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Genetic information, variation and relationships between organisms
(AQA AS Biology) PART 1 of 6 TOPICS
DNA, genes and chromosome:
Prokaryotic cells have short, circular DNA molecules that are not associated with proteins known as
Eukaryotic cells have long, linear DNA molecules which are associated with histone proteins.
Chromosomes are formed from two DNA strands.
DNA molecules that are short and circular are found in mitochondria and DNA which are found in
A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide and also
codes for functional RNA (transfer RNA known as tRNA).
Protein synthesis happens in two stages:
Transcription: DNA molecule splits by DNA helicase which breaks the hydrogen bonds. RNA
polymerase moves in the 5' (5 end) to 3' (3 end) of one DNA strand called the cistron where
complementary bases of RNA are matched with bases of the DNA strand. Three bases of the
DNA are called triplet and three bases of the RNA are called codon. NB: Remember that RNA
does not have thymine as a base so when a complementary base of RNA is needed to pair
up with adenine of DNA, uracil is used. The bases of RNA make a molecule called
pre-messenger RNA or pre-mRNA. DNA reforms once pre-mRNA has finished being made. The
introns are cut out of the pre-mRNA molecule leaving the exons which are bases that can code
for amino acids. These are spliced together to make mRNA and this leaves the nucleus via the
pores of the nuclear membrane into the cytoplasm.
Translation: The mRNA reaches the ribosomes to make polypeptides. tRNA molecules which
have three bases known as anti-codons attaches to a complementary codons on the mRNA.
The tRNA molecules have an amino acid attached to them which all join by condensation
reactions to form peptide bonds to create a polypeptide chain.
tRNA molecules are small in length compared to mRNA and are clover-leaf shaped because of
hydrogen bonding between complementary bases on different strands to one another. mRNA
molecules are single stranded. tRNA molecules have an amino acid attached to them.
Prokaryotes do not make pre-mRNA as they do not have introns.
Amino acids need activation in the form of ATP to attach to tRNA molecules.
A gene occupies a specific location on a DNA called a locus.
The genetic code has characteristics such as :
It is universal: Every organism has this