. The mRNA carries with it the unique code for the synthesis of a protein, each amino acid is inserted by three nucleotide sequence know as a codon. Each triplet codes for one amino acid and successive amino acids are added in similar manner. The mRNA carries the genetic code which is read by the ribosomes from 5' to 3' direction. The ribosome can be described as a 'factory' where the amino acids are assembled into polypeptide chain. The assembly of the amino acids is done by the help of tRNA (transfer RNA) which are small non coding RNA chains about 74-93 nulcelotides and transports amino acids to the ribosome. tRNAs have a site for amino acid attachment, and a site called an anticodon. The anticodon is an RNA triplet complementary to the mRNA triplet that codes for their cargo amino acid. A tRNA with an amino acid is called 'loaded' or charged tRNA.
DNA Replication
Semi conservative replication
DNA replication is among the fundamental aspects of hereditary, it is the basis for biological inheritance. It is commenced by one double stranded DNA which gives raise to two identical copies of the molecules. The two identical copies of DNA consists of a new strand and an old strand of DNA. This is known as semi conservative replication. Cellular proofreading and error checking mechanisms all ensure that the replication goes smoothly and without any error.
The replication beings by a section of the DNA unwinding by the help of the enzyme helicase and forms a replication fork. Two strands form from this, one is the leading strand and the other is the lagging strand.
Leading strand
This strand moves in such a way that the replication fork moves along the template strand in the 3' to 5' direction which allows the for the newly synthesized strand to go in the 5' to 3' which is in the same direction to the movement of the replication fork. In here the enzyme DNA polymerase continually adds nucleotide in the correct order.
Lagging strand
This strand is in such a way that the replication fork moves in a 5' to 3' manner. Due to this the DNA polymerase cannot function in this orientation, it function in the opposite e.g when the template strand is running 3' to 5' like the leading strand and the enzyme adds nucleotide in 5' to 3' direction (DNA polymease adds nucleotides in the 5' to 3' manner but the replication fork is also going that way.) Because of this the lagging strand is more complicated. To overcome this challenge the template strand forms a loop where RNA primers are added at the start of the loop, each time a primer is added it segments it and forms fragments of various lengths, the fragments are known as okazaki fragments. Then DNA polymerase adds complementary nucleotides in the 5' to 3' direction. The primers are then removed and the fragments joined by the help of DNA ligase.
The replication beings by a section of the DNA unwinding by the help of the enzyme helicase and forms a replication fork. Two strands form from this, one is the leading strand and the other is the lagging strand.
Leading strand
This strand moves in such a way that the replication fork moves along the template strand in the 3' to 5' direction which allows the for the newly synthesized strand to go in the 5' to 3' which is in the same direction to the movement of the replication fork. In here the enzyme DNA polymerase continually adds nucleotide in the correct order.
Lagging strand
This strand is in such a way that the replication fork moves in a 5' to 3' manner. Due to this the DNA polymerase cannot function in this orientation, it function in the opposite e.g when the template strand is running 3' to 5' like the leading strand and the enzyme adds nucleotide in 5' to 3' direction (DNA polymease adds nucleotides in the 5' to 3' manner but the replication fork is also going that way.) Because of this the lagging strand is more complicated. To overcome this challenge the template strand forms a loop where RNA primers are added at the start of the loop, each time a primer is added it segments it and forms fragments of various lengths, the fragments are known as okazaki fragments. Then DNA polymerase adds complementary nucleotides in the 5' to 3' direction. The primers are then removed and the fragments joined by the help of DNA ligase.