Ocr A2 Biology - Regulating Gene Expression (Part 1)

Front 1

Genes can be switched...

Back 1

...on or off.

Front 2

All the cells in an organism carry...

Back 2

...the same genes (DNA) but the structure and function of different cells varies.

Front 3

How do all cells carry the same DNA but have different structures and functions?

Back 3

• This is because not all genes in a cell are expressed (transcribed and used to make a functional protein) - they are selectively switched on or off.
  
• Because cells show different gene expression, different proteins are made and these proteins modify the cell - they determine cell structure and control cell processes (including the expression of more genes, which produce more proteins).

Front 4

Name the three levels gene expression can be controlled at.

Back 4

• Transcriptional level
• Post-transcriptional level
• Post-translational level

Front 5

[Transcriptional level] Gene expression can be controlled at the transcriptional level by...

Back 5

...altering the rate of transcription of genes. E.g. increased transcription produces more mRNA, which can be used to make more protein.

Front 6

[Transcriptional level] What controls the rate of transcription in a cell?

Back 6

• Transcription factors.
• These are proteins that bind to the DNA and switch genes on or off by changing the rate of transcription.

Front 7

[Transcriptional level] What are the names for transcription factors and increase or decrease the rate of transcription?

Back 7

• Transcription factors that increase the rate of transcription are called activators.
• Transcription factors that decrease the rate of transcription are called repressors.

Front 8

[Transcriptional level] What factor affect whether a transcription factor can bind to DNA, what affects the factor and what does this mean?

Back 8

• The shape of a transcription factor affects whether it can bind to DNA.
• This shape can be altered by the binding of a molecule e.g. hormones or sugars.
• This means the amount of a certain molecule in an environment can control the synthesis of some proteins by affect transcription factor binding.

Front 9

[Transcriptional level] State where transcription factors bind in eukaryotes and prokaryotes.

Back 9

• In eukaryotes, transcription factors bind to specific DNA sites near the start of their target genes - the genes they want to control the expression of.
• In prokaryotes, the bind to operons.

Front 10

[Transcriptional level] What is an operon?

Back 10

An operon is a section of DNA that contains a cluster of structural genes, that are transcribed together, as well as control elements and sometimes a regulatory gene.

Front 11

[Transcriptional level] What are structural genes, control elements and regulatory genes?

Back 11

• Structural genes code for useful proteins, such as enzymes.
• Control elements include a promoter (a DNA sequence located before the structural genes that RNA polymerase binds to) and an operator (a DNA sequence that transcription factors bind to).
• The regulatory gene codes for an activator or repressor.

Front 12

[Transcriptional level] State and explain an example of an operon.

Back 12

• The lac operon in E. coli
• E. coli is a bacterium that respires glucose, but it can use lactose if glucose is not available.
• The genes that produce the enzymes needed to respire lactose are found in the lac operon.
• The lac operon has three structural genes - lazZ, lacY and lacA, which produce proteins that help the bacteria digest lactose (including B-galactosidase and lactose permease).

Front 13

[Transcriptional level] What occurs at the lac operon when there is no lactose present?

Back 13

• The regulatory gene (lacl) produces the lac repressor, which is a transcription factor that binds to the operator site.
• This blocks transcription as RNA polymerase cannot bind to the promoter.

Front 14

[Transcriptional level] What occurs at the lac operon when there is lactose present?

Back 14

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

Front 15

[Post-transcriptional level] mRNA is ......... at a post-transcriptional level.

Back 15

edited

Front 16

[Post-transcriptional level] Genes in eukaryotic DNA contain sections that don't...

Back 16

...code for amino acids.

Front 17

[Post-transcriptional level] What are the sections that don't code for amino acids called? What are the sections that do code for amino acids called?

Back 17

• Sections that don't code for amino acids are called introns.
  
• Sections that do code for amino acids are called exons.

Front 18

[Post-transcriptional level] What occurs during transcription in terms of introns and exons and what is formed?

Back 18

• During transcription, both introns and exons are copied into mRNA.
• mRNA strands that contain both introns and exons are called primary mRNA strands (or pre-mRNA).

Front 19

[Post-transcriptional level] ........ are removed from pre-mRNA strands in a process called ........ - ....... are removed and ........ joined, forming ........ ........ strands. This takes place in the ......... .

Back 19

1. Introns

2. splicing

3. introns

4. exons

5. mature mRNA

6. nucleus

Front 20

[Post-transcriptional level] What occurs after splicing?

Back 20

The mature mRNA leaves the nucleus for the next stage of protein synthesis.

Front 21

[Post-translational level] .......... activates some .......... at the post-translational level.

Back 21

1. cAMP

2. proteins

Front 22

[Post-translational level] What is cAMP?

Back 22

• cAMP is a secondary messenger.
• It relays the message from the control molecule e.g. the hormone, to the inside of the cell.

Front 23

[Post-translational level] Some proteins are not ........... after they have been synthesised - they need to be .......... to work.

Back 23

1. functional

2. activated

Front 24

[Post-translational level] Protein activation is controlled by...

Back 24

...molecules e.g. hormones and sugars

Front 25

[Post-translational level] How do the molecules that activate protein activation work?

Back 25

• They bind to cell membranes and trigger the production of cyclic AMP (cAMP) inside a cell.
• cAMP then activates proteins inside the cell by altering their 3D structure.

Front 26

[Post-translational level] How does changing the 3D shape of a protein activate it?

Back 26

Altering the 3D structure can change the shape of the active site of an enzyme, making it become more or less active.

Front 27

[Post-translational level] Give and explain an example of a protein being activated by cAMP.

Back 27

• cAMP activates protein kinase A (PKA).
• When cAMP is not bound to the protein, the four units are bound together and are inactive.
• When cAMP binds, it causes a change in the 3D shape of the enzyme, releasing the activate subunits - PKA is now active.