3.4.1 DNA, genes and chromosomes Flashcards
what is a gene?
a base sequence of DNA that codes for the amino acid sequence of a polypeptide and a functional RNA
what is a locus?
the fixed position of a gene on a particular DNA molecule
what is an allele?
one of a number of alternative forms of a gene
what is meant by a homologous pair of chromosomes?
they have exactly the same genes but might have different alleles
how is DNA stored in eukaryotes?
- stored as chromosomes inside the nucleus
- chromosomes are linear in shape
- DNA is tightly wound around proteins called histones
- this complex of DNA wrapped around a histone is called a nucleosome
how is DNA stored in prokaryotes?
- DNA molecules are shorter and circular
- DNA is not associated with a protein
- instead, DNA is supercoiled to fit in the cell
how is DNA in mitochondria and chloroplasts stored?
- similar to the DNA of prokaryotes
- short, circular and not associated with proteins
what is a start codon?
the three bases (triplet) at the start of every gene which initiates translation
what is a stop codon?
- the three bases (triplet) at the end of every gene that do not code for an amino acid
- the stop codons mare the end of a polypeptide chain and cause ribosomes to detach
- this therefore stops translation
what are the three features of the genetic code?
- degenerate
- universal
- non-overlapping
what is meant by the genetic code being ‘degenerate’?
most amino acids are coded for by more than one triplet of bases
what is the advantage of the genetic code being degenerate?
if a point mutation (substitution) occurs, the triplet of bases will be different but it may still code for the same amino acid and therefore will have no effect
what is meant by the genetic code being ‘universal’?
the same triplet of bases codes for the same amino acid in all organisms
what is the advantage of the genetic code being universal?
it means that genetic engineering is possible (e.g., inserting the human gene for insulin into bacteria)
what is meant by the genetic code being ‘non-overlapping’?
- each base in a gene is only part of one triplet of bases that codes for one amino acid
- therefore each codon, or triplet of bases, is read as a discrete unit