2.7-7.2 Transcription and Gene Expression Flashcards
The promotor as an example of non-coding dna with a function
- non-coding regions have important functions
- The promoter is a DNA sequence located near a gene. It acts as the binding site for RNA polymerase
- The adjacent gene is transcribed into mRNA but the promoter region is not
Gene expression is regulated by proteins that bind to specific base sequences in DNA
One example of the regulation of gene expression by proteins is the metabolism of lactose in the E.coli bacterium
- Operator is a region of DNA that can regulate transcription, typically inhibiting transcription through silencer sequences
- The repressor protein is bound to the operator preventing RNA polymerase from transcribing the gene
- The consequence of the inhibition of lactose metabolism is that the concentration of undigested lactose now increases in E.coli
- Lactose binds to the repressor protein inhibiting it: the repressor can no longer bind to the operator
- RNA polymerase binds with the promoter and express the genes by transcribing them, which in turn synthesizes lactase
- With the synthesis of lactase the lactose is broken down, as its concentration decreases the inhibition of the repressor molecules will decrease silencing the gene again.
Summary of the common types of regulating proteins and associated sequences found in eukaryotes
DNA sequence: Enhancers
Binding Proteins: Activators
- activator proteins bind to enhancer sequences of DNA to greatly increase the rate of transcription of a gene
DNA sequence: Silencer
Binding Proteins: Repressor
- Repressor proteins bind to non-coding regions of DNA to either block or reduce the transcription of a gene
DNA sequence: Promoter
Binding Proteins: RNA polymerase
- A region of DNA located close to a specific gene. Once bound to the sequence RNA polymerase transcribes the gene.
The environment of an organism impacts gene expression part 1
- Human hair and skin colour are impacted by the exposure to sunlight and high temperatures
- Similarly pigments in the fur of Himalayan rabbits are regulated by temperature
- Gene C controls fur pigmentation in Himalayan rabbits. The gene is active when environmental temperatures are between 15 and 25 degrees C. At higher temperatures the gene is inactive
- In lower temperatures gene C becomes active in the rabbits colder extremities and produces a black pigment
- In warm weather no pigment is produced and the fur is white
The environment of an organism impacts gene expression part 2
This is a complex area of genetics, but can be outlined as follows:
- Only a small number of genes are involved in determining body patterns during embryonic development.
- The expression of these genes is regulated by a group of molecules referred to as morphogens
- morphogens diffuse across the surfaces of cells from a concentrated source. Therefore different embryonic cells get different concentrations of morphogens.
- Morphogens regulate the production of
transcription factors in a cell. - This results in the activation and inhibition of different genes in different cells. This in turn controls how long your fingers should be, where your nose is on your face, and other specific about body structure.
Nucleosomes help to regulate transcription in eukaryotes
- Methylation is the addition of methyl groups to DNA
- Methylation of DNA inhibits transcription
- Processes that inhibit transcription bind the DNA more tightly to the histone making it less accessible to transcription factors (forming heterochromatin)
- Chromatin is a complex of DNA, RNA, and protein. Tightly packed chromatin which cannot be transcribed is referred to as heterochromatin.
- Acetylation is the addition of Acetyl groups to histones
- Acetylation promotes transcription
- Processes that promote transcription bind the DNA more loosely to the histone making it more accessible to transcription factors (forming euchromatin)
- Chromatin is a complex of DNA, RNA, and protein. Loosely packed chromatin which can be transcribed is referred to as euchromatin.
epigenetics
Changes in the environment affect the cell metabolism, this in turn can directly or indirectly affect processes such as Acetylation and Methylation
- Methylation and acetylation mark the DNA to affect transcription. These markers are known as epigenetic tags. (The branch of genetics concerned with heritable changes not caused by DNA is called epigenetic.)
- For a new organism to grow it needs unmarked DNA that can develop into lots of different specialized cell types.
- Reprogramming scours the genome and erases the epigenetic tags to return the cells to a genetic “blank state.”
- For a small number genes, epigenetic tags make it through this process unchanged hence get passed from parent to offspring.
Analysis of changes in the DNA methylation patterns
One study compared the methylation patterns of 3-year old identical twins with 50-year old identical twins
- Methylation patterns were dyed red on one chromosome for one twin and dyed green for the other twin on the same chromosome
- Chromosome pairs in each set of twins were digitally superimposed. The result would be a yellow colour if the patterns were the same.
- Differences in patterns on the two chromosomes results in mixed patterns of green and red patches.
- This was done for 4 of the 23 chromosome pairs in the genome
Transcription
In the nucleus, the cells machinery copies the gene sequence into messenger RNA, a molecule that is similar to DNA. Like dna, mRNA has 4 nucleotide bases, but in mRna the uracil replaces thymine.
Transcription is the process by which an RNA sequence is produced from a DNA template
Three main types of RNA are predominantly synthesized
Messenger RNA - a transcript copy of a gene used to encode a polypeptide
Transfer RNA - A clover leaf shaped sequence that carries an amino acid
Ribosomal RNA - a primary component of ribosomes
Summary of transcription
- the enzyme RNA polymerase binds to the promoter site of the DNA at the start of a gene ( the sequence of DNA that is transcribed into RNA is called a gene)
- RNA polymerase separates the DNA strands and synthesises a complementary RNA copy from the antisense DNA strand
- Transcription occurs in a 5’ to 3’ direction
RNA polymerase adds the 5 end of the free RNA nucleotide to the 3 end of the growing mRNA molecule ( RNA polymerase moves along the antisense strand in a 3 to 5 direction) - It does this by covalently bonding ribonucleoside triphosphate that align opposite their exposed complementary partner ( using the energy from the cleavage of additional phosphate groups to join them together )
- Once the terminator sequence is reached, transcription ends
Once the RNA sequence has been synthesized
- RNA polymerase will detach from the DNA molecule
- RNA detaches from the DNA
- The double helix reforms
- Transcription occurs in the nucleus (where DNA is) and mRNA moves to cytoplasm to be translated.
Eukaryotic cells modify mRNA after transcription
Eukaryotic genes (unlike prokaryotes) contain base sequences that aren’t translated into polypeptides
- Exons are the coding sections of the gene
- introns are non-coding sections of the gene
- the splicesome forms and causes introns to form loops which allows the exons to be joined
- Introns are removed and then broken down back into nucleotides ready for use
- mature mRNA contains only exons and leaves the nucleus to be translated into polypeptides.
Splicing of mRNA increases the number of different proteins an organism can produce
the splicing process can happen in different ways to the same gene. Particular exons of a gene may be included or excluded from mature mRNA.
- this means that multiple proteins can be produced by a single gene
- each protein produced will vary in its biological function
- an example of this is the IgM gene which produces different immunoglobulins (antibodies) to fight different pathogens.
