Module 6 Section 1 - Cellular Control

Question 1

Define mutation.

Answer 1

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

Question 2

What are the three types of gene mutations?

Answer 2

• SUBSTITUTION
• DELETION
• INSERTION

Question 3

What is a substitution mutation?

Answer 3

When one or more bases are swapped for another, e.g. ATGCCT becomes ATTCCT

Question 4

What is a deletion mutation?

Answer 4

When one or more bases are removed, e.g. ATGCCT becomes ATCT

Question 5

What is a insertion mutation?

Answer 5

When one or more bases are added, e.g. ATGCCT becomes ATGACCT

Question 6

How does a mutation cause a modification in a protein (3) ?

Answer 6

1. The order of DNA bases in a gene determines the order of amino acids in a particular protein.
2. If a mutation occurs in a gene, the primary structure of the protein it codes for could be altered.
3. This may change the tertiary structure of the protein so it doesn’t work properly, e.g. active sites denatured.

Question 7

Why can some mutations have a neutral effect on a protein’s function? Give 3 ways.

Answer 7

1. Although the base in a triplet changes, the amino acid that the triplet codes for doesn’t change. Some amino acids coded for by more than one triplet. (e.g. TAT and TAC = tyrosine)
2. The mutation produces a triplet that codes for a different amino acid, but the amino acid is CHEMICALLY SIMILAR to the original.
3. The mutated triplet codes for an amino acid NOT INVOLVED with the protein’s function.

Question 8

Give an example in which a mutation can have a beneficial effect?

Answer 8

e.g. some bacterial enzymes can be mutated so that it BREAKS DOWN a wide range of antibiotics, so causes bacteria to be resistant to these antibiotics and increases its chance of survival.

Question 9

Give an example in which a mutation can have a harmful effect?

Answer 9

e.g. cystic fibrosis is caused by a deletion of three bases in the gene that codes for CFTR which means that it beaks down and leads to excess mucus production.

Question 10

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

Answer 10

• TRANSCRIPTIONAL Level
• POST-TRANSCRIPTIONAL level
• POST-TRANSLATIONAL level

Question 11

What is gene expression controlled by at the transcriptional level?

Answer 11

Transcription Factors

Question 12

What are Transcription Factors?

Answer 12

Proteins that bind to DNA and switch genes on or off by increasing or decreasing the rate of transcription.

Question 13

What are Transcription Factors that increase the rate of transcription called? And vice versa?

Answer 13

Increases rate of transcription = ACTIVATORS

Decreases rate of transcription = REPRESSORS

Question 14

How can the shape of a transcription factor that binds to DNA be altered?
What does this mean in terms of the how the synthesis of some proteins are controlled?

Answer 14

By binding of some molecules, e.g. some hormones and sugars

This means the amount of certain molecules in an environment or a cell can control the synthesis of some proteins by affecting transcription factor binding.

Question 15

How do transcription factors perform in EUkaryotes?

Answer 15

They bind to specific DNA sites near the start of their TARGET genes - the genes they control the expression of.

Question 16

How do transcription factors perform in PROkaryotes?

Answer 16

They bind to OPERONS

Question 17

What are operons?

Answer 17

A section of DNA that contains a cluster of STRUCTURAL genes, as well as CONTROL elements,and sometimes a REGULATORY gene.

Question 18

What is the role of structural genes in an operon?

Answer 18

Codes for USEFUL proteins, e.g. enzymes

Question 19

What is the role of control elements in an operon?

Answer 19

Includes a PROMOTER ( DNA sequence located BEFORE the structural genes that RNA Polymerase binds to) and an OPERATOR (a DNA sequence that transcription factors bind to)

Question 20

What is the role of the regulatory gene in an operon?

Answer 20

Codes for an ACTIVATOR or a REPRESSOR.

Question 21

Describe how gene expression is regulated when lactose is NOT present in E. coli.

Answer 21

• the REGULATORY gene (LacI) produces the lac repressor which is a transcription factor that binds to the OPERATOR site when there’s NO lactose is present.
• This BLOCKS transcription because RNA Polymerase cannot bind to the PROMOTER site.

Question 22

Describe how gene expression is regulated when lactose IS present in E.Coli.

Answer 22

• Lactose binds to the REPRESSOR, changing the repressor’s shape so that it can no longer bind to the OPERATOR site.
• RNA Polymerase can then begin transcription of the structural genes (i.e. LacZ,LacY,LacA which produces the lactose-digesting enzymes (e.g. lactose permease))

Question 23

How is gene expression regulated at the POST-transcriptional level in eukaryotic DNA (4)?

Answer 23

• eukaryotic DNA caontain INTRONS which are sections that don’t code for amino acids. EXONS are sections which do code for amino acids.
• When mRNA is produced, both the introns and exons are copied. An mRNA strand that contain introns and exons are called ‘primary mRNA transcripts’ (pre-mRNA)
• Introns are then removed from pre-mRNA by SPLICING. Exons are joined which form MATURE mRNA strands. This all takes place in the nucleus via endonuclease enzymes.
• The mature mRNA then LEAVES the nucleus for translation.

Question 24

How is gene expression regulated at the post-translational level? (3)

Answer 24

• Some proteins that need to be activated work by binding to cell membranes and trigger the production of cyclic AMP (cAMP).
• cAMP then activates proteins inside the cell by altering its tertiary structure.
• This can then produce an effect, e.g. active site changing to become more/less active.

Question 25

Using the example of PKA, describe how gene expression is regulated at the post-translational level? (3)

Answer 25

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

Question 26

Why is cAMP a secondary messenger?

Answer 26

It relays the message from the control molecule, e.g. the hormone, to the inside of the cell.

Question 27

What are the genes that code for the proteins that control body plan development called?

Answer 27

Hox genes

Question 28

What do Hox genes contain?

Answer 28

A DNA sequence called Homeobox sequence (180 base pairs long)

Question 29

What does it mean when Homeobox sequences are ‘highly conserved’?

Answer 29

The sequences have changed very little during the evolution of different organisms that possess these homeobox sequences

Question 30

How do Hox genes control body plan development (3)?

Answer 30

1. Homeobox sequences code for a part of the 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 body plan development.

Question 31

Define apoptosis.

Answer 31

Programmed cell death

Question 32

Describe the series of steps that arise when apoptosis has been triggered (3)?

Answer 32

1. Enzymes inside the cell break down the cell components, e.g. proteins in the cytoplasm and DNA in the nucleus.
2. The cell begins to shrink and breaks up into fragments
3. The cell fragments are engulfed by phagocytes and digested

Question 33

What process other than apoptosis is crucial for controlling body plan development?

Answer 33

Mitosis - differentiates creates the bulk of the body parts and apoptosis refines the parts by removing unwanted structures.

Question 34

Give an example of an internal stimulus that the genes that regulate apoptosis/mitosis can respond to.

Answer 34

DNA damage - it’s detected during the cell cycle and this can result in the expression of genes which cause the cycle to be paused and can even trigger apoptosis.

Question 35

Give an example of an external stimulus that the genes that regulate apoptosis/mitosis can respond to.

Answer 35

Stress caused by a lack of nutrient availability - can result in gene expression that prevents cells from undergoing mitosis.
Gene expression which leads to apoptosis being triggered can also be caused by an external stimulus such as an attack by a pathogen.