These are my notes for the fourth unit of alevel biology - genetic information, variation and relationships between organisms!
I got an A in my alevel AQA biology exam using these notes.
Happy revising :)
Genetic information,
variation and relationships
between organisms
,What is a gene?
• GENE = section of DNA that contains coded information for making
polypeptides and RNA
• Located at a particular position called a locus on a DNA molecule
,The genetic code
• Minimum of three bases (triplet) that coded for each amino acid
• Only four different bases (adenine, thymine, guanine and cytosine)
are present in DNA
• Three bases (4 x 4 x 4 = 64) different codes which is more than
enough to satisfy the requirement of 20 amino acids
,Features of the genetic code
1. Non-overlapping = each triplet is only read once
2. Degenerate = more than one triplet codes for the same amino ac
this reduces the number of mutations (a change in base sequenc
of DNA). This can be harmful as the mutations can result in cystic
fibrosis
3. Universal = same sequence of bases codes for the same amino ac
in all organisms
4. Start and stop codons
,• EXON- a segment of DNA containing information coding for a prote
or a polypeptide chain
• INTRON- a segment of DNA which doesn’t code for a protein or a
polypeptide chain
• GENOME- the complete set of genes in the cell
• PROTEOME- full range of proteins it can provide
,DNA and chromosomes
• PROKARYOTIC CELLS = the DNA molecules are shorter and form a
circle. They are not associated with protein molecules and do not
have chromosomes
• EUKARYOTIC CELLS = the DNA molecules are longer and form a line
(linear). They are associated with protein molecules called histones
form structures called chromosomes. The mitochondria and
chloroplasts of eukaryotic cells also contain DNA which is short,
circular and isn’t associated with proteins (like the DNA of
prokaryotes)
,Homologous chromosomes
• HOMOLOGOUS CHROMOSOMES = two chromosomes that carry t
same genes which are not identical as they carry different alleles o
the same gene (allele is different forms of the same gene)
,What is an allele?
• ALLELE = alternative forms of the same gene
,Transcription
, Transcription
• TRANSCRIPTION = a molecule of mRNA is made in nucleus
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