8) DNA, genes and protein synthesis Flashcards
What is a gene?
A gene is a section of DNA that contains the coded information for making polypeptides and functional RNA.
The coded information is in the form of a specific sequence of bases along the DNA molecule.
A gene is a section of DNA located at a particular position, called a locus, on a DNA molecule.
What are polypeptides?
Polypeptides make up proteins and so genes determine the proteins of an organism.
What are enzymes?
Enzymes are proteins. As enzymes control chemical reactions they are responsible for an organism’s development and activities.
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⚫ the amino acid sequence of a polypeptide
⚫ or a functional RNA, including ribosomal RNA and transfer RNAS (Topic 8.3).
One DNA molecule carries many genes.
Explain why there must be a min of three bases that coded for each amino acid.
⚫ Only 20 different amino acids regularly occur in proteins.
⚫ Each amino acid must have its own code of bases on the DNA.
⚫ Only four different bases (adenine, guanine, cytosine and thymine) are present in DNA.
⚫ If each base coded for a different amino acid, only four different amino acids could be coded for.
⚫ Using a pair of bases, 16 (42) different codes are possible, which is still inadequate.
⚫ Three bases produce 64 (43) different codes, more than enough to satisfy the requirements of 20 amino acids.
As the code has three bases for each amino acid, each one is called a triplet. As there are 64 possible triplets and only 20 amino acids, it follows that some amino acids are coded for by more than one triplet.
Give some features of the genetic code.
A few amino acids are coded for by only a single triplet.
⚫ The remaining amino acids are coded for by between two and six triplets each.
⚫ The code is known as a ‘degenerate code’ because most amino acids are coded for by more than one triplet.
⚫ A triplet is always read in one particular direction along the DNA strand.
⚫ The start of a DNA sequence that codes for a polypeptide is always the same triplet. This codes for the amino acid methionine. If this first methionine molecule does not form part of the final polypeptide, it is later removed.
⚫ Three triplets do not code for any amino acid. These are called ‘stop codes’ and mark the end of a polypeptide chain. They act in much the same way as a full stop at the end of a sentence.
⚫ The code is non-overlapping, in other words each base in the sequence is read only once. Thus six bases numbered 123456 are read as triplets 123 and 456, rather than as triplets 123, 234, 345, 456. ⚫ The code is universal, with a few minor exceptions each triplet codes for the same amino acid in all organisms. This is indirect evidence for evolution.
Much of the DNA in eukaryotes does not code for polypeptides. For example, between genes there are non-coding sequences made up of multiple repeats of base sequences. Even within genes, only certain sequences code for amino acids. These coding sequences are called exons. Within the gene these exons are separated by further non-coding sequences called introns. Some genes code for ribosomal RNA and transfer RNAs.
