Introduction To Genetics Flashcards
What is molecular genetics?
- Expression and regulation of genes at a molecular level.
- investigative approach that involves the application of laboratory methods and research strategies.
- Sub discipline of genetics. Genes are units of heredity. Molecular genetics is concerned with the physical and chemical properties.
- Understands the nature of inherited characteristics at a molecular level in terms of DNA + RNA structure end sequence and factors that influence gene expression.
- The branch of genetics that deals with hereditary transmission and variation on the molecular level.
Advances in molecular genetics:
- Forensics
- Genetic testing for disease
- Identification of horse meat in British food.
- Ancestral DNA lineage
Flow of Genetic Information
Prokaryotes
DNA - mRNA - Protein
Through transcription and translation.
Flow of Genetic Information
Eukaryotes
DNA
Pre-mRNA
mRNA
Protein
What is transcription?
Copies one of the DNA strands by making a complimentary RNA strand.
5’ to 3’ sequence - based on location of carbon atoms.
Does not require a primer.
What enzyme is involved with transcription?
RNA Polymerases
What does mRNA code for?
Codes for protein and other RNA’s.
What is rRNA?
Part of ribosome
What is tRNA?
Carries amino acids to the ribosome.
What is the level of gene product controlled by?
Transcription regulation
Where does transcription take place?
Eukaryotes
In the nucleus
What happens with transcription in eukaryotes?
After transcription the RNA product travels to its next destination.
mRNA goes to the cytosol where ribosomes translate it.
Where does transcription take place?
Prokaryotes
No nuclear membrane.
mRNA can be translated as soon as made.
What are RNA polymerases?
Enzymes that catalyse the formation of phosphodeister bonds between ribonucleotides.
Form polymers of RNA by copying a DNA template.
Prokaryotic RNA polymerase
What is it made up of?
5 different core subunits and a sigma factor.
β+ and β = 2 large sub units
α = 2 small subunits
ω = 1 small subunit (smallest)
1 sigma factor (various)
Each subunit is a polypeptide chain of amino acids
How is transcription regulated?
Not all genes are expressed all the time.
They can be switch on / off depending on the conditions.
E.g. in response to nutrients, temp or during development. Or to confer specific properties on a cell within a tissue or organ.
What do sigma factors do?
Allow the prokaryotic polymerase to recognise and bind different promoter sequences.
O70 - most prokaryotic promotors
054 - nitrogen-regulates promoters
032 - heat shock regulation promoters
How do species differ with sigma factors?
Have different number of sigma factors.
Typically more sigma factors improves response to changing conditions.
What is the promoter recognition by the bacterial RNA polymerase holoenzyme?
Core RNA polymerase associates with a sigma factor that binds to a specific promoter sequence (green) in the DNA.
Contacting sequences centered at -10 and -35 region.
The α terminal domains contact the up element (orange), which is located upstream of the -35 region on some promoters.
The tighter the bind to the sequence in transcription equals
The higher level of transcription
Key features of prokaryotic transcription
- only 1 RNA polymerase
- sigma factor
- transcription and translation are coupled
- genes contain NO introns
- transcripts can be polycistronic
Key features of eukaryotic transcription?
- 3 RNA polymerases Pol I, Pol II, Pol III
- No sigma factor
- Transcription and translation are NOT coupled
- Genes frequently contain introns
- Transcripts are monocistronic
Steps of transcription
1) initiation
2) elongation
3) termination
Transcription
1) initiation
The most important and complex
Transcription steps
2) Elongation
RNA polymerase continues to add nucleotides to the growing chain.
Transcription steps
3) Termination
RNA polymerase reaches the end of the gene and reads a signal to stop transcribing.
Where does transcription start?
RNA polymerase initiates transcription at specific sites near the start of the gene.
These sequences are called promoters.
What are the two transcription sequence promoters?
Prokaryotic promoters
Eukaryotic promoters
There are two important prokaryotic promoter elements for RNAP binding
- 35 region e.g. TTGACAT
- 10 region e.g. TATAAT
Known as consensus sequences
What is a consensus sequence?
A sequence of nucleotides found in different genes which are similar or identical.
Companion of several similar sequences leads to an “average” sequence.
E.g.
TATGAT - sequence 1
TATAAT - sequence 2
TAATAT - sequence 3
TATATT - sequence 4
TATAAT - consensus sequence
Stages of initiation
1) RNA polymerase holoenzyme bunds to the promoter region, forming the closed complex.
2) The enzyme then separates the template and non-template strand, forming the open complex.
3) Synthesis of an RNA strand is initiated, using base-pairing interactions with the template strands.
4) RNA polymerase frequently synthesises only a few base pairs at first and then releases the product, which is known as abortive initiation.
5) When an RNA product of around 9-11 nucleotides is successfully synthesised, RNA polymerase undergoes a conformational change and loosens it’s grip on the stigma subunit to become an elongating comped that continues to synthesise RNA as it translocates along the DNA template.
Prokaryotic Transcription Initiation
Summarised
- RNA polymerase walks on the DNA
- RNA polymerase (sigma factor) binds to promoter.
- DNA strands separate and one strand is copied.
- Sigma factor dissociates after 10 nucleotides have been added.
- Core RNA polymerase continues to end.
Termination bacteria
Two mechanisms
Rho-dependent and rho-independent
Rho = protein
Bacteria is typically rho-independent
