6.1 - cellular control

Question 1

Types of mutation

Answer 1

• Mutations can be harmful (e.g. lead to genetic diseases), beneficial (lead to evolution - new allele gives advantage against environmental pressures) or neutral (e.g. attached/detached ear lobes)
• There are 3 types of mutations – substitution, insertion and deletion

Substitution mutations (point mutations) - one base pair replaces another (same number of amino acids result). Can have 3 effects: silent, missense and nonsense mutations

• Silent mutations - the change in base pair will still code for the same amino acid (because of the redundancy of genetic code) = same protein = no affect
• Missense mutations - change in base pair causes a change in amino acid = change in primary and tertiary structure of protein = change in shape/function
• Nonsense mutations - Change in base pair codes for a premature stop codon = early termination of polypeptide chain = change in shape/function of protein

Insertions and deletions (indel mutations)

• Extra base pairs are inserted or some are deleted - if this results in the number of b.p not being in multiples of 3, this is causes a frameshift. Frameshifts alter all of the subsequent DNA codons = large change in primary and tertiary structure of protein = large change in shape/function
• If 3 bases are inserted/deleted, then no frame shift occurs - 1 extra/less amino acid

Question 2

Why is the genetic code described as non-overlapping (unambiguous) and degenerate (redundant)?

Answer 2

• Non-overlapping - no codon codes for more than one amino acid
• Redundant - more than one codon codes the same amino acid.

Question 3

Lac operon – regulation of gene expression at transcriptional level in prokaryotes

Answer 3

Background

• E. coli normally metabolises glucose
• If glucose is absent but lactose is present, the lactose causes 2 enzymes to be made:
• lactose permease - acts as a carrier protein for lactose to enter cell (coded for by lacY)
• B-galactosidase - hydrolyses lactose into glucose and galactose (coded for by lacZ)

In the absence of lactose (most of the time)…

• Regulatory gene (I) transcribed and translated into repressor protein
• Repressor protein prevents RNA polymerase binding at promoter region and so prevents transcription of the lactose permease and B-galactosidase genes (genes switched off)

In the absence of glucose and presence of lactose (inducer)…

• Lactose binds to repressor protein which changes the shape of repressor and so it is no longer able to bind to the operator region
• RNA polymerase binds at promoter region and transcription occurs which results in the production of lactose permease and beta – galactosidase
• Lactose permease inserts into the membrane and will transport more lactose into the cell (positive feedback)
• B-galactosidase will hydrolyse lactose into glucose and galactose

Question 4

Control of gene expression at transcriptional level in eukaryotes

Answer 4

• All somatic cells contain all chromosomes but certain genes are only expressed in certain cells e.g. mucus production in goblet cells
• Transcription factors (proteins) control which genes are switched on or off
• Transcription factors bind to specific promoter regions of DNA for the gene they control - they can help or prevent RNA polymerase from binding and transcribing the gene

Question 5

Control of gene expression at post-transcriptional level in eukaryotes

Answer 5

• Within a gene there are introns (non-coding regions) and exons (expressed/coding regions)
• Both introns and exons are transcribed to produce primary mRNA which is then spliced by an endonuclease enzyme (breaks phosphodiester bonds) to remove the introns
• This leaves behind the exons, now joined together to produce mature mRNA which will leave the nucleus to be translated

Question 6

Control of gene expression at post-translational level

Answer 6

• Once a protein has been made, it may be activated by cAMP- this can involve adding functional groups e.g. many proteins are phosphorylated
• Signalling molecule (first messenger) binds to receptor on cell surface membrane
• This causes a G-protein to activate adenyl cyclase which converts ATP into cAMP (second messenger)
• cAMP activates PKA (an enzyme) which phosphorylates proteins to activate them

Question 7

Homeobox genes

Answer 7

• A homeobox gene is a type of homeotic gene (control morphogenesis) that contains a 180 bp homeobox sequence that codes for a 60 amino-acid sequence called a homeodomain sequence within a protein. These proteins are transcription factors.
• The homeodomain sequence’s shape is specific to part of the enhancer region on DNA so it binds to the DNA to initiate/stop transcription to switch genes on or off. This controls the development of the body plan.
• Homeobox genes are master genes - they switch on/off many other genes.

Question 8

Homeobox genes are highly conserved

Answer 8

• Homeobox sequences in animals, plants and fungi are very similar and are highly conserved (found in all plant, animal and fungal species from a common ancestor throughout history).
• Organisms have very similar base sequences in the homeobox genes.
• Across a wide range of animals there are very few mutations in these genes because they are very important and mutations would have large effects on the body plan. These mutations would have likely been selected against as they would have killed organisms.

Question 9

Hox genes

Answer 9

• Hox genes are a type of homeotic genes only found in animals.
• They control the formation of anatomical features in the correct locations of the body plan.
• In embryonic development Hox genes are expressed one by one along the anterior-posterior axis which causes the development of particular body parts in this particular order
• Hox genes can switch on other genes that promote apoptosis and mitosis.

Question 10

Stages of apoptosis (programmed cell death)

Answer 10

1. Cytoskeleton broken down by enzymes
2. The cell shrinks, the cell surface membrane forms blebs (small protrusions) and chromatin condenses
3. DNA breaks up and the nuclear envelope breaks down into fragments. Blebs form vesicles containing organelles
4. Vesicles are engulfed and digested by phagocytes so the old cell and its contents can cause no damage to other cells

Question 11

Importance of mitosis and apoptosis as mechanisms controlling the development of body form.

Answer 11

• Mitosis causes growth from zygote to adult. When cells reach their Hayflick limit (after dividing about 50 times) they are destroyed by apoptosis
• The rates of mitosis and apoptosis should be equal otherwise tumours will form (higher rate of mitosis) or tissues will degenerate (more apoptosis)
• Apoptosis is vital in body development e.g. causes separation of digits in body development, causes loss of vestigial tail in humans
• The genes which regulate the cell cycle and apoptosis are able to respond to internal and external cell stimuli e.g. stress which can determine the development of the bod

Question 12

Regulatory gene

Answer 12

makes repressor protein / transcription factor or idea that product switches (structural / another) gene on / off

Question 13

Structural gene

Answer 13

makes enzyme / polypeptide / protein