6.1 Cellular Control Flashcards
What is a mutation?
a change in the base sequence of DNA
What causes a mutation?
They occur randomly, yet can be caused by:
1. UV light
2. Ionising radiation e.g. x-rays
3. Chemicals - carcinogens
What is a Deletion mutation?
A base has been deleted / removed from the code.
Effects of a deletion mutation?
Frameshift - all the bases move down.
Now all the subsequent codons will code for different amino acids, changing the overall shape of the protein
What is degenerate code?
Code where multiple codons code for the same amino acid
What is an Insertion mutation?
When a nucleotide is randomly inserted into the DNA sequence
Effects of insertion mutations?
Changing the triplets (groups of three bases) further on in the DNA sequence
This is sometimes known as a frameshift mutation
This may change the amino acid sequence produced and the ability of the polypeptide to function
What is a substitution mutation?
Where one base is substituted for another
Effects of a substitution mutation?
Will not have a knock-on effect
Will only change the amino acid which the triplet codes for
What are 3 types of substitution mutation?
Silence - the mutation does not alter the amino acid sequence (due to degenerate code)
Missense - mutation alters a single amino acid
Nonsense - mutation creates a premature stop codon
What is a stop codon?
A signal for the cell to stop translation of the mRNA molecule into an amino acid sequence
Beneficial mutations
A significantly altered polypeptide shape, yet the mutation causes an altered characteristic which has beneficial effects for the organism
—> e.g. Pale skin synthesising vitamin D from decreased melanin production
Harmful mutations
Causes an altered characteristic which has harmful effects for the organism
—> e.g. cystic fibrosis
Neutral mutations
No selective advantage or disadvantage
- A mutation does not alter the polypeptide
- A mutation only alters the polypeptide slightly so that its structure or function is not changed
- A mutation alters the structure or function of the polypeptide but the resulting difference in the characteristic of the organism provides no particular advantage or disadvantage to the organism
What are regulatory mechanisms?
They make sure the correct genes are expressed in the correct cell at the correct time
3 main types of regulatory mechanism?
- Transcriptional level
- Post-transcription
- Post-translation
What does a structural gene do?
codes for a protein that has a function within a cell e.g. enzymes
What do regulatory genes do?
code for proteins that control the expression of structural genes
What is the lac operon an example of?
a regulatory mechanism at the transcriptional level
What is an ‘operon’?
a group or a cluster of genes that are controlled by the same promoter
Where is the lac operon found, and what does it control?
Bacteria
It controls the production of lactase.
Lactase can be used to break down lactose which can be used as an energy source for the cell
Why is lactase referred to as an inducible enzyme?
it is only synthesized when lactose is present
What happens when lactose is absent?
- Regulatory gene is transcribed and translated to produce lac repressor protein
- lac repressor binds to operator region
- RNA polymerase cannot bind to promoter region
- transcription of structural genes does not take place
- no lactase is synthesised
(obvs, when there is no lactose, no lactase is needed to break it down)
What happens when lactose is present?
- uptake of lactose by bacteria
- lactose binds to 2nd binding site on repressor protein and distorts its shape
- Repressor protein cannot bind to operator region
- RNA polymerase binds to promoter region
- transcription takes place
- lactase enzyme is produced
- Lactase breaks lactose down into galactose and glucose which can be used as a substrate.
What are exons and introns?
Exons - coding sequence of DNA
Introns - non-coding sequence of DNA
Why shouldn’t introns be translated?
if any introns were included in the mature mRNA, the resulting protein would not be formed properly and may not function as it should
What is splicing?
- Removal of introns from primary mRNA
- exons are then all fused together to form a continuous mRNA molecule called mature mRNA that is ready to be translated
What is splicing an example of?
Post-transcriptional modification
Why is activation needed at the post-transcriptional level?
Some proteins aren’t functional straight after they have been synthesised, so they need to be activated in order to work
How is protein activation controlled?
Controlled by molecules e.g. hormones / sugars
1. These bind to cell membranes and trigger production of cAMP inside cell
2. cAMP activates proteins by altering 3D structure
—> e.g. this could change the active site, making the protein more or less active
Example of cAMP and PKA
- PKA is a protein with 4 sections
- When cAMP isn’t bound, the 4 sections are bound together and it is inactive
- When cAMP binds to PKA, it causes a change in the 3D structure of the protein. releases the subunits
- PKA is now active
What is a homeobox?
a DNA sequence that codes for a protein transcription factor
These transcription factors attach to DNA at specific locations and regulate the transcription of genes by turning various different genes on and off in the correct order
What is a homeobox gene?
any gene that contains a homeobox sequence
Homeobox in plants, fungi and animals are…
similar and highly conserved
i.e. they remain unchanged when going back in time
Why are the homeobox genes highly conserved?
Mutations that cause changes in these homeobox sequences can lead to organisms that are not viable so they are not favoured by natural selection. This strong negative selection pressure explains why the sequences are highly conserved
What are homeobox genes responsible for?
the genetic control of the development of body plans in different organisms
they control the basic pattern of the organism
What 3 things does the homeobox control?
- The pattern of the body of the organism
- The polarity of the organism
- Segmentation into distinct body parts i.e. development into wings or limbs, as well as which organs are present
What are hox genes?
Subset of homeobox genes
- determine the identity of embryonic body regions along the anterior-posterior axis (i.e. the head-tail axis)
What is apoptosis?
Programmed cell death
What is the importance of apoptosis in the body plan?
some cells that are produced by mitosis earlier on in development may no longer be needed
As a result, these cells are destroyed by apoptosis as part of the development of the organism
—> e.g. structures like fingers and toes first develop as a single unit and then are separated later via apoptosis of the cells in between the digits
What are the uses of mitosis?
- growth
- replacement of cells
- tissue repair
Which two genes control mitosis and apoptosis?
- Proto-oncogenes are genes that stimulate cell division
- Tumour-suppressor genes which reduce cell division
Functions of Tumour-suppressor genes
Stimulate apoptosis in cells with damaged DNA that cannot be repaired
This protects the body as it ensures that any cells that are genetically damaged (and that could, therefore, lead to cancer) are destroyed
Which stimuli can the genes which control apoptosis and mitosis respond to?
External and Internal
Which examples of external stimuli can genes can respond to?
- The presence of cell signalling molecules such as cytokines from the immune system, hormones and growth factors
- Viruses and bacteria, harmful pollutants or ultraviolet light
Which examples of internal stimuli can genes can respond to?
- Irreparable genetic damage
- RNA decay
These factors can all initiate apoptosis in cells that are undergoing cell stress
Importance of mitosis in controlling body plan
Constantly replaces destroyed cells in early development
