6.1: Cellular Control

Question 1

What is a gene?

Answer 1

A length of DNA that codes for a polypeptide or a length of RNA that is involved in regulating gene expression

Question 2

What is a mutation?

Answer 2

Random change in DNA base sequence caused by mutagens

Question 3

What is a point mutation?

Answer 3

One base pair replaces another

Question 4

What are the three types of point mutations?

Answer 4

• Silent mutation
• Misense mutation
• Nonsene mutation

Question 5

What is a silent mutation?

Answer 5

One of the bases change
No change to polypeptide and it has no effect due to the degenerate code of DNA
This means it produces the same amino acid

Question 6

What is a misense mutation?

Answer 6

• Change in amino acid sequence
• Changes the primary, secondary and tertiary structure
• Changes the polypeptide
• Degenerate code has failed and cannot carry out usual function

Question 7

What is a nonsense mutation?

Answer 7

A codon changes to a STOP codon
Results in a truncated protein that will not function

Question 8

What is an indel mutation?

Answer 8

One or more nucleotides are inserted/deleted from a length of DNA

Question 9

What happens in an indel mutation?

Answer 9

A base is inserted or deleted and the base triplets are altered
A frameshift occurs
Due to the bases being read as non overlapping
Primary and tertiary structure is altered
Protein cannot carry out its normal function

Question 10

What is a beneficial mutation?

Answer 10

Helped to drive evolution through natural selection

Question 11

What are two examples of benefical mutations?

Answer 11

Mutation of blue eyes:

• Helps people see bettwe in less bright light in temperate zones
• Dangerous in areas where sunlight intensity is higher - lack of iris pigmentation could lead to lens cataracts

Melanin:

• High concentration of melanin protected early humans in Africa from sunburn and skin cancer
• In temperate conditions paler skin would be an advantage enabling vitamin D to be made with a lower intensity of sunlight - protects form heart disease and cancer

Question 12

What are neutral mutations?

Answer 12

Neither harmful or benefical

Question 13

What are two examples of neutral mutations?

Answer 13

• Inability to smell different flowers
• Differently shaped ear lobes

Question 14

What happens in glucose is absent in E. coli and lactose is present?

Answer 14

E. coli normally metabolises glucose as a respiratiory substrate
When it is not present and lactose is present it induces the production of two enzymes:

• Lactose permease
• Beta galactosidase

Question 15

What does lactose permease do?

Answer 15

Allows lactose to enter the bacterial cell

Question 16

What does beta galactosidase?

Answer 16

Hydrolyses lasctose to glucose and galactose

Question 17

What is the lac operon and what does it consist of?

Answer 17

Where gene expression is regulated in E. coli
Consists of:

• Regulatory gene - codes for repressor protein
• Promoter region
• Lac O - length of DNA containing an operator region
• Structural genes lac Z and Lac Y - code for beta galactosidase and lactose permease

Question 18

What happens when lactose is not present?

4 steps

Answer 18

1. Regulatory gene is expressed and produces repressor protein
2. This binds to operator region, preventing RNA polymerase from binding to the promotor region
3. Prevents lac Z and lac Y from being transcribed
4. Enzymes for lactose metabolism is not made

Question 19

What happens when lactose is present?

4 steps

Answer 19

1. Lactose binds to repressor gene
2. Alters the shape of the repressor protein and prevents it from binding to the operator
3. RNA polymerase can bind to the promotor region and start transcribing the structural genes into mRNA
4. This wil then be translated into the two enzymes

Question 20

What are transcription factors?

Answer 20

A protein or region of short non coding RNA that can combine with a specific site on a length of DNA and inhibit/activate transcription of the gene e.g repressor protein

Question 21

What are introns?

Answer 21

Non-coding regions of DNA

Question 22

What are exons?

Answer 22

The coding o expressed region of DNA

Question 23

What is an example of post transcriptional gene regulation?

Answer 23

Alternative splicing

Question 24

What happens in alternative splicing?

4 steps

Answer 24

1. Both introns and exons are transcribed (primary mRNA)
2. Primary mRNA is edited and RNA introns are removed
3. The remaining mRNA exons are joined together
4. Endonuclease enzyme may be involved in the editing and splicing process

Question 25

What is an example of post translational level of gene regulation?

Answer 25

Enzyme phosphorylation

Question 26

What is enzyme phosphorylation? | 8 steps

Answer 26

1. A signalling molecule binds to a receptor on the plasma membrane of the target cell 2. Activated a transmembrane protein which then activates a G protein 3. Activated G protein activated adenyl cyclase enzymes 4. Activated adenyl cyclase enzymes catalyse the formation of many molecules of cAMP form ATP 5. cAMP actiavtes PKA (protein kinase A) 6. Activated PKA catalyses the phosphorylation of various proteins, hydrolysing ATP in the process. This phosphorylation activates many enzymes in the cytoplasm 7. PKA may phosphoylate another protein (cAMP, CREB response element binding) 8. This then enters the nucleus and acts as a transcription factor to regulate transcription

Question 27

What are hox genes | also referred to as homeobox genes in plant, animal and fungi kingdoms

Answer 27

Control the development of body plans They code for polypeptides that act as transcription factors at different stages throughout development

Question 28

What is colinearity?

Answer 28

Sequence of genes on the chromosome = sequence of body parts (and time of expression)

Question 29

What are homeobox sequences/regions?

Answer 29

Hox genes contain a homeobox sequence * Sequence = 180 base pairs found within genes * Involved in regulating patterns of anatomical development in animals, fungi and plants * Highly conserved in plants, animals and fungi

Question 30

What are homeobox proteins?

Answer 30

Act as transcription factors Can change: * Rate of cell cycle * Apoptosis

Question 30

What are homeodomains?

Answer 30

Coded by homeobox regions * HTH (helix turn helix) - good at binding to DNA * A 60 amino acid sequence forms a polypeptide involved in DNA binding

Question 31

What is apoptosis?

Answer 31

Programmed cell death e.g for development Can be triggered in response to other things: * Intracellular signals such as DNA damage - mitosis checkpoint has failed * External signals such as t killer cells

Question 32

What occurs in apoptosis? | 5 steps

Answer 32

1. Enzymes break down the cytoskeleton 2. The cytoplasm becomes dense with tightly packed organelles 3. The cell surface membrane changes and small protrusions called blebs form 4. Chromatin condenses, the nuclear envelope breaks and DNA breaks into fragments 5. The cell breaks into vesicles that are ingested by phagocytic cells, so that cell debrs does not damage any other cells or tissues

Question 33

How is apoptosis controlled?

Answer 33

Cell signalling molecules that may be released by cells when genes that are involved in regulating the cell cycle and apoptosis respond to internal cell stimuli and external stimuli such as stress For example: * Nitric oxide can induce apoptosis by making the inner mitochondrial membrane more permeable to hydrogen ions and dissipating the proton gradient * Proteins are released into the cytoplasm where they bind to apoptosis inhibitor proteins, allowing apoptosis to occur

Question 34

What happens in too little or to much apoptosis

Answer 34

Too little: Leads to formation of tumours Too much: Leads to cell loss and degeneration (Alzhiemer's)