8.1 Genes and triplet code Flashcards
What is a gene
A gene is a section of DNA located at a specific place called the locus, that contains the coded information for making polypeptides and functional RNA
This coded information is in the form of a specific sequence of bases along the DNA molecule
What are polypeptides and what do they do
Polypeptides are chains of amino acids bonded in condensation reactions making peptide bonds
Polypeptides make up proteins,
and as genes contain the coded information for making polypeptides, genes determine the proteins of an organism
Proteins may be enzymes, what do these do
They change the rate of chemical reactions in the body, so are responsible for the organisms development
What do genes code for specifically
. The amino acid sequence of polypeptide
. Functional RNA, including rRNA and tRNA
What is the reasoning behind there being 3 DNA bases that code for an amino acid
. Only 20 amino acids occur in proteins
. Each amino acid must have its own code of bases on DNA ( different sequence)
. There are only 4 bases present in DNA (ATCG)
. With 2 bases, 4^2 =16 so only 16 different codes are possible which isn’t adequate for 20 amino acids
. With 3 bases, 4^3 = 64 different codes , which is more than enough to satisfy the requirements of the 20 amino acids
What are 3 bases called, and how is this read
What do these make up
A triplet is read in one direction along the DNA strand
Amino acid
Why is the genetic code degenerate
As there are 64 possible sequences for amino acids, and only 20 amino acids, multiple triplets can each code for the same amino acid.
Eg ATC and GAT can both code for the same amino acid
However a few amino acids are coded for by only one triplet
What is significant about the start of a DNA sequence that codes for a polypeptide
The same triplet is always at the start
This triplet codes for amino acid methionine
It may be removed after if it doesn’t form part of the final polypeptide
What are stop codes
There are 3 triplets that don’t code for any amino acid
So these mark the end of the polypeptide chain
How is the genetic code non-overlapping
. Each base in a sequence is only read once
So eg 6 bases numbered 123456 will be read
123 and 456, not 123, 234, etc
How is the genetic code universal
What is it evidence for
. The genetic code is found in all biological organisms,
and each triplet codes for the same amino acid in all organisms (there are a couple exceptions)
Evolution
What are introns and where are they found
Are they found in prokaryotic organisms?
Found in eukaryotic organisms
they are sequences of bases in DNA that don’t code for proteins.
They are non coding sequences of bases
They aren’t found in prokaryotic organisms
What are exons
Where are they found
Triplet of bases that code for polypeptide
Found in both eukaryotes and prokaryotes
