6.1 - Cellular Control

Question 1

What is an exon?

Answer 1

Coding part of a gene.

Question 2

What is an intron?

Answer 2

Non-coding part of a gene.

Question 3

Why is the term ‘junk’ DNA misleading?

Answer 3

Junk implies this DNA has no function – in truth we do not yet know the function.

Question 4

What is a mutation?

Answer 4

A change to the base sequence.
Substitution: One or more bases are swapped for one another.
Deletion: One or more bases are removed.
Insertion: One or more bases are added.

Question 5

How might a deletion of bases mutation affect protein functionality?

Answer 5

Deletion leads to frameshift. Changes amino acid sequence. Affects hydrogen/ionic/sulphur bonds. Changes tertiary structure of protein – so cannot function.

Question 6

How might a mutation result in a non-functional protein?

Answer 6

Mutation causes a stop codon so transcription stopped prematurely. Mutation codes for a different amino acid to be produced - protein may/may not function.

Question 7

When may a mutation not result in a non-functional protein?

Answer 7

Mutation codes for same amino acid to be produced due to the degenerate nature of the genetic code. Mutations that occur in the non-coding DNA regions – the introns will be removed by splicing & not be translated so have no effect on protein produced.

Question 8

What are the causes of mutations?

Answer 8

Spontaneous during DNA replication. Mutagens - Chemicals that alter the DNA structure or high energy radiation.

Question 9

What are the three levels that gene expression can be controlled at?

Answer 9

Transcriptional level, post-transcriptional level & post-translational level.

Question 10

What is a promotor?

Answer 10

A region of DNA that initiates transcription (upstream) of a particular gene. Where RNA polymerase binds to initiate transcription.

Question 11

What is a transcription factor?

Answer 11

A protein that binds to DNA to activate or repress transcription (turn genes on or off).

Question 12

What determines whether a transcription factor can bind to DNA or not?

Answer 12

Shape – sometimes this can be altered by the binding of some molecules e.g. certain hormones & sugars.

Question 13

Can transcription factors be controlled?

Answer 13

Yes – by the amount of certain molecules in an environment affecting transcription factor binding.

Question 14

What are the functions of transcription factors (TF) in eukaryotic cells?

Answer 14

TF regulate gene expression - by either activating or inhibiting the binding of RNA polymerase to promoter region.
TF make sure that only certain genes in specific cell types are expressed.
TF play a role in cell differentiation & help to regulate the cell cycle and cell division.

Question 15

What is an operon?

Answer 15

A group of genes that function as a single transcription unit.

Question 16

What is the lac operon composed of?

Answer 16

A promoter, an operator and 3 structural genes, lacZ, lacy & lacA (produce proteins that help bacteria digest lactose).

Question 17

In the presence of glucose (absence of lactose):
What binds to the operator?
What binds to the promoter?
Does transcription of structural genes, lacZ, Y & A occur?

Answer 17

Lac repressor protein (a transcription factor).
Nothing – RNA polymerase is blocked from binding due to the repressor protein being bound to the operator.
No.

Question 18

In the presence of lactose:
Where does lactose bind & the effect?
What binds to the promoter?
What enzymes are produced as a result of transcription of lacZ, Y & A?

Answer 18

Lactose binds to the Lac repressor protein, changing its shape so can no longer bind to operator site.
RNA polymerase can now bind & begin transcription of structural genes.
β-galactosidase & lactose permease.

Question 19

What is splicing?

Answer 19

Where introns are removed from primary mRNA to make mature mRNA.

Question 20

Why is the length of the section of DNA that codes for a protein longer than the corresponding mRNA used for translation?

Answer 20

DNA contains introns – non-coding bases which are spliced out of primary mRNA in transcription to form mature mRNA.

Question 21

Why is cAMP needed by some proteins to function?

Answer 21

Some proteins are not functional straight after they have been synthesised – they need to be activated by cAMP.

Question 22

How does cAMP activate protein kinase A (PKA)?

Answer 22

PKA is an enzyme mode of 4 subunits & is inactive in this state. When cAMP binds, it causes a change in enzymes 3D structure, releasing the active subunits - PKA is now active.

Question 23

How is the production of cAMP triggered inside the cell?

Answer 23

Signalling molecule (often a hormone) binds to receptor on cell surface as complementary in shape. Receptor activates G protein. G protein activates adenyl cyclase. Activated adenyl cyclase catalyses the production of large amounts of cAMP from ATP within the cell. This results in the amplification of the first message by the secondary messenger. cAMP activates a cascade (a chain of reactions) of enzyme reactions. One signalling molecule produces many cAMP molecules, resulting in the amplification of the original signalling message.

Question 24

What are homeotic genes?

Answer 24

A group of genes that control the formation of anatomical structures in animals, plants & fungi.

Question 25

What is meant by a homeobox gene?

Answer 25

A regulatory (homeotic) gene containing a 180 base pair homeobox sequence. The homeobox codes for the homeodomain (60 amino acids) on the protein. The gene product acts as a transcription factor & binds to DNA to switch genes on or off to control body plan development.

Question 26

What are hox genes?

Answer 26

A group (cluster) of genes that control body plan development in animals (only) from anterior to posterior.

Question 27

What do the homeobox genes do?

Answer 27

Gene product binds to DNA at the start of developmental genes. This activates or represses transcription (acting as a transcription factor). This controls the production of proteins involved in the development of the body plan.

Question 28

What would be the likely result of a mutation in the homeobox gene?

Answer 28

These genes are very important - a mutation would alter body plan & is likely to be lethal.

Question 29

Name two processes that hox gene products can control.

Answer 29

Mitosis and apoptosis.

Question 30

What is meant by spatial and temporal colinearity?

Answer 30

The arrangement of hox genes on the chromosome is in the same order as they appear along the body from anterior to posterior (spatial). The genes also become active in this order (temporal).

Question 31

Why should the rate of apoptosis in an adult equal the rate of mitosis?

Answer 31

Adults are not growing.

Question 32

What happens if rate of apoptosis and mitosis are not balanced?

Answer 32

Formation of a tumour (too much mitosis).

Degeneration (too much apoptosis).