Topic 8—A: Mutations and Gene expression- 7. Regulation of transcription and translation

Question 1

What is the gene responsible for synthesising mRNA from DNA?

Answer 1

• RNA polymerase

Question 2

What is transcription?

Answer 2

• its when a gene is copied from DNA into messenger RNA (mRNA)

Question 3

What do all the cells in an organism carry?

Answer 3

• the same genes (DNA) but the structure ans function of different cells varies

Question 4

Why does the structure and function of different cells varies ?

Answer 4

• because not all the genes in a cell are expressed (transcribed and used to make a protein)

Question 5

What happens when different genes are expressed?

Answer 5

• different proteins are made and these proteins modify the cell- they determine the cell structure and control cell processes (including the expression of more genes, which produce more proteins

Question 6

Transcription factors

Answer 6

• transcription of genes is controlled by these protein molecules

Question 7

In eukaryotes, where do transcription factors move from and to?

Answer 7

From the cytoplasm to the nucleus

Question 8

In the nucleus where do transcription factors bind to?

Answer 8

Specific DNA sites called promoters

Question 9

Promotors

Answer 9

• found near the start of their target genes (genes they control the expression of)

Question 10

Promotor region

Answer 10

• a DNA sequence that tells the enzyme RNA polymerase where to start producing mRNA

Question 11

How do transcription factors control expression?

Answer 11

• by controlling the rate of transcription

Question 12

Activator

Answer 12

• a transcription factor that increases the rate of transcription

Question 13

How do activators increase the rate of transcription?

Answer 13

• they help rna polymerase bind to the start of the target gene and activate transcription

Question 14

Repressor

Answer 14

A transcription factor that inhibits or decreases the rate of transcription

Question 15

How do depressors inhibit or decrease the rate of transcription?

Answer 15

• they bind to the start of the target gene preventing rna polymerase from binding, stopping transcription

Question 16

What other molecule can affect the expression of genes?

Answer 16

Oestrogen

Question 17

What hormone is oestrogen?

Answer 17

A steroid hormone

Question 18

How can oestrogen affect transcription?

Answer 18

• by binding to a transcription factor called an oestrogen receptor forming an oestroge-oestrogen receptor complex
• the complex moves from the cytoplasm to the nucleus where it binds to specific DNA sites near the start of the target gene
• the complex can act as an activator of transcription e.g. helping RNA polymerase bind to the start of the target gene

Question 19

In eukaryotes what is gene expression also affected by?

Answer 19

RNA interference (RNAi)

Question 20

RNAi

Answer 20

• It’s where small, double-stranded RNA molecules stop mRNA from target genes being translated into proteins

Question 21

What are the molecules involved in RNAi?

Answer 21

• siRNA (small interfering RNA)
• miRNA (microRNA)

Question 22

How does RNAi work? (SiRNA and miRNA in plants)

Answer 22

• once mRNA has been transcribed, it leaves the nucleus for the cytoplasm
• in the cytoplasm, double-stranded siRNA associates with several proteins and unwinds
• One of the resulting single strands of siRNA is selected and the other strand is degraded
• the single strand of siRNA then binds to the target mRNA
• the base sequence of the siRNA is complementary to the base sequence in sections of the target mRNA
• the proteins associated with the siRNA cut the mRNA into fragments- so it can no longer be translated
• the fragments then move into a processing body, which contains ‘tools’ to degrade them

Question 23

MiRNA (plants)

Answer 23

• similar process to siRNA
• Like siRNA the base sequence of plant miRNA is complementary to its target MRNA sequence and so binding results in the cutting up and degradation of the mRNA
• However its production in the cell is similar to that of mammalian miRNA

Question 24

miRNA in mammals

Answer 24

• in mammals, the miRNA isn’t usually fully complementary to the target mRNA
• This makes it less specific than siRNA and so it may target more than one mRNA molecule
• When miRNA is first transcribed, it exists as a long, folded strand
• it is processed into a double strand and then into two single strands by enzymes in the cytoplasm
• Like siRNA, one strand associates with proteins and binds to target mRNA in the cytoplasm
• Instead of the proteins associated with miRNA cutting mRNA into fragments, the miRNA-protein complex physically blocks the translation of the target mRNA
• The mRNA is then moved into a processing body where it can either be stored or degraded
• when its stored it can be returned and translated at another time