3.4.1 DNA, Genes and Chromosomes Flashcards
What is a gene?
A section of DNA that contains the coded information for making polypeptides and functional RNA
Why is it necessary for three bases to code for one amino acid?
Only 20 different amino acids regularly occur in proteins
Each amino acid must have its own code of bases on the DNA
There are only four different bases that 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 different codes are possible which is still inadequate
Three bases produce 64 different codes which are more than enough to create 20 amino acids
What is a DNA triplet?
Each amino acid is a triplet, consisting of 3 bases
There are 64 possible triplets so some amino acids are coded for by more than one triplet
A triplet is always read in one particular direction along the DNA strand
Three triplets don’t always code for an amino acid- some are stop codes which mark the end of a polypeptide chain
What is degenerate code?
Because most amino acids are coded by more than one triplet
How is the DNA code universal?
The code is universal with a few minor exceptions as each triplet codes for the same amino acid in all organisms
Why is DNA non-overlapping?
Because each base is read once
What are introns and exons?
Exons are coding sequences which code for amino acids
Introns are non-coding sequences that separate exons
What is a chromosome?
DNA molecules which form a line (are linear) and occur in association with proteins called histones
Found in eukaryotic cells
What are homologeous chromosomes?
Chromosomes consisting of two chromatids with identical DNA
What is the structure of a chromosome?
DNA is in a double helix
Helix is wound around histones to fix it in position
DNA-histone complex is then coiled
Coil is in turn looped and further coiled before being packed into the chromosome
What is an allele?
An allele is one of a number of alternative forms of a gene
What causes mutation?
Any changes in the base sequence of a gene produces a new allele of that gene (mutation)
What is a gene locus?
A fixed position on a chromosome
How are introns/exons different in eukaryotic and prokaryotic cells?
Introns are more likely to be in eukaryotic cells
Prokaryotes do not have noncoding DNA so splicing does not occur
