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
Down’s syndrome is due to a mistake that happens in…
meiosis
Polyploidy
Organisms having more than 2 sets of chromosomes. Occurs almost exclusively in plants.
The sugar-phosphate backbone is joined together via…
phosphodiester bonds
How does the genetic code work?
3 bases code for an amino acid (primary structure) and is called a codon
Universal & degenerate
There are start & stop codons
The three stages of protein synthesis in order are…
- Transcription
- Splicing
- Translation
Transcription in a nutshell
A copy of the gene is made out of RNA.
This can leave the nucleus and go to the ribosome in the cytoplasm.
Transcription in detail
- Original DNA molecule has a coding strand and a template strand
- DNA molecule separates
- An RNA molecule is built using the template strand
- RNA polymerase joins the RNA nucleotides together
- The pre mRNA molecule detaches from the DNA molecule
What’s unique about splicing?
Only in eukaryotic cells
What is splicing?
Making modifications to the pre mRNA before it leaves the nucleus (i.e. removing bits that don’t code for proteins)
What is pre mRNA?
A copy of the gene containing introns and exons
What are introns?
Regions that don’t code for a protein in eukaryotes
What are exons?
Regions that code for a protein in eukaryotes
What is mature mRNA?
The end product of splicing
How does the mRNA leave the nucleus after splicing?
Via the nuclear pores
Structure of tRNA
Amino acid at the top, anticodon at the bottom
2 things the anticodon on tRNA is complementary to
- the codon for the amino acid attached at the other end of the tRNA
- the codon of the mRNA
What are ribosomes made of (cells recap!)
Proteins and ribosomal RNA (rRNA)
Translation in detail
- Small subunit of ribosome binds to mRNA
- tRNA molecule arrives
- Large subunit arrives
- Anticodon of tRNA binds with codon of the mRNA that it is complementary to
- Another tRNA arrives at the ribosome
- A peptide bond forms between the two amino acids once the codon and anticodon have joined together
- tRNA leaves the ribosome leaving the amino acid behind
- Ribosome shifts along by one codon after the bond has formed
- Process continues until the stop codon is reached
Where exactly is the mRNA during translation?
BETWEEN the large & small subunit, NOT DIRECTLY ATTACHED TO THE SMALL SUBUNIT!!
What happens when the STOP codon is reached during translation?
The ribosome detaches & so does the polypeptide chain
What needs to happen to a protein after it has been synthesised on a ribosome so that it is ready to carry out its job?
- Add prosthetic groups (some - eg. haemoglobin)
- Folded up into specific 3D structure (tertiary)
- Bind to a carbohydrate (glycoproteins)
Where do the modifications to the protein after translation take place?
Some in the cytoplasm
Can also happen in the Golgi (when molecules need to be added)
T or F: only one ribosome can work on one mRNA molecule
FALSE!
Several ribosomes can work on an mRNA molecule at the same time
