4. DNA, Genes & Chromosomes Flashcards
DNA
Deoxyribonucleic acid - a molecule. Genes and chromosomes are made of DNA.
Gene
A section of DNA that codes for 1 polypeptide
Chromosome
A long chain of genes. May also contain non-coding DNA.
Allele
Different version of a gene
Chromosomes are found in homologous pairs. What does this mean?
They have the same genes, but not necessarily the same alleles.
Loci
The place on a chromosome where a gene is found
DNA is packaged with…
proteins called histones
only in the nucleus of eukaryotic cells!!!
Breakdown of a chromosome in an eukaryotic cell
See Notion
How is DNA different in a prokaryotic cell?
1 loop of DNA No homologous pairs No histones No nucleus DNA is shorter than in eukaryotic cells
How does DNA work?
- DNA stores information
- The information is in the order of the bases
- That information is a code for how to make proteins
- One gene is a stretch of DNA that contains the information to make one polypeptide
- The order of bases in the gene is a code for the order of amino acids in the protein
Base triplet code quick summary
A sequence of 3 bases is a code for an amino acid. The three bases are referred to as a codon.
Key features of the base triplet code
Non-overlapping
Includes start and stop codons
Degenerate - some amino acids have more than one codon
Universal - all organisms use the same code
Mutation
a change in the structure of the DNA
Gene mutation
a change in the sequence of bases in a gene
When do mutations usually happen?
During DNA replication before mitosis/meiosis
Germ line mutation
Mutations in gametes due to mistakes in meiosis
Somatic mutation
Mutations in somatic cells due to mistakes in mitosis
Most cancers are linked to which type of mutation?
Somatic mutations
Why are germ line mutations arguably worse than somatic mutations?
Germ line mutations are passed onto the next generation - potentially subsequent generations too!
Gene mutations lead to…
changes in the sequences of bases in the DNA, and therefore a change in the primary structure of the protein encoded by that gene
Substitution
This can be silent (no consequences).
One base gets swapped out for another.
They can lead to a change in the primary structure of a protein –> change in bonds forming between R groups –> change tertiary structure –> change function.
Substitution can also lead to a protein being cut short (mutates into a stop codon)
Deletion
Leads to a frameshift - all codons after the mutations are going to be affected.
Insertion of a base can also lead to a frameshift.
Examples of mutagens
Chemicals
- nitrous acid: changes C into U by removing a chemical group
- benzopyrene: makes G unable to pair with C, so DNA polymerase inserts any other base instead
Radiation
- ionising radiation creates free radicals which alter the shape of bases so that DNA polymerase can’t act on them
Chromosome mutation
Changes in whole sets of chromosomes
