Cellular control

Question 1

What are transcription factors?

Answer 1

Proteins that bind to DNA and switch genes on or off by increasing or decreasing the rate of transcription. Factors that increase the rate are called activators and those that decrease the rate are called repressors.

Question 2

What are the different levels of gene control?

Answer 2

Transcriptional- lac operon and transcriptional factors in eukaryotes
Post-transcriptional level- the editing of primary mRNA and the removal or introns to produce mature mRNA
Post-translational level- the activation of proteins by cyclic AMP

Question 3

What is an operon?

Answer 3

A section of DNA that contains a cluster of structural genes (code for useful proteins) that are transcribed together, as well as control elements and sometimes a regulatory gene (codes for an activator or repressor).

Question 4

What is E.coli?

Answer 4

A bacterium that respires glucose, but it can use lactose if glucose isn’t available.

Question 5

The lac operon in E.coli: Where are the genes that produce enzymes needed to respire lactose found?

Answer 5

On an operon called lac operon

Question 6

The lac operon in E.coli: What structural genes does the lac operon have?

Answer 6

lacZ, lacY, lacA, which produce proteins that help bacteria digest lactose

Question 7

The lac operon in E.coli: What happens when lactose is not present?

Answer 7

The regulatory gene (lacL) produces the lac repressor which is a transcription factor that binds to the operator site when no lactose is present.
This blocks transcription because RNA polymerase can’t bind to the promoter.

Question 8

The lac operon in E.coli: What happens when lactose is present?

Answer 8

Lactose binds to the repressor, changing the repressor’ shape so that it can no longer bind to the operator site.
RNA polymerase can now begin transcription of the structural genes.

Question 9

Post-transcription level: What are introns?

Answer 9

The sections of eukaryotic DNA that don’t code for amino acids.

Question 10

Post-transcription level: What are exons?

Answer 10

All the bits of DNA that do code for amino acids

Question 11

Post-transcription level: What happens to introns and exons during transcription?

Answer 11

Introns and exons are both copied into mRNA. mRNA strands containing introns and exons are called primary mRNA transcripts (or pre-mRNA).

Question 12

Post-transcription level: What happens to primary mRNA strands?

Answer 12

Introns are removed from primary mRNA strands by a process called splicing- introns are removed and exons joined, forming mature mRNA strands. This takes place in the nucleus.

Question 13

Post-transcription level: What happens to the mature mRNA?

Answer 13

Then leaves the nucleus for the next stage of protein synthesis (translation).

Question 14

Post-translation level: What do some proteins need after they have been synthesised?

Answer 14

Some proteins aren’t functional straight after they have been synthesised- they need to be activated to work (to become a functional protein).
Protein activation is controlled by molecules e.g. hormones and sugars.

Question 15

Post-translation level: How do some molecules work in protein activation?

Answer 15

Work by binding these molecules to cell membranes and triggering the production of cyclic AMP (cAMP) inside the cell.

Question 16

Post-translation level: What does cAMP do?

Answer 16

Activates proteins inside the cell by altering their three-dimensional (3D) structure.
For example, altering the 3D structure can change the activate site of an enzyme and make it more or less reactive.

Question 17

Post-translation level: How does cAMP activate protein kinase A (PKA)?

Answer 17

1. PKA is an enzyme made of 4 subunits.
2. When cAMP isn’t bound, the 4 units are bound together and are inactive.
3. When cAMP binds, it causes a change in the enzyme’s 3D structure, releasing the active subunits- PKA is now active.

Question 18

What is a body plan?

Answer 18

The general structure of an organism .

Question 19

What controls the development of a body plan?

Answer 19

Proteins- they help set up the basic body plan so everything is in the right place, e.g. the legs grows where the legs should grow.

Question 20

What are the proteins that control body plan development coded by?

Answer 20

Genes called Hox genes. e.g. two Hox gene clusters control the developement of the Drosophila body plan- one controls the development of the head and anterior thorax and the other controls the development of the posterior thorax and abdomen.

Question 21

What do Hox genes have?

Answer 21

Regions called homeobox sequences, which are high conserved- this means that these sequences have changed very little during the evolution of different organisms that posses these homeobox sequences.

Question 22

How do Hox genes control the development?

Answer 22

1. Homeobox sequences code for a part of a protein called the homeodomain.
2. The homeodomain binds to specific sites on DNA, enabling the protein to work as a transcription factor.
3. The proteins bind to DNA at the start of developmental genes, activating or repressing transcription and so altering the production of proteins involved in the development of the body plan.

Question 23

What is apoptosis?

Answer 23

When some cells die and break down as a normal part of development- this is a highly controlled process.

Question 24

What happens once apoptosis has been triggered?

Answer 24

The cell is broken down in a series of steps:
1) Enzymes inside the cell break down important cell components such as proteins in the cytoplasm and DNA in the nucleus.
2) As the cell’s contents are broken down it begins to shrink and breaks up into fragments.
3) The cell fragments are engulfed by phagocytes and digested.

Question 25

What does mitosis do?

Answer 25

Mitosis and the differentiation create the bulk of the body parts and then apoptosis refines the parts by removing unwanted structures. For example, as tadpoles develop into frogs, their tail cells are removed by apoptosis and when hands and feet first develop in humans they are connected- they’re only seperated when cells in the connecting tissue undergo apoptosis

Question 26

How can the genes that regulate apoptosis respond to internal stimuli?

Answer 26

An internal stimulus could be DNA damage. If DNA damage is detected in the cell cycle, this can result in the expression of genes which cause the cell cycle to be paused and can even trigger apoptosis.

Question 27

How can the genes that regulate apoptosis respond to external stimuli?

Answer 27

An external stimulus, such as stress caused by a lack of nutrient availability, could result in gene expression that prevents the cell cycle from undergoing mitosis. Gene expression which leads to apoptosis being triggered can also be caused by an external stimulus suck as attack by a pathogen.

Question 28

What is a mutation?

Answer 28

Any change to the base (nucleotide) sequence of DNA.

Question 29

Mutations: What is substitution?

Answer 29

One or more bases are swapped for another, e.g. ATG becomes ATT

Question 30

Mutations: What is deletion?

Answer 30

One or more bases are removed e.g. ATGCCT becomes ATCT

Question 31

Mutations: What is insertion?

Answer 31

One or more bases are added, e.g. ATG becomes ATGT

Question 32

What do mutations cause?

Answer 32

The order of DNA bases in a gene determines the order of amino acids in a particular protein. If a mutations occurs in a gene, the primary structure (amino acid chain) of the protein it codes for could be altered. This may change the final 3D shape of the protein so it doesn’t work properly, e.g. active sites in enzymes may not form properly, meaning substrates can’t bind to them.

Question 33

Why do some mutations have a neutral effect on a protein’s function?

Answer 33

• The mutation changes a base in te triplet, but the amino acid that the triplet codes for doesn’t change. This happens because some amino acids are coded for by more than one triplet.
• The mutation produces a triplet that codes for a different amino acid, but the amino acid is chemically simialr to the orginal so it functions like the orginal amino acid.
• The mutated triplet codes for an amino acid not involved with the protein’s function, e.g. one that’s located far away from the enzyme’s active site, so the protein works as normal.

Question 34

Why do some mutations have beneficial effects?

Answer 34

• Have an advantageous effect on an organism, i.e. they increase its chance of survival
• E.g. some bacterial enzymes break down certain antibiotics. Mutations in the genes that code for these enzymes could make them work on a wider range of antibiotics. This is beneficial to bacteria because antibiotic resistance can help them to survive.

Question 35

Why do some mutations have harmful effects?

Answer 35

• Have disadvantageous effects on an organism, i.e. they decrease the chance of survival
• E.g. cystic fibrosis (CF) can be caused by the deletion of three bases in the gene that codes for the CFTR (cystic fibrosis transmembrane conductance regulator) protein. The mutated CFTR protein fold incorrectly, so it’s broken down. This leads to excess mucus production, which affects the lungs of CF suffers.