Module 6 Section 1 - Cellular Control

Question 1

What levels is gene expression controlled at?

Answer 1

Transcriptional, post transcriptional, and post translational

Question 2

What happens at transcriptional level control?

Answer 2

The rate of transcription of genes is altered eg increased transcription produces more mRNA, which can be used to make more protein. This is controlled by transcription factors. The shape of the transcription factor determines whether it can bind to DNA or not, and can sometimes be altered by the binding of some molecules eg hormones.

Question 3

What are transcription factors?

Answer 3

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

Question 4

What are two types of transcription factors?

Answer 4

Activators- factors that start transcription
Repressors - factors that stop transcription

Question 5

What do transcription factors bind to in eukaryotes and prokaryotes?

Answer 5

Eukaryotes - transcription factors bind to specific dna sites near the start of their target genes.

Prokaryotes- transcription factors bind to operons

Question 6

What is an operon?

Answer 6

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

Structural genes code for useful proteins, such as enzymes. The 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 a repressor.

Question 7

How does the lac operon work in e.coli?

Answer 7

E.coli respires glucose, but when glucose runs out, uses lactose. The genes that produce enzymes needed to respire lactose are found on a lac operon. The lac operon has 3 structural genes (lacZ, lacY and lacA) which produce proteins that help bacteria digest lactose.

The regulatory gene produces the lac repressor, which is a transcription factor that binds to the operator site when there’s no lactose present. This blocks transcription becahxe RNA polymerase can’t bind to the promoter.

When lactose is present, it binds to the repressor, changing the respressor’s shape so that it can no longer bind to the operator site. RNA polymerase can now begin transcription of structural genes.

Question 8

How is gene expression controlled at the post-transcriptional level?

Answer 8

After transcription, mRNA is eukaryotic cells is edited because there are genes in the eukaryotic dna that don’t code for amino acids. This sections of DNA are called introns. All sections that code for amino acids are called exons.

During transcription the introns and exons are both copied to mRNA. mRNA strands containing introns ans exons are called primary mRNA transcripts. Introns are removed by splicing and exons are joined together to form mature mRNA strands. This takes place in the nucleus.

Question 9

How is gene expression controlled at the post-translational level?

Answer 9

Some proteins are not functional straight after being synthesised and need to be activated. This is controlled by molecules eg certain molecules.

Some molecules that control protein activation work by binding to cell membranes and triggering the production of cyclic AMP inside the cell. cAMP then activates proteins inside the cell by altering their three dimensional structure. Changing the 3D shape can change the active site of an enzyme, making it less or more active.

Question 10

How does the activation of protein kinase A (PKA) by cAMP?

Answer 10

PKA is an enzyme made of 4 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 11

What is a body plan?

Answer 11

A body plan is the general structure of an organism

Question 12

What are hox genes?

Answer 12

The proteins that control body plan development are coded for by genes called hox genes.

Question 13

What are homeobox sequences?

Answer 13

These are regions of hox genes that code for a protein called the homeodomain. The homeodomain binds to specific sites on DNA, enabling the protein to work as a transcription factor.

Question 14

What is apoptosis?

Answer 14

Cell death

Question 15

What are the main steps of apoptosis?

Answer 15

1) enzymes inside the cell break down cell components eg proteins

2) the cell shrinks and begins to fragment

3) phagocytes engulf and digest the cell fragments.

Question 16

What is the role of mitosis and apoptosis in development?

Answer 16

Mitosis and differentiation create the bulk of the body parts and then apoptosis refines the parts by removing unwanted structures.

During development genes that control mitosis and genes that control apoptosis are switched on and off in appropriate cells.

Question 17

Stimuli that mitosis and apoptosis respond to?

Answer 17

Both internal and external stimuli

Question 18

What is a mutation?

Answer 18

Any change to the base sequence of DNA

Question 19

What are the types of mutation?

Answer 19

1) substitution - one or more bases are swapped for another base
2) deletion- one or more bases are removed
3) insertion- one or more bases are added

Question 20

What is a framshift mutation?

Answer 20

Adding or deleting bases changes the number of bases present, causing a shift in all the base triplets that follow. The earlier the framshift mutation, the more amino acids are affected.

Question 21

Why are there mutations that don’t affect an organism?

Answer 21

1) the mutation changes a base in a triplet, but the amino acid that the triplet codes for doesn’t change. This can happen as more than one triplet codes for an amino acid,

2) the mutation produces triplet that codes for a different amino acid buy the amino acid is chemically similar to the original so it functions like the original amino acid.

3) the mutation triplet codes for an amino acid not involved with the proteins function

Question 22

How do some mutations affect organisms?

Answer 22

1) beneficial- it can lead to an advantageous effect for an organism and increases its chance of survival eg antibiotic resistance. This is passed through generations by natural selection.

2) negative- can decrease chance of survival eg cystic fibrosis is caused by the deletion of three bases in a gene that codes for the CFTR protein. This leads to excess mucus production and affects the lungs.