6.1 GENETICS OF LIVING SYSTEMS

Question 1

What does degenerate mean?

Answer 1

Degenerate is the multiple ways of coding for each amino acid, as amino acids can be coded for by more than one triplet.

Question 2

What does non-overlapping mean?

Answer 2

Non-overlapping is where each base is only part of one codon, as each base is only read once in the sequence.

Question 3

What does universal code mean?

Answer 3

Universal code means that everyone/everything has the same code, all organisms have the same triplets/amino acids.

Question 4

What are the types of gene mutation?

Answer 4

Types of gene mutation:
- substitution (nonsense, missense, silent)
- insertion
- deletion
- frameshift

Question 5

What is substitution gene mutation?

Answer 5

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)

Question 6

What is sickle cell anaemia?

Answer 6

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.

Question 7

What is insertion gene mutation?

Answer 7

Insertion gene mutation is where an extra nucleotide is added to the sequence.

Question 8

What is deletion gene mutation?

Answer 8

Deletion gene mutation is where a nucleotide is removed from the sequence.

Question 9

What is frame shift?

Answer 9

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)

Question 10

What are the effects of mutations?

Answer 10

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

Question 11

What are the causes of mutations?

Answer 11

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

Question 12

What is gene expression?

Answer 12

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

Question 13

What is gene regulation?

Answer 13

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

Question 14

What are the types of genes?

Answer 14

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

Question 15

What is an operon?

Answer 15

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

Question 16

What is the structure of the lac operon?

Answer 16

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

Question 17

What is beta-galactosidase?

Answer 17

Beta-galactosidase is an enzyme that hydrolyses lactose into glucose and galactose.

Question 18

What is lactose permease?

Answer 18

Lactose permease is the transport protein that becomes embedded in the E.coli membrane and helps transport more lactose into the cell

Question 19

What happens when E.coli is placed into a lactose substrate?

Answer 19

When placed in a lactose substrate, E.coli increases the synthesis of the two proteins by 100x. lactose triggers the enzymes production = inducer molecule

Question 20

What is the account of the lac operon?

Answer 20

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.

Question 21

What is a transcription factor?

Answer 21

Transcription factors are proteins that bind to specific regions of DNA to control the transcription of genes.

Question 22

Why are transcription factors important?

Answer 22

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.

Question 23

Where do transcription factors often bind and what possible effects can this have on transcription?

Answer 23

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

Question 24

Why can oestrogen enter cells easily?

Answer 24

Oestrogen can enter cells easily as it is lipid-soluble and can diffuse through the plasma membrane.

Question 25

Where does oestrogen bind to in the nucleus and what effect does binding have?

Answer 25

In the nucleus, oestrogen binds to the oestrogen receptor, this causes a change in the shape of the receptor. Which causes the receptor to move away from the protein complex and bind to a promotor.

Question 26

Does the transcription factor (oestrogen receptor) turn genes on or off?

Answer 26

The oestrogen receptor turns genes on.

Question 27

What is heterochromatin?

Answer 27

Heterochromatin is found during cell division. Histone proteins are tightly packed and RNA polymerase cannot access DNA.

Question 28

What is euchromatin?

Answer 28

Euchromatin is found during interphase. Histone proteins not tightly packed and RNA polymerase can access DNA.

Question 29

What is transcriptional control?

Answer 29

Transcriptional control - chromatin remodelling:
- DNA is along a molecule that must be wound around histone proteins to allow it to be packaged in the nucleus, this combination of DNA and histones in chromatin
- chromatin found in two forms based on how tightly the DNA is wrapped around the histones (heterochromatin and euchromatin)

Question 30

What are histone modifier proteins?

Answer 30

Histone modifier proteins:
methylation - prevents transcription (packed more tightly)
acetylation - allows transcription (packed more loosely)
phosphorylation - allows transcription (packed more loosely)

Question 31

What is post-transcriptional control?

Answer 31

Post-transcriptional control - introns and exons:
- the editing of primary mRNA and the removal of introns to produce mature mRNA
intron = section of DNA that does not code for an amino acid
exon = does code for amino acids
- when the gene is transcribed, only the exons are left, the introns are removed

Post transcriptional control- splicing:
- need to remove introns
- in pre-mRNA of eukaryotic cells the introns are removed and the functional exons are joined together (splicing)
- the spliceosome causes the intron to from a loop shape
- the intron is exercised and the exons are then spliced together
- the mRNA may then leave the nucleus into the cytoplasm for the next stage of protein synthesis

Post-transcriptional control- RNA processing and editing:
. processing
- cap = added to 5’ end
- tail = long chain of adenine nucleotides added to the 3’ end
- stabilise mRNA and delay degradation in the cytoplasm
- cap aids binding of mRNA to ribosomes
. editing
- addition, deletion or substitution of bases
- same effect as point mutations
- increases the range of proteins that can be produced from a single mRNA strand/gene

Question 32

What is translational control?

Answer 32

Translational control:
- degradation of mRNA = the more stable the mRNA the more protein is made
- binding of inhibitory proteins = can prevent mRNA binding to ribosomes
- initiation factors = when activated can aid mRNA binding to ribosomes

Question 33

What is post-translational control?

Answer 33

Post-translational control:
- addition of non-protein groups
- modifying amino acids and formation of bonds
- folding/shortening of proteins
- modification by cAMP
= this is a second messenger inside cells formed from ATP
= stimulated to act by adenyl cyclase
e.g activation of protein kinase A (PKA) by cAMP. PKA is an enzyme made of 4 subunits. when cAMP isn’t bound, the 4 units are bound together and are inactive. When cAMP binds, it causes a change in the enzymes 3D structure, releasing the active subunits - PKA is now active.