Chapter 19 - Genetics of Living Systems Flashcards
In lac operon, what happens when there is no lactose present?
- The repressor protein binds to the operator
- This means that RNA polymerase cannot bind to the promoter, so no transcription of the enzymes occurs
In lac operon, what does the lac Z gene code for?
B-galactosidase
In lac operon, what does the lac Y gene code for?
Lactose permease
What does B-galactosidase do to lactose?
It breaks it down into more easily metabolisable sugars (glucose and galactose)
What does lactose permease do in lac operon?
It helps to transfer lactose into the cell
In what organisms are lac operons found?
Prokaryotes- common example is E. coli
What is an operon?
An operon is a group of genes under the control of the same regulatory mechanism and are expressed at the same time
In lac operon, what is the role of allolactose?
Allolactose will also be present when lactose is present, and will bind to the repressor protein. This will prevent the repressor protein from binding to the operator
In lac operon, what happens when the repressor protein is no longer bound to the operator?
It allows RNA polymerase to bind to the promoter and so allows for the transcription of the genes which code for the enzymes necessary to metabolise lactose
What is the role of cAMP in lac operon?
If cAMP is present, it will bind to the CAP. The CAP will then bind to the CAP site, which increases the rate of transcription, allowing the required quantity of enzymes needed to efficiency metabolise lactose to be produced.
What is the relationship between cAMP and glucose and what does this mean in a regulatory sense?
Glucose inhibits the creation of cAMP, therefore when glucose is present, cAMP will not be (and visa versa)
This means that when there are high levels of glucose, the rate of transcription will be much lower (even if lactose is present), as there is no need to metabolise lactose when the preferred sugar (glucose) is present.
What are the 3 structural genes in lac operon and what is their role?
LacZ, lacY and lacA. They code for three different enzymes, that are involved in the metabolism of lactose
What is the role of the regulator gene (lacI), which is found near the lac operon?
It codes for the repressor protein that inhibits transcription from occurring when lactose is not present
What is it called when only one nucleotide is affected by mutation?
Point mutation
What does insertion or deletion of a nucleotide lead to?
A frameshift mutation, which will shift the reading frame of the sequences of bases, hence changing every successive codon from the point of mutation
What is the effect of damaging mutations?
The phenotype of an organism is affected in a negative way, as proteins are no longer synthesised, or the proteins synthesised are non-functional
How are some mutations beneficial (with an example)?
Very rarely, a protein is synthesised that results in a new and useful characteristic in the phenotype.
For example a mutation in a protein in cell surface membranes of human cells can give HIV immunity
Where do mutations often spontaneously occur?
During DNA replication
What is a mutagen?
An agent that increases the rate/likelihood of mutations
What are the 3 types of mutagens?
Physical, chemical and biological
Give an example of a physical mutagen
Ionising radiation such as x-rays
Give an example of a chemical mutagen
Deaminating agents
How do deaminating agents work?
Chemically alter bases in DNA, changing the base sequence
Give an example of a biological mutagen
A virus
How are viruses biological mutagens?
Viral DNA may insert itself into a genome, changing the base sequence
What does it mean for a mutation to be silent?
The mutations do not change any proteins, or the activity of any proteins synthesised. Therefore they have no effect on the phenotype
Why are most mutations silent?
- They can occur in the non-coding regions of DNA
- Due to the degenerate nature of genetic code, mutated codons can code for the same amino acid
What are nonsense mutations?
Nonsense mutations result in a codon becoming a stop codon instead of coding for an amino acid. This creates a shortened often non-functional protein
What are missense mutations?
Result in the incorporation of an incorrect amino acid into the primary structure during protein synthesis. The missense mutation can be conservative or non-conservative
What is a conservative missense mutation?
When the amino acid change leads to an amino acid that has similar properties to the original being added to the primary structure, meaning the mutation is less severe
What is a non-conservative missense mutation?
When the amino acid change leads to an amino acid that has different properties to the original being added to the primary structure, meaning the mutation is more severe
What is the difference between gene and chromosome mutations?
Gene mutations occur in single genes or sections of DNA, whereas chromosome mutations affect the whole chromosome or a number of chromosomes
When do chromosome mutations often take place?
During meiosis
What 4 ways can chromosome mutations occur?
Deletion
Duplication
Translocation
Inversion
In chromosomal mutation, what is deletion?
Where a section of chromosome breaks of and is lost within the cell
In chromosomal mutation, what is duplication?
Where sections get duplicated on a chromosome
In chromosomal mutation, what is translocation?
Where a section of one chromosome breaks off and joins another non-homologous chromosome
In chromosomal mutation, what is inversion?
Where a section of chromosome breaks off, is reversed, and then rejoin the chromosome
What are housekeeping genes?
Genes that code for enzymes that are constantly required (e.g. enzymes for respiration)
What are the 4 levels of gene regulation control?
Transcriptional
Post-transcriptional
Translational
Post-translational
What is transcriptional control?
The control of gene expression by regulating the transcription stage (i.e. regulating the conversion of DNA to RNA)
What is an example of transcriptional control?
Chromatin remodelling
How does chromatin remodeling work to control gene expression?
DNA winds around proteins called histones, forming a chromatin.
The tightness of the winding of the DNA around the histone dictates whether transcription can occur
What is it called when the DNA is tightly wound around the histone, and what is the effect on transcription?
Heterochromatin, which makes the transcription of genes not possible as RNA polymerase cannot access the genes
What is it called when the DNA is loosely wound around the histone, and what is the effect on transcription?
Euchromatin, which allows the genes to be freely transcribed
What is the purpose of chromatin remodeling?
- Ensures proteins necessary for cell division are synthesised
- Prevents protein synthesis from occuring when cells are actually dividing
What is histone modification?
Histones can be modified to increase or decrease the degree of packing and the coiling of DNA
What is the effect of acetylation in histone modification?
The addition of acetyl groups makes histones more negative, causing DNA (which is negative) to coil more loosely
What is the effect of methylation in histone modification?
The addition of methyl groups makes histones less polar so they bind more tightly to one another, causing DNA to bind more tightly
What are introns?
Non-coding sections of DNA
What are exons?
Coding sections of DNA
What is epigenetics?
A term that is used to describe the control of gene expression through the modification of DNA
What is the product of transcription, and what does this turn into?
The product of transcription is the precursor molecule pre-mRNA. This is then modified to form mature mRNA
What is a cap in post-transcriptional control?
A modified nucleotide
What is a tail in post-transcriptional control?
A long chain of adenine nucleotides
What is the 5’ end of DNA?
The 5’ end of the DNA is the one with the phosphate group on the 5’ carbon of the deoxyribose
What is the 3’ end of DNA?
The 3’ end of the DNA is the end with the ending OH group on the 3’ carbon of the deoxyribose
In what order is DNA read?
From 5’ to 3’
What is splicing in post-transcriptional control?
Splicing is where the the RNA is cut at specific points. The introns are removed and the exons are joined together
How is pre-mRNA turned into mature mRNA?
A cap is added to the 5’ end and a tail to the 3’ end of the RNA. Then splicing occurs, removing the introns and joining the exons together
What are the effects of adding a cap and tail to pre-mRNA?
They both act to help stabilise and delay degradation. The cap also aids in binding of mRNA to ribosomes
Where does the processes turning pre-mRNA into mature mRNA take place?
In the nucleus
What is RNA editing?
In post-transcriptional control, mRNA molecules can be changed through base addition, deletion or substitution.
What is the effect of RNA editing?
It increases the range of proteins that can be produced from a single mRNA molecule
What is translational control?
Mechanisms that regulate the process of protein synthesis translation
What is the effect of inhibitory proteins binding to mRNA?
Prevents it from binding to ribosomes and hence prevents the synthesis of proteins
What is the effect of initiation factors on mRNA?
Aids the binding of mRNA to ribosomes, initiating protein synthesis
What are inhibitory proteins binding to mRNA and initiation factors examples of?
Translational control
What do protein kinases do?
They are enzymes that catalyse the addition of phosphate groups to proteins
What does post-translational control involve?
Modifications to proteins that have been synthesised
Give two examples of post-translational control
- Addition of non-protein groups to the protein
- Folding or shortening of proteins
Give an example of gene control in lac operon
In the lac operon cAMP binds to the cAMP receptor protein, increasing the rate of transcription of the genes
What are homeobox genes?
Regulatory genes that control body development
What do homeobox genes code for?
The homeodomain, a highly conserved part of a protein 180 base pairs long
What does highly conserved mean?
Very similar in plants, animals and fungi
What is an example of a homeobox gene and what does it control?
Pax6 is a homeobox gene that controls eye development
What can homeobox genes do in response to stimuli?
Homeobox genes can induce mitosis or apoptosis in response to external and internal stimuli
What are hox genes?
Homeobox genes found only in animals
What can stress on an organism be defined by?
The condition produced when the homeostatic balance is upset
