Cellular Control Flashcards
Does the nucleus of every cell in the human body contain the same genes?
Yes
Is every gene in every cell expressed?
No and not all of these genes are expressed all the time
What are the mechanisms that make sure the correct genes are expressed in the correct cell at the correct time called?
regulatory mechanisms
What are the three main types of regulatory mechanisms?
- regulation at the transcriptional level (i.e. regulatory mechanisms that occur during transcription)
- regulation at the post-transcriptional level (i.e. regulatory mechanisms that occur after transcription)
- regulation at the post-translational level (i.e. regulatory mechanisms that occur after translation)
What are regulatory mechanisms controlled by?
many different regulatory genes
What is a structural gene?
a structural gene codes for a protein that has a function within a cell (e.g. enzymes, membrane carriers, hormones etc.)
What are regulatory genes?
regulatory genes code for proteins (or various forms of RNA) that control the expression of structural genes
Do regulatory genes control structural genes or structural genes control regulatory genes?
regulatory genes control structural genes and their levels of protein production
Can regulatory genes have control over several structural genes at once?
sometimes
Give an example of a regulatory mechanism at the transcriptional level.
the lac operon
What is a homeobox?
a DNA sequence that codes for a protein transcription factor
What is a mutation in a gene?
a change in the base sequence of the DNA
When and how often do gene mutations occur?
they randomly occur and during DNA replication which is in the S phase of the cell cycle
What makes random mutations more likely to occur?
exposure to mutagenic agents
What do mutagenic agents interfere with?
DNA replication
Give some examples of mutagenic agents
high energy radiation (UV light), ionising radiation (Gamma rays and X rays) and chemicals (carcinogens such as mustard gas and cigarette smoke)
What are the three different types of gene mutation that could occur?
deletion, insertion and substitution
What does it mean when we describe the genetic code as degenerate?
multiple codons can code for the same amino acid
How may a base substitution be silent?
the new codon still codes for the same amino acid
Which can be more harmful: a base substitution or a base deletion or insertion?
base deletion or insertion as removing or adding a base changes all of the subsequent codons (a frameshift) so multiple amino acids may be incorrectly coded for
How do gene mutations affect the protein structure?
- because mutations alter the gene, they can result in a different amino acid sequence (primary structure) in the encoded polypeptide
- if the amino acid sequence changes, then the folding and coiling of the secondary structure will be different
- when the protein is modified into the tertiary structure it will form hydrogen and ionic bonds in different places and fold differently
- this will result in a different 3D shape, and therefore a non-functioning protein could be made
Are all mutations harmful?
No as sometimes the new protein made may be beneficial such as the mutation that resulted in antibiotic resistance in bacteria. Also, some effects are neutral, if they do not code for a protein that alters the survival chances of that organism
When will transcription of a gene occur?
only occur when a molecule from the cytoplasm enters the nucleus and binds to the DNA in the nucleus
What are transcription factors?
proteins and each one can bind to different base sequences on DNA and therefore initiate transcription of genes
What happens in the absence of a bound transcription factor to a gene?
the gene is inactive and the protein won’t be made
How can genes be turned on/off in eukaryotes?
transcription of target genes can be stimulated or inhibited when specific transcriptional factors move from the cytoplasm into the nucleus
Turning on/off genes allows what to happen?
only certain proteins are produced in that particular cell - this is what enables them to become specialised
What is an operon?
a group of simultaneously controlled genes that are either all expressed or not
Are operons more common in prokaryotes or eukaryotes?
in prokaryotes
Where can the lac operon be found?
in E. coli
What is the lac operon?
it is a sequence of 3 genes that collectively aid lactose digestion
Does bacteria require more or less energy to digest glucose than lactose?
less energy to digest glucose so glucose is the favourable respiratory substrate
When, in bacteria, will lactose be digested?
when glucose isn’t present
When are the proteins produced by the lac operon needed?
if glucose is absent and lactose is present
What are the three genes in the lac operon?
lacZ, lacY and lacA
What is LacI?
a regulatory gene
Where is LacI found?
near the operon
What does the gene LacI code for?
a repressor protein that inhibits transcription when there is no lactose present
When lactose is absent, what does LacI do?
constantly produce the repressor protein
What does the repressor protein do when lactose is absent?
binds to the operator (a region close to the operon) which prevents RNA polymerase from binding to the DNA at the promoter region - in this way transcription is inhibited
If lactose is present, what does the repressor protein do?
the lactose binds to the repressor protein and this causes it to change shape - this change in shape prevents the repressor protein from binding to the operator and therefore RNA polymerase can bind to the promoter and transcription of the lac operons occurs
What increases the rate of transcription of the lac operon?
cyclic AMP (cAMP)
In order for sufficient enzymes to be produced by lacZ, lacY and lacA, what must bind?
cAMP receptor protein (CRP)
CRP can only bind and increase the rate of transcription once it has bound to what?
cAMP
Plants, animals and fungi all have what gene sequences to control the development of their body?
homeobox gene sequences
What are homeobox genes?
sequences of genes which create proteins that regulate the expression of other genes that are involved in the formation of the body in the early stages of development as an embryo
What are hox genes?
a type of homeobox gene found in animals
What are hox genes responsible for?
the correct body development and positioning of body parts
What does mitosis result in?
an increase in the number of cells, resulting in growth
What does apoptosis do?
removes unwanted cells (programmed cell death)
What is an operon?
a cluster of genes under the control of a promoter
Describe the base sequence in Hox genes
similar in all animals
What is the product of Hox genes?
polypeptide
What is the effect of mutations of Hox genes?
often lethal
What is the level at which gene expression is being controlled when DNA is more tightly wound around histones during cell division?
transcriptional
State what structural detail of a polypeptide is altered by gene mutations
primary structure/order of amino acids
Explain how it is possible for a mutation to have no effect on the protein produced from that gene
some triplets code for the same amino acid so the amino acid sequence is not altered
Explain how a mutation could alter the protein so that it no longer performs its correct function in the cell
insertion/deletion cause a frame shift so all triplets downstream will be different
State the type of adaptation represented by the tiger’s stripes
anatomical
