DNA, genes and protein synthesis

what is a gene?

a section of DNA that contains the coded information for making polypeptides and functional RNA

what is the coded information in the form of?

a specific sequence of bases along the DNA molecule

what do polypeptides make up and what does this mean?

they make up proteins, so genes determine the proteins of an organism

what are enzymes?

proteins

why are enzymes responsible for an organisms development and activites?

they control chemical reactions

what do genes (and other environmental factors) therefore determine?

the nature and development of all organisms

what is a gene? (location based)

a section of DNA located at a particular position (locus) on a DNA molecule

what does a gene (base sequence of DNA) code for?

• the amino acid sequence of a polypeptide

• or a functional RNA, including rRNA and tRNA

what did scientists suggest when trying to discover how DNA bases code for amino acids?

there must be a minimum of 3 bases that coded for each amino acid

what was their reasoning?

• only 20 amino acids regularly occur in proteins

• each amino acid must have its own code of bases on the DNA

• only 4 different bases are present in DNA

• if each base coded for a different amino acid, only 4 different amino acids could be coded for

• using a pair of bases, 16 different codes are possible (still inadeuqate)

• 3 bases produce 64 different codes, more than enough to satisfy the requirements of 20 amino acids

what is each set of 3 bases for each amino acid called?

a triplet

what is the case as there are 64 possible triplets and only 20 amino acids?

some amino acids are coded for by more than one triplet

what have further experiments revealed about the features of the genetic code?

• a few amino acids are coded for by only 1 triplet

• the remaining amino acids are coded for by 2-6 triplets

• the code is known as a degenerate code as 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 doesnt form part of the final polypeptide, it is later removed

• 3 triplets dont 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

• the code is non overlapping - each base is only read once (eg. 123456, 123, 456, not 123 234 345 …)

• the code is universal, with few exceptions each triplet codes for the same amino acid in all organisms - indirect evidence for evolution

what does much of the DNA in eukaryotes do?

not code for polypeptides

within genes…

only certain sequences code for amino acids

what are these coding sequences called?

exons

what are the non coding sequences called?

introns

what do some genes code for?

rRNA and tRNA

extra info: interpreting the genetic code