6.1.1 Cellular control Flashcards
Explain how the functions of cells are controlled by the nucleus
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.
Summarise how mutations can affect the function of cells
A mutation is a change in the base sequence of DNA
During transcription, this can cause a change in the codons of mRNA
Changes in mRNA codons can result in a different primary structure
This can change the tertiary structure of the protein (3D shape)
A different tertiary structure can affect the function of a protein and therefore the functioning of a cell
Identify the types of mutation that can occur in DNA
Substitutions
Frameshift (insertion)
Frameshift (deletion)
Explain the extent to which substitution mutations alter protein structure
Substitution is the replacement of one base with another
This alters one triplet in the DNA and one codon in the mRNA
This would change one amino acid in the polypeptide
However, due to the degenerate nature of genetic code
The new codon may still code for the same amino acid
This will cause no change to the protein structure, and so no effect on protein function
Explain the extent to which insertion mutations alter protein structure
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
Explain the extent to which deletion mutations alter protein structure
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
Describe the range of effects that mutations can have on protein function
Mutations can have no effect on protein structure and function
For example substitutions that do not affect the primary structure of the protein
Mutations can be damaging
Single amino acid changes, or frameshifts
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)
Explain what gene expression is
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
Describe the advantages of regulating gene expression
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
State the levels at which gene expression can be regulated
Transcription initiation
Post-transcriptional
Post-translational control
Describe how transcription initiation can be regulated by DNA-binding proteins
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
Summarise the function of the Lac operon
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
Describe the structure of the Lac operon
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
Describe the function of the Lac operon in the absence of lactose
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
Describe the function of the Lac operon in the presence of lactose
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
Describe how mRNA splicing is a post-transcriptional gene regulatory mechanism
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
Describe how gene expression can be controlled at the post-translational level
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
Explain how and why fruitflies (Drosophila melanogaster) are used to study morphogenesis
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
Explain what homeobox genes are
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
Explain how homeobox genes control the body plan of an organism
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
Explain why mitosis and apoptosis are both required during development
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
