Flashcards in F215: Cellular Control Deck (103)
What is a gene?
A length of DNA that codes for one or more polypeptides.
What is a polypeptide?
A polymer consisting of a chain of amino acid residues joined by peptide bonds.
What is a genome?
The entire DNA sequence of that organism.
What is a protein?
A large polypeptide. Some have a single polypeptide chain and some have numerous.
What is a gene locus?
Where the gene is located in the chromosome.
What is a genetic code?
The sequence of nucleotide bases on a gene.
Describe the genetic code.
A triplet code. 3 nucleotide bases code for an amino acid.
A degenerate code, all amino acids have more than one code.
Some codes indicate a stop to the polypeptide chain rather than an amino acid.
What is transcription?
The creation of a single stranded mRNA copy of the DNA coding strand.
How does transcription occur?
One strand (template strand) of the length Of DNA is used as a template.
Hydrogen bonds between complementary base pairs break.
RNA nucleotides bind to the exposed bases with hydrogen bonds.
Bond joining is catalysed by RNA polymerase.
Two extra phosphates are released.
The mRNA produced is complementary to the base sequence on the coding strand.
The mRNA is passed through a pore in the nuclear envelope to a ribosome.
What is translation?
The assembly of polypeptides at ribosomes.
Why is the sequence of amino acids in a protein critical?
It forms the primary structure, which also determines the tertiary structure.
The tertiary structure determines the functioning of a protein. If it is altered the protein no longer functions.
Describe an anticodon.
On tRNA (transfer).
There are three exposed bases at one end whee an amino acid can bind.
Each anticodon can bind temporarily with it's complementary codon.
How is the polypeptide assembled?
A molecules of mRNA binds to a ribosome,
What is a mutation?
A change in the amount or arrangement of the genetic material in a cell.
What is a chromosome mutation?
Changes to parts of or whole chromosomes
What are DNA mutations?
Changes to genes due to changes in nucleotide base sequences.
What are the two main classes of DNA mutations?
Point mutations/substitutions - where one base pair replaces another.
Insertion/deletion mutations - one or more nucleotide pairs are inserted or deleted from a length of DNA. These cause a frame shift.
Point mutations have a lesser effect since they do not cause a frame shift.
What is an allele?
An alternative version of a gene, it is still at the same locus and codes for the same polypeptide but the alteration to the DNA base sequence may alter the proteins structure.
How can mutations have no effect on the organism?
If the mutation is in a non coding region of the DNA.
It is a silent mutation, if the changed triplet still codes for the same amino acid.
How is a mutation neutral?
If there is a mutation an causes a change that us not her advantageous or disadvantageous.
What is an operon?
A length of DNA made up of structural genes and control sites. The structural genes code for proteins, such as enzymes. The control sites are the operator region and a promoter region.
What do the operator and promoter have in common?
Both genes since they are lengths of DNA but do not code for polypeptides.
Describe how E. coli adapts depending on it's environment - glucose and lactose.
If taken from a culture with no lactose and placed into one with lactose, they cannot metabolise it at first.
They need beta galactosidase and lactose permease.
After a few minutes these enzymes are made, lactose is thus the inducer.
What two enzymes does E. coli need to respire lactose.
Beta galactosidase - catalysed the hydrolysis of lactose to glucose.
Lactose permease - transports lactose into the cell.
What are the sections of the lac operon and their functions?
The structural genes:
Z codes for beta galactosidase,
Y codes for lactose permease.
Operator region: O is a length of DNA next to the structural genes, which can switch them on and off.
Promoter region: P is a length of DNA to which the enzyme RNA polymerase binds to begin to the transcription of the structural genes Z and Y.
Where is the regulator gene situated?
Not part of the operon, far from the lac operon.
What happens when lactose is absent from the growth medium?
The regulator gene is expressed (transcribed & translated) and the repressive protein is synthesised.
It has two binding sites, one to lactose and one to the operator region.
The repressor protein binds to the operator region, and covers part of the promoter region, where RNA polymerase usually attaches.
RNA polymerase cannot attach to the promoter region so the structural genes cannot be transcribed into mRNA.
Without mRNA the genes cannot be translated and the enzymes beta galactosidase and lactose permease cannot be synthesised.
What is a repressor protein?
Protein that can bind to the operator region, and RNA polymerase binds to the promoter region to transcribe the structural genes.
What happens when lactose I added to the growth medium?
Lactose molecules (inducer) bind to the other site on the repressor protein. Causing the molecules of repressor protein to change shape so it's other binding site cannot bind to the operator region, the repressor then dissociates from the operator region.
This leaves the promoter region unblocked, and RNA polymerase can bind to it and initiate the transcription of mRNA for Z and Y.
E. coli can then respire using lactose.
The operator-repressor-inducer system is effectively a molecular switch.