2.5 – Transcription and Translation
2.5.1 – Compare the structure of RNA and DNA
2.5.2 – Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase
A complimentary copy of the DNA is made in the nucleus to form the mRNA. This process is catalysed by the enzyme RNA polymerase. To copy the mRNA, the DNA double helix is unwound by DNA helicase, with the hydrogen bonds breaking between the base pairs to be copied. The DNA opens at the transcription site, or position of the gene that needs to be copied.
The coding strand, or the sense strand, is the template for the mRNA. However, the mRNA is actually built against the anti-sense strand. It has the same pattern as the opposite strand due to complimentary base pairing.
The free nucleotides pair with the DNA nucleotides. The only difference is that uracil replaces thymine, bonding to adenine. The RNA polymerase forms the phosphodiester bonds to make the backbone of the mRNA molecule. The mRNA then detaches and leaves the nucleus via the nuclear pores in the membrane. It enters the cytoplasm for reading at the ribosomes. The DNA double helix reforms.
2.5.3 – Describe the genetic code in terms of codons composed of triplets of bases
Each sequence of three bases codes for one amino acid, called a triplet code. These groups of three are called codons.
For every amino acid, it has two or three triplets which code for them. Other triplets act as the ‘start’ or ‘stop’ codons, which define where to begin and end the polypeptide sequence.
There are also multiple triplets which code for these ‘punctuation’ codons.
2.5.4 – Explain the process of translation, leading to polypeptide formation
The amino acids are activated by combining with tRNA (transfer RNA) in the cytoplasm. tRNA molecules are in the shape of a clover leaf. Each molecule binds to a specific amino acid codon, the other end binding to the amino acid. The other end has an anticodon, which
is the complimentary codon for the mRNA. The tRNA binds to the amino acid, catalysed by an enzyme. This process uses ATP.
Once the mRNA molecule has been transcribed, it is sent to the ribosome in the cytoplasm or endoplasmic reticulum for translation. The protein is formed from the polypeptides, which are built up at the ribosomes. The ribosomes move along the mRNA the ‘read’ the code, beginning at the start codon.
From here, the tRNA molecules, with their amino acids, find their complimentary codon on the mRNA. The amino acids are bound in the ribosomes to form the polypeptide chains. The tRNA then separates from the amino acid and the mRNA, and is sent back to the cytoplasm to find more amino acids. This process continues until a stop codon is reached, at which point the polypeptide chain is released.
In order to provide enough free amino acids for translation, heterotrophs consume them in the protein of their diet.
The first codon on the mRNA molecule is AUG, the start codon, which bonds to the anti codon [UAC] on the tRNA molecule. This tRNA molecule carries the amino acids Methionine. Codon to anti-codon binding is anti-parallel.
The process continues until the complete polypeptide is formed.
2.5.5 – Discuss the relationship between one gene and one polypeptide
The theory is that one gene forms one polypeptide. This is true in most cases, however there are a few exceptions:
- Some genes code for types of RNA that do not produce polypeptides
- Some control the expression of other genes