6.1.1 Cellular control

Question 1

Define mutation

Answer 1

A change in a DNA base sequence

Question 2

What can cause mutations?

Answer 2

• Mutations can occur spontaneously if DNA is misread during replication.
• Mutagens can increase the rate of mutation (e.g. ultraviolet/ionising radiation, chemicals and viruses).

Question 3

What are the three types of mutation?

Answer 3

Substitution
Deletion
Insertion
(Of one or more nucleotides)

Question 4

Describe substitution

Answer 4

One base is replaced by another

Question 5

Describe deletion

Answer 5

One base is removed from the sequence

Question 6

Describe insertion

Answer 6

One base is added to the sequence

Question 7

Effect of mutations

Answer 7

May change the amino acid sequence coded for by the gene. This results in a different polypeptide and a different tertiary structure.

Question 8

Why don’t all mutations result in a change to the amino acid sequence of a protein?

Answer 8

The genetic code is degenerate. Many amino acids are coded for by more than one triplet.

Question 9

Why are deletions and insertions (=indel mutations) much more likely to cause a change in the amino acid sequence of a protein?

Answer 9

They cause a frameshift.

Question 10

Describe frameshift

Answer 10

The entire sequence shifts by one base. This changes all triplets after deletion/insertion so changes the amino acid sequence.

Question 11

What are transcription factors?

Answer 11

Proteins that bind to specific DNA sequences to initiate the transcription of genes into mRNA.

Question 12

What happens when a gene is ‘switched off’?

Answer 12

Transcription factors cannot bind to DNA. This prevents the transcription process.

Question 13

What modifications can promote transcription in eukaryotes? Why?

Answer 13

• Acetylation and phosphorylation
• Reduce the positive charge on histones DNA more loosely coiled.

Question 14

What modifications can inhibit transcription in eukaryotes? Why?

Answer 14

• Methylation
• Increases hydrophobic interactions of histones making DNA more tightly coiled.

Question 15

What two forms can chromatin exist in?

Answer 15

Heterochromatin and Euchromatin

Question 16

Describe heterochromatin

Answer 16

Densely packed making it difficult for RNA polymerase to access genes, reducing transcription

Question 17

Describe euchromatin

Answer 17

Loosely packed, facilitating RNA polymerase access to genes increasing transcription

Question 18

What is chromatin?

Answer 18

Negatively charged DNA wraps around positively charged histones to produce chromatin.
(Can be remodelled for gene expression)

Question 19

What is an operon?

Answer 19

A cluster of genes controlled by a single promoter allowing for coordinated expression

Question 20

Key components of operons

Answer 20

Regulatory gene(s)
Promoter region
Operator region
Structural gene(s)

Question 21

Describe regulatory gene(s)

Answer 21

Code for proteins that regulate the expression of structural genes.

Question 22

Describe promoter region

Answer 22

Site where RNA polymerase binds to initiate transcription

Question 23

Describe operator region

Answer 23

Sequence where regulatory proteins can bind

Question 24

Describe structural genes

Answer 24

Genes that code for proteins (e.g. enzymes)

Question 25

Name and describe the structural genes in lac operon

Answer 25

lacZ - codes for B-galactosidase, breaks down lactose to glucose and galactose lacY - codes for lactose permease, transports lactose into the cell lacA - codes for transacetylase, modifies lactose/its by products

Question 26

Name and describe the regulatory gene in lac operons

Answer 26

lacI Codes for a repressor protein which can inhibit and control lac operons activity

Question 27

Describe the steps for how lac operons functions when lactose is absent

Answer 27

1. Repressor protein binds to operator region 2. RNA polymerase blocked from the promoter region 3. RNA polymerase cannot transcribe structural genes 4. Enzymes for lactose metabolism aren't produced

Question 28

Describe how lac operons functions when lactose is present

Answer 28

1. Lactose binds to the repressor protein 2. Repressor protein changes shape and is released from the operator region 3. RNA polymerase can bind to the promoter region and initiate transcription 4. RNA polymerase transcribes structural genes leading to production of enzymes necessary for lactose metabolism

Question 29

How does the presence of glucose indirectly inhibit the lac operon?

Answer 29

Via a signalling molecule, cyclic AMP (cAMP)

Question 30

Describe what happens when only lactose is present

Answer 30

1. cAMP levels increase and cAMP binds to the cAMP receptor protein (CRP) 2. The CRP-cAMP complex upregulates the transcription of the lac operon 3. Lactose metabolism is optimised.

Question 31

Describe what happens when both glucose and lactose are present

Answer 31

1.Glucose reduces cAMP levels 2. The CRP-cAMP complex cannot form 3. lac operon transcription is downregulated 4. Lactose metabolism enzymes not produced

Question 32

What are introns and exons?

Answer 32

Introns - non coding regions of DNA Exons - Coding regions of DNA

Question 33

How can pre-mRNA be processed (post transcriptional control)?

Answer 33

- Addition of a 5' cap: stabilises mRNA, delays degradation, assists in ribosomes binding - Addition of a 3' poly-A tail: stabilises mRNA, delays degradation - Splicing: Introns removed and exons joined together providing protein sequence

Question 34

How is gene expression controlled at the post translational level?

Answer 34

Proteins are activated by cAMP. e.g. protein kinase (adds phosphate groups to proteins by phosphorylation)

Question 35

Define body plan

Answer 35

Basic structured arrangement of an organism's parts (determined by genetic and developmental factors)

Question 36

Define homeobox genes

Answer 36

A group of regulatory genes with a conserved DNA sequence that guides development of body plans

Question 37

Define Homeobox sequence

Answer 37

Highly conserved and similar DNA sequence (in plants, animals and fungi) crucial for development of body plan

Question 38

Define Hox genes

Answer 38

Subset of homeobox genes (in animals) containing homeobox sequence essential for correct positioning of body parts.

Question 39

How do hox genes control development?

Answer 39

- Homeobox sequence codes for the homeodomain (part of a protein that binds to DNA) - Homeodomain acts as a transcription factor so It binds to DNA, switching genes on/off - This modifies transcription of proteins necessary for development of body plans

Question 40

Why are homeobox genes so conserved?

Answer 40

- A mutation would have large effects by altering body plan - Many other genes would also be affected by a mutation in a homeobox gene - Mutations are likely to be lethal and selected against

Question 41

What can genes that regulate mitosis/ the cell cycle and apoptosis respond to?

Answer 41

Internal and external stimuli

Question 42

Importance of mitosis and apoptosis in body plan development

Answer 42

Refine body morphology (alongside cell differentiation)