Chapter 20: Gene Expression Flashcards
Learn all about what controls gene expression, how genes can be switched on or off
Define the word mutation.
A mutation is a change to the sequence of nucleotide bases in DNA
Name the 6 different types of mutations.
Deletion Substitution Addition Duplication Inversion Translocation
mutagenic agents
factors that can increase the rate of mutation
substitution
a base is replaced by another base
this can lead to the production of a non-functional protein or to no change in the amino acid sequence of the polypeptide
How can substitution result in the production of a non-functional protein?
Each of the triplets in a gene code for a particular amino acid e.g. ACT-AGG-TTC
- the base that gets substituted could be the same as the original base so the triplets stay the same e.g. ACT-AGG-TTC
- the base that gets substituted could be different than the original e.g. AAT-AGG-TTC
- the new triplet AAT still codes for the same amino acid (as the genetic code is degenerate) and therefore there is no change to the sequence of amino acids in the polypeptide
- the new triplet AAT codes for a different amino acid and therefore leads to a change in the amino acid sequence of the polypeptide which may mean the protein cannot fold into a specific shape and therefore cannot carry out its function
silent mutation
A mutation that doesn’t cause a change in the amino acid order
Give 4 examples of mutagenic agents.
ultraviolet radiation, ionising radiation, some chemicals and viruses
What is a deletion mutation and what effect does it have?
Where a base is removed from the base sequence.
This causes a frame shift to the left, affecting all the bases afterwards
Because the genetic code is non-overlapping, the bases are still read in 3’s which means the triplets are different and so will code for different amino acids to the original.
This changes the primary structure of the polypeptide possibly leading to a non-functional protein
duplication
Where one or more bases are repeated
e.g. ATGCCT becomes ATCCCCT
addition
Where a bases is added to the sequence
inversion
Where a sequence of bases is flipped over or reversed
e.g. ATGCCT becomes ACCGTT
translocation
Where a group of bases are removed from one place and added at another location and this could be movement within the same chromosome or movement from one chromosome to another
What is the epigenome?
The epigenome is the layer of chemicals surrounding the genome
What does it mean for a gene to be expressed?
When a gene is said to be expressed/switched on in an organism it means the gene is producing the polypeptide it codes for
Give 3 ways mutagenic agents can increase the rate of mutations
- Some types of radiation can change the structure of DNA
- some chemicals can alter or delete bases
- types of chemicals called base analogs can substitute a base during replication
Mutations in particular genes can lead to cancer. How?
Proto-oncogenes mutate into oncogenes which can cause them to be permanently switched on which leads to cancer
Proto-oncogenes normally stimulate a cell to divide by producing proteins. When mutated they can become overactive and
- produce excess amount of growth factors
- the receptor proteins can be permanently activated
Mutations in tumour suppressor genes cause them to become inactive which leads to cancer
Tumour suppressor genes are involved in slowing down cell division and in carrying out apoptosis (programmed cell death) when the cell’s DNA gets damaged.
If they are inactive it means they can’t do their job so cells divide uncontrollably
Gene expression is regulated by _____ which is controlled by _____
i. transcription
ii. transcriptional factors
Describe how a hormone such as oestrogen is able to control transcription and therefore gene expression.
- oestrogen (steroid hormone) diffuses into cytoplasm
- binds to an oestrogen receptor forming oestrogen-oestrogen receptor complex
- receptor is attached to a DNA binding site
(receptor and DNA binding site together make up transcriptional factor) - complex moves into nucleus from cytoplasm
- DNA binding site binds to specific DNA sites near start of gene
- can act as an activator or a repressor depending on the type of cell and the target gene
What are transcriptional factors?
proteins that control the transcription of target genes
How do transcriptional factors control transcription and therefore gene expression?
- control the rate of transcription
- by binding to specific DNA sites near the start of their target gene
- some stimulate transcription or increase its rate, called activators
- help RNA polymerase bind to start of target gene which leads to transcription
- some inhibit transcription or decrease its rate, called repressors
- prevent RNA polymerase binding to start of target gene which stops transcription
All the cells in an organism carry the same what?
genes
How does RNA interference with siRNA work?
- involves siRNA (small double stranded RNA)
- enzyme separates the 2 strands
- single strand of siRNA binds to target mRNA
- base sequence of siRNA complimentary to some of the bases on mRNA strand
- enzyme cuts mRNA into fragments
- mRNA cannot be translated into a protein so expression of that gene stops
How does RNA interference with miRNA work?
- miRNA (small single strandee)
- less specific than siRNA can target more than one mRNA molecule
- binds to target mRNA
- prevents translation of mRNA
How does decreased acetylation of histones allow gene expression to be controlled epigenetically?
increases the association between histones and DNA
so DNA-histone complex is more tightly packed
transcriptional factor cannot access DNA
cannot bind to initiate transcription
gene is switched off
