6.1 GENETICS OF LIVING SYSTEMS Flashcards
What does degenerate mean?
Degenerate is the multiple ways of coding for each amino acid, as amino acids can be coded for by more than one triplet.
What does non-overlapping mean?
Non-overlapping is where each base is only part of one codon, as each base is only read once in the sequence.
What does universal code mean?
Universal code means that everyone/everything has the same code, all organisms have the same triplets/amino acids.
What are the types of gene mutation?
Types of gene mutation:
- substitution (nonsense, missense, silent)
- insertion
- deletion
- frameshift
What is substitution gene mutation?
Substitution gene mutation:
- changes a nucleotide within a codon
- could code for a new amino acid but the code is degenerate so the new codon may still code for the same amino acid
- or may code for a stop codon
nonsense substitution- the mutation results in one of the three stop codons (e.g TAG, TAA, TGA)
missense substitution- results in a different amino acid sequence being coded for (e.g GTC changes to GAC = valine to aspartic acid)
silent substitution- although it is a different codon, the same amino acid is coded for (e.g GTC to GTT = valine)
What is sickle cell anaemia?
Sickle cell anaemia is a genetic disease caused by missense substitution. The 6th codon in the gene for beta-hb chain contains substitution so that the codon codes for valine and not glutamic acid.
What is insertion gene mutation?
Insertion gene mutation is where an extra nucleotide is added to the sequence.
What is deletion gene mutation?
Deletion gene mutation is where a nucleotide is removed from the sequence.
What is frame shift?
Frame shift:
- substitution, insertion and deletion are all point mutations
- however insertion and deletion lead to frameshift mutation, where the addition or removal or a nucleotide moves the reading frame of the sequence of bases
- this is very serious as it effects all further amino acids (unless the number of bases added or removed is 3)
What are the effects of mutations?
Effects of mutations:
neutral - normal functioning proteins still synthesised = phenotype of organism is unchanged. most mutations are of this type and occur in non-coding DNA
harmful - proteins not synthesised or are non-functional = phenotype of organism is negatively impacted. e.g oncogene mutations, sickle cell, Huntington’s, albinism
beneficial - protein synthesised with a new and useful characteristic in the phenotype = very rare
What are the causes of mutations?
Causes of mutations:
can occur naturally but their appearance can be increased
- x-rays
- high energy radiation (e.g gamma)
- chemicals in cigarette smoke
- chemicals in caffeine
- UV light
What is gene expression?
Gene expression:
- every somatic cell contains the same genes but not all need to be expressed in every cell all of the time
- genes can be switched off when they are not needed
- this prevents cellular resources being wasted
- typical human cells express 3-5% of their genes at any given time
- cancer results when genes don’t turn off properly
What is gene regulation?
Gene regulation is regulatory mechanisms that ensure that the correct gene is expressed in the correct cell at the correct time
occurs at 4 points:
- transcriptional
- post-transcriptional
- translational
- post translational
regulatory mechanisms are controlled by regulatory genes
What are the types of genes?
Types of genes:
structural gene - codes for a protein that has a function within a cell (e.g enzyme membrane carrier, hormones)
regulatory gene - codes for a protein (or form of RNA) that controls the expression of structural genes
What is an operon?
An operon is an example of transcriptional control.
- a section of DNA that contains a cluster of structural genes that are all transcribed together as well as control elements ands a regulatory gene
- more common in prokaryotes than eukaryotes
- saves resources as these genes can be ‘switched’ off together e.g lac operon
- E.coli only produces the enzymes needed to metabolise lactose when lactose is present
- if lactose is not present then a ‘repressor’ is produced to stop lactose being produced
What is the structure of the lac operon?
Lac operon structure:
. regulator
- codes for a protein called a repressor protein
- repressor protein has 2 binding sites, one for the operator and one for the the lactose. if lactose binds the protein changes shape and cannot bind to the operator
. promotor
- where RNA polymerase bind to catalase the transcription of mRNA from the structural genes
. operator
- where the repressor protein binds. if the operator is free, then the promotor is available for the RNA polymerase to bind to, so transcription of the gene occurs
. gene for beta-galactosidase
- enzyme that hydrolyses lactose
. gene for lactose permease
- enzyme that enable the uptake of lactose
What is beta-galactosidase?
Beta-galactosidase is an enzyme that hydrolyses lactose into glucose and galactose.
What is lactose permease?
Lactose permease is the transport protein that becomes embedded in the E.coli membrane and helps transport more lactose into the cell
What happens when E.coli is placed into a lactose substrate?
When placed in a lactose substrate, E.coli increases the synthesis of the two proteins by 100x. lactose triggers the enzymes production = inducer molecule
What is the account of the lac operon?
An operon is a length of DNA containing genes that code for one or more proteins, along with base sequences that control whether or not the genes will be expressed. The genes that code for the proteins are called structural genes.
The bacterium E.coli contains the lac operon. The functioning of this operon is a good example of an interaction between genes and the environment. The lac operon has genes encoding two enzymes, as well as an operator region and a promotor region. One enzyme, called lactose permease, enables the bacterial cell to take up lactose, acting as a pump in the cell surface membrane. The second enzyme, called beta-galactosidase, breaks down lactose into its constituent monosaccharides, alpha glucose and galactose.
Normally, when E.coli is grown on a nutrient agar jelly that contains glucose but no lactose, it is not necessary to switch on the genes for the two enzymes, because this would result in a waste of both amino acids and energy/ATP. To prevent the prevent the structural genes from being expressed, a repressor protein is synthesised. This protein is coded for by yet another gene located elsewhere on the bacterial DNA, called a regulatory gene. The repressor protein has two binding sites. One binds to the operator region of the DNA. This prevents the enzyme RNA polymerase from binding to the promotor region. This means that the mRNA for the two enzymes will not be transcribed and the enzymes will not be synthesised.
When E.coli is grown on agar jelly containing only lactose, some lactose enters the cell by diffusion. The lactose fits into the second binding site on the repressor. Binding alters the tertiary structure of the repressor, changing its shape. The repressor will now no longer bind with the operator region of the operon. The enzyme RNA polymerase can now bind to the promotor and start transcribing the genes for the two enzymes, so the bacterium can use the lactose in the growth medium.
What is a transcription factor?
Transcription factors are proteins that bind to specific regions of DNA to control the transcription of genes.
Why are transcription factors important?
Transcription factors are important as they control the expression of genes to allow organisms to respond to their environment and allows some hormones to achieve their effect.
Where do transcription factors often bind and what possible effects can this have on transcription?
Transcription factors often bind to the promotor region of a gene. This binding can either allow or prevent the transcription of the gene from taking place. Switch specific genes on and off to induce or inhibit transcription.
- increase transcription = activator
- decrease transcription = preventor
Why can oestrogen enter cells easily?
Oestrogen can enter cells easily as it is lipid-soluble and can diffuse through the plasma membrane.
