8.1 Genes and the triplet code Flashcards
What is a gene?
A gene is a section of DNA that contains the coded information for making polypeptides and functional RNA
What do the genes code for?
The amino acid sequence of a polypeptide. A functional RNA, including ribosomal RNA and transfer RNAs.
What do genes and environmental factors do together?
Genes, along with environmental factors, determine the nature and development of all organisms. The coded information is in the form of a specific sequence of bases along the DNA molecule. Polypeptides make up proteins and so genes determine the proteins of an organism. Enzymes are proteins and as enzymes control chemical reactions they are responsible for an organism’s development and activities.
What is the significance of the order of the bases?
If the code is different then a different sequence of amino acid is produced, therefore it may cause the polypeptide to become non-functional depending on the amino acid produced.
How is DNA organised inside cells?
DNA is organised into a double helix strand that is tightly packed in the nucleus of a cell. The DNA is stretched out during replication.
What is a locus?
A locus is the position of a gene on a chromosome/DNA molecule.
What is the genetic code?
Sequences of bases in our DNA which codes for a specific order of amino acids to make a specific protein. The order of/sequence of nucleotide bases in the mRNA that code for specific amino acids is referred to as the genetic code.
How does the genetic code code for a protein?
The bases are arranged in triplets. Each triplet codes for a specific amino acid. The bases’ sequence determine the primary sequence of the protein.
Why is there a minimum of 3 bases that coded for each amino acid?
There are 20 different amino acids that regularly occur in proteins and each amino acid must have its own code of bases on the DNA. There are four different bases (adenine, thymine, cytosine, guanine) that are present in DNA and if each base coded for a different amino acid, there would only be four different possible amino acids coded for and if pairs of bases are used, 16 (42) different codes are possible, which is still inadequate. Therefore, three bases are used as the produce 64 (43) different codes, more than enough to satisfy the requirements of 20 amino acids.
What are the three bases that code for an amino acid called?
Triplet
What are the features of the genetic code?
- They are degenerative.
- The code is universal.
- It is non-overlapping.
- The start of the DNA sequence is always the same triplet.
- There are three codons which doesn’t code for any amino acid. They are called ‘stop codons’ and they mark the end of a polypeptide chain.
What is meant by the genetic code being degenerative?
The code is known as a ‘degenerate code’ because most amino acids are coded for by more than one triplet.
What is meant by the code being universal?
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.)
What is meant by the code being non-overlapping?
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.
How many amino acids would be coded for by 15 bases?
5 amino acids
Deduce the sequence of amino acids in the order in which they would occur in the resultant polypeptide using the table:
- ATGCGTTAAGGCAGT
- GCTAAGTTTCCAGAT
- try-ala-ile-pro-ser
- arg-phe-lys-gly-leu
