DNA Replication And The Genetic Code Flashcards
Intro
Cells divide to produce more cells needed for growth or repair of tissues. The two daughter cells produced as a result of cell division are genetically identical to the parent cell and to each other. In other words, they contain DNA with a base sequence identical to the original parent cell.
When a cell prepares to divide, the two strands of DNA double helix separate and each strand serves as a template for the creation of a new double-stranded DNA molecule. The complementary base pairing rules, ensure that the two new strands are identical to the original. This process is called DNA replication.
Semi-conservative replication
For DNA to replicate, the double helix structure has to unwind and then separate into two strands, so the hydrogen bonds holding the complementary bases together must be broken. Free DNA nucleotides will then pair with their complementary bases, which have been exposed as the strands separate. Hydrogen bonds are formed between them. Finally, the new nucleotides join to their adjacent nucleotides with phosphodiester bonds.
In this way, two new molecules of DNA are produced. Each one consists of one old strands of DNA and one new strand. This is known as semi-conservative (meaning half the same) replication.
Roles of enzymes in replication
DNA replication is controlled by enzymes, a class of proteins that act as catalysts for biochemical reactions. Enzymes are only able to carry out their function by recognising and attaching to specific molecules or particular parts of the molecules.
Before replication can occur, the unwinding and separating of the two strands of the DNA double helix is carried out by the enzyme DNA helicase. It travels along the DNA backbone, catalysing reactions that break the hydrogen bonds between complementary base pairs as it reaches them. This can be thought of as strand ‘unzipping’.
Free nucleotides pair with the newly exposed bases on the template strands during the ‘unzipping’ process. A second enzyme, DNA polymerase catalyses the formation of the phosphodiester bonds between nucleotides.
Replication errors
Sequences of bases are not always matched exactly, and an incorrect sequence may occur in the newly-copied strand. These errors occur randomly and spontaneously and lead to a change in the sequence of bases, known as a mutation.
Genetic code
DNA is contained within the cells of all organisms and scientists determined that this molecule was the means by which genetic information was passed from one generation to the next. Scientists understood that DNA must carry the ‘instructions’ or ‘blueprint’ needed to synthesise the many different proteins needed by these organisms. Proteins are the foundation for the different physical and biochemical characteristics of living things. They are made up of a sequence of amino acids, folded into complex structures. Therefore DNA must code for a sequence of amino acids. This is called the genetic code.
A triplet code
The instructions that DNA carries are contained in the sequence of bases along the chain of nucleotides that make up the two strands of DNA. The code in the base sequences is a simple triplet code. It is a sequence of three bases, called a codon. Each codon codes for an amino acid.
A section of DNA that contains the complete sequence of bases (codons) to code for an entire protein is called a gene.
The genetic code is universal-all organisms use this same code, although the sequences of bases coding for each individual protein will be different.
Degenerate code
Four different bases, meaning there are 64 different base triplets or codons possible (4x4x4). This includes one codon that acts as the start codon when it comes at the beginning of a gene, signalling the start of a sequence that codes for a protein. If it is in the middle of a gene, it codes for the amino acid methionine. There are also three ‘stop’ codons that do not code for any amino acids and signal the end of the sequence.
Having a single codon to signal the start of a sequence ensures that the triplets of bases (codons) are read ‘in frame’. In other words the DNA base sequence is ‘read’ from base 1, rather than base 2 or 3. So the genetic code is non-overlapping.
As there are only 20 different amino acids that regularly occur in biological proteins, there are a lot more codons than amino acids. Therefore, many amino acids can be coded for by more than one codon. Due to this, the code is known as degenerate.
