6.1.1 Cellular control

Question 1

Explain how the functions of cells are controlled by the nucleus

Answer 1

The functions of cells (such as metabolism) are carried out by proteins

The function of proteins are determined by their tertiary structure

The tertiary structure is dependent on the primary structure

The primary structure is translated from the codons of the mRNA

The mRNA is transcribed from the alleles of a gene in the chromosomal DNA found in the nucleus.

Question 2

Summarise how mutations can affect the function of cells

Answer 2

1. A mutation is a change in the base sequence of DNA
2. During transcription, this can cause a change in the codons of mRNA
3. Changes in mRNA codons can result in a different primary structure
4. This can change the tertiary structure of the protein (3D shape)
5. A different tertiary structure can affect the function of a protein and therefore the functioning of a cell

Question 3

Identify the types of mutation that can occur in DNA

Answer 3

1. Substitution
2. Frameshift (insertion)
3. Frameshift (deletion)

Question 4

Explain the extent to which substitution mutations alter protein structure

Answer 4

1. Substitution is the replacement of one base with another
2. This alters one triplet in the DNA and one codon in the mRNA
3. This would change one amino acid in the polypeptide
4. However, due to the degenerate nature of genetic code
5. The new codon may still code for the same amino acid
6. This will cause no change to the protein structure, and so no effect on protein function

Question 5

Explain the extent to which insertion mutations alter protein structure

Answer 5

Insertions are the addition of bases into the original DNA sequence

This changes the triplet where it is added, but also the reading frame from that point onwards

This can alter every single triplet/codon after the insertion or even introduce a premature STOP codon

Therefore can have a very significant effect on protein structure and function

However, insertions of multiples of three do not change the reading frame

They will only add some amino acids to the protein structure

Question 6

Explain the extent to which deletion mutations alter protein structure

Answer 6

deletions are the removal of bases from the original DNA sequence

This changes the triplet where it is occurs, but also the reading frame from that point onwards

This can alter every single triplet/codon after the deletion or even introduce a premature STOP codon

Therefore can have a very significant effect on protein structure and function

However, deletions of multiples of three do not change the reading frame

They will only remove some amino acids from the protein structure

Question 7

Describe the range of effects that mutations can have on protein function

Answer 7

SOME mutations can have no effect on protein structure and function (SILENT, NEUTRAL)

For example substitutions that do not affect the primary structure of the protein (degenerate genetic code)

Most Mutations are DAMAGING

Single amino acid changes (MISSENSE), or frameshifts (NONSENSE)

Can change the protein structure so that it does not function

This can affect cell, tissue and even organ function

Rarely, mutations can be BENEFICIAL (evolution, yo!)

Changes to protein structure

May result in improved or new functionality (depending on the selection pressure)

Question 8

Explain what gene expression is

Answer 8

Gene expression is the term that describes synthesis of a protein from its gene

Genes are located on chromosomes in the DNA stored in the nucleus

For a gene to be expressed, it must first be transcribed (by RNA polymerase) to produce mRNA

The mRNA of the gene must then translated (by ribosomes) to produce the functional protein

This protein will affect the function of the cell

Question 9

Describe the advantages of regulating gene expression

Answer 9

The expression of genes can switched on or off

This allows prokaryotic cells to respond to changes in their environment

Allows cells to only express genes when they are needed (saving energy, resources)

Allows cells to express genes specific to their function (differentiation, specialisation)

Allows cells/tissues to make responses to chemical signals (hormones, paracrine signalling)

Allow gene expression to change through time, so that organisms can develop

Question 10

State the levels at which gene expression can be regulated

Answer 10

Transcription initiation

Post-transcriptional

Post-translational control

Question 11

Describe how transcription initiation can be regulated by DNA-binding proteins

Answer 11

Transcription is affected by DNA binding proteins

These bind to the specific regulated gene through their DNA-binding domain

These proteins can enhance or prevent RNA polymerase from binding to gene

Transcription factors enhance the gene’s transcription

Repressor proteins inhibit the gene’s transcription

And so gene expression can be activated or inhibited

Question 12

Summarise the function of the Lac operon

Answer 12

Bacterial cells can use different sugars for respiration

Different genes need to be expressed to use different sugars

It is wasteful to express genes for the use of different sugars at the same time

The Lac operon is a group of genes that are activated by the presence of the sugar lactose

Thus ensuring that genes for lactose metabolism are only expressed when lactose is available

This saves energy and resources of bacterial cells

Question 13

Describe the structure of the Lac operon

Answer 13

The Lac operon is a stretch of DNA that contains genes required for lactose metabolism

The operon contains three structural genes

The LacY gene encodes a channel protein that allows lactose to enter bacterial cells (lactose permease)

The LacZ gene encodes the enzyme b-galactosidase, that hydrolyses the disaccharide lactose into glucose and galactose

The operon also contains the regulatory gene, LacI, which encodes a repressor protein

Between the regulatory and structural genes are the Promoter and Operator regions of DNA

Question 14

Describe the function of the Lac operon in the absence of lactose

Answer 14

In the absence of lactose the repressor protein binds the operator region

This prevents RNA polymerase from binding the promoter

lac operon structural genes are not transcribed

The genes for lactose metabolism are not expressed

Question 15

Describe the function of the Lac operon in the presence of lactose

Answer 15

In the presence of lactose, lactose binds to the repressor protein

This prevents the repressor from binding the operator region

RNA polymerase can bind the promoter

lac genes are transcribed and expressed

Lactose diffuses into bacterial cells through lactose permease

Lactose metabolism is initiated by b-galactosidase

Lactose can be used as a respiratory substrate

Question 16

Describe how mRNA splicing is a post-transcriptional gene regulatory mechanism

Answer 16

A gene contains coding exons, and non-coding introns

Transcription results in pre-mRNA which also contains exons and introns

The introns are removed and during a process called splicing, the exons are joined together to produce the final mRNA

At this stage, signalling can result in the exons being spliced together in different sequences (alternative splicing)

This produces different mRNAs, with different sequences of codons

Which will result in proteins with different shapes and functions being produced

Question 17

Describe how gene expression can be controlled at the post-translational level

Answer 17

Post-translational means after the polypeptide has been made

Post-translational modifications can change the activity of the protein (and therefore the characteristic determined by the gene)

Post-translational modifications include:

Carbohydrate and lipid modifications

Cutting off parts of the protein to make it active, eg zymogens to enzymes

Adding and removing a phosphate group

Binding of second messengers such as cyclic AMP

Question 18

Explain how and why fruitflies (Drosophila melanogaster) are used to study morphogenesis

Answer 18

Morphogenesis is the anatomical development of an organism

Fruit-flies are small, easy to maintain and have a short life cycle

Individuals with abnormal development can easily be identified

Their DNA can be compared with those with normal development

To identify the genes that are involved in regulating morphogenesis

Question 19

Explain what homeobox genes are

Answer 19

Studies with fruit-flies and other animals identified genes that, when mutated, caused developmental abnormalities

These genes all had part of their gene sequence very similar, called the homeobox domain

When expressed these genes all gave rise to DNA-binding proteins

They bind to specific genes and regulate their expression

Ensuring the correct gene expression to form specific anatomical structures

Question 20

Explain how homeobox genes control the body plan of an organism

Answer 20

There are multiple homeobox genes in an organism’s genome

During development, the embryo is divided into sections called somites

Each somite expresses a different homeobox gene

Each homeobox gene controls a different set of genes from the genome

So each somite has a different gene expression pattern

So each somite develops different anatomical structures

Homeobox genes also control mitosis and apoptosis

Question 21

Explain why mitosis and apoptosis are both required during development

Answer 21

As new anatomical structures are formed, and as organisms grow, more cells are required

An increase in the number of cells is possible due to mitosis

However, in order to shape the structures properly, cell death is also needed

This is carried out in a controlled way by the process of apoptosis

Homeobox genes regulate mitosis and apoptosis to control morphogenesis