chapter 19 p2

Question 1

housekeeping genes.

Answer 1

the genes that code for enzymes which are necessary for reactions present in metabolic pathways like respiration and are constantly required

Question 2

Protein-based hormones

Answer 2

(required for the growth and development of an organism or enzymes) are only required by certain cells at certain times to carry out a short-lived response - They are coded for by tissue-specific genes.

Question 3

The entire genome of an organism is present in

Answer 3

• every prokaryotic cell, or eukaryotic cell that contains a nucleus.
• This includes genes not required by that cell so the expression of genes and the rate of synthesis of protein products like enzymes and hormones has to be regulated.
• Genes can be turned on or off, and the rate of product synthesis increased or decreased depending on demand.

Question 4

Bacteria are able to

Answer 4

respond to changes in the environment because of gene regulation.
Expressing genes only when the products are needed also prevents vital resources being wasted.
Gene regulation is fundamentally the same in both prokaryotes and eukaryotes.
However, the stimuli that cause changes in gene expression and the responses produced are more complex in eukaryotes.

Question 5

Multicellular organisms not only have to

Answer 5

respond to changes in the external environment but also the internal environment.
Gene regulation is required for cells to specialise and work in a coordinated way.

Question 6

There are a number of different ways in which genes are regulated, categorised by the level at which they operate:

Answer 6

Transcriptional
Post-transcriptional
Translational
Post-translational

Question 7

Transcriptional

Answer 7

genes can be turned on or off

Question 8

Post-transcriptional

Answer 8

mRNA can be modified which regulates translation and the types of proteins produced

Question 9

Translational

Answer 9

translation can be stopped or started

Question 10

Post-translational

Answer 10

• proteins can be modified after synthesis which changes their functions.

Question 11

Transcriptional control:
There are a number of mechanisms that can affect the transcription of genes:

Answer 11

Chromatin remodelling
Histone modification
Lac operon
Role of cyclic AMP

Question 12

Chromatin remodelling
p1

Answer 12

• DNA is a very long molecule and has to be wound around proteins called histones in eukaryotic cells, in order to be packed into the nucleus of a cell.
• The resulting DNA/protein complex is called a chromatin.
• Heterochromatin is tightly wound DNA causing chromosomes to be visible during cell division whereas euchromatin is loosely wound DNA present during interphase.
• The transcription of genes is not possible when DNA is tightly wound because RNA polymerase cannot access the genes.

Question 13

Chromatin remodelling
p2

Answer 13

• The genes in euchromatin, however, can be freely transcribed.
• Protein synthesis does not occur during cell division but during interphase between cell divisions.
• This is a simple form of regulation that ensures the proteins necessary for cell division are synthesised in time.
• It also prevents the complex and energy-consuming process of protein synthesis from occurring when cells are actually dividing.

Question 14

Histone modification
p1

Answer 14

• DNA coils around histones because they are positively charged and DNA is negatively charged.
• Histones can be modified to increase or decrease the degree of packing (or condensation).
• The addition of acetyl groups (acetylation) or phosphate groups (phosphorylation) reduces the positive charge on the histones (making them more negative) and this causes DNA to coil less tightly, allowing certain genes to be transcribed.

Question 15

Histone modification
p2

Answer 15

The addition of methyl groups (methylation) makes the histones more hydrophobic so they bind more tightly to each other causing DNA to coil more tightly and preventing transcription of genes.
Epigenetics is a term that is increasingly used to describe this control of gene expression by the modification of DNA.
It is sometimes used to include all of the different ways in which gene expression is regulated.

Question 16

Lac operon
p1

Answer 16

• An operon is a group of genes that are under the control of the same regulatory mechanism and are expressed at the same time.
• Operons are far more common in prokaryotes than eukaryotes owing to the smaller and simpler structure of their genomes.
• They are also a very efficient way of saving resources because if certain gene products are not needed, then all of the genes involved in their production can be switched off.
• Glucose is easier to metabolise and is the preferred respiratory substrate of Escherichia coli and many other bacteria.

Question 17

Lac operon
p2

Answer 17

• If glucose is in short supply, lactose can be used as a respiratory substrate.
• Different enzymes are needed to metabolise lactose.
• The lac operon is a group of three genes, lacZ, lacY, and lacA, involved in the metabolism of lactose.
• They are structural genes as they code for three enzymes (B-galactosidase, lactose permease, and transacetylase) and they are transcribed onto a single long molecule of mRNA.
• A regulatory gene, lacl, is located near to the operon and codes for a repressor protein that prevents the transcription of the structural genes in the absence of lactose

Question 18

Lac operon
p3

Answer 18

• The repressor protein is constantly produced and binds to an area called the operator, which is also close to the structural genes.
• The binding of this protein prevents RNA polymerase binding to DNA and beginning transcription - This is called down regulation.
• The section of DNA that is the binding site for RNA polymerase is called the promoter.
• When lactose is present, it binds to the repressor protein causing it to change shape so it can no longer bind to the operator.
• As a result RNA polymerase can bind to the promoter, the three structural genes are transcribed, and the enzymes are synthesised.

Question 19

lac operon diagram

Answer 19

Question 20

Role of cyclic AMP

Answer 20

• The binding of RNA polymerase still only results in a relatively slow rate of transcription that needs to be increased or up-regulated to produce the required quantity of enzymes to metabolise lactose efficiently.
• This is achieved by the binding of another protein, cAMP receptor protein (CRP), that is only possible when CRP is bound to CAMP (a secondary messenger)
• The transport of glucose into an E. coli cell decreases the levels of CAMP, reducing the transcription of the genes responsible for the metabolism of lactose.
• If both glucose and lactose are present then it will still be glucose, the preferred respiratory substrate, that is metabolised.

Question 21

Post-transcriptional/pre-translational control:

Answer 21

RNA processing:
RNA editing:

Question 22

RNA processing:

Answer 22

• The product of transcription is a precursor molecule, pre-mRNA.
• This is modified forming mature mRNA before it can bind to a ribosome and code for the synthesis of the required protein.
• A cap (a modified nucleotide) is added to the 5’ end and a tail (a long chain of adenine nucleotides) is added to the 3’ end.
• These both help to stabilise mRNA and delay degradation in the cytoplasm.
• The cap also aids binding of mRNA to ribosomes.
• Splicing also occurs where the RNA is cut at specific points - the introns (non-coding DNA) are removed and the exons (coding DNA) are joined together. Both processes occur within the nucleus.

Question 23

diagram RNA processing:

Answer 23

Question 24

RNA editing:

Answer 24

The nucleotide sequence of some mRNA molecules can also be changed through base addition, deletion, or substitution.
These have the same effect as point mutations and result in the synthesis of different proteins which may have different functions.
This increases the range of proteins that can be produced from a single mRNA molecule or gene.