Transcription Flashcards
What distinguishes the information content of DNA?
It’s sequence of nucleotides.
What are proteins considered to be?
The links between genotype and phenotype.
If proteins are the link between genotype and phenotype, then how does inherited DNA lead to specific traits?
The inherited DNA dictates synthesis of proteins and RNA molecules involved in protein synthesis that lead to specific traits.
What is gene expression? What are the two stages of gene expression?
This is the process of DNA directing protein synthesis.
First stage: transcription
Second stage: translation
Do some RNA molecules function inside the cell independently of synthesizing proteins? Example?
YES
Ribosomal RNA
What is transcription? Why is it called transcription?
Transcription is the synthesis of RNA using information from DNA.
It is called this because RNA and DNA use the same language, so the information is transcribed from DNA to an RNA molecule.
What is translation? Why is it called translation?
This is the synthesis of a polypeptide using information from mRNA.
the language of RNA and the language of peptides are different, so the information is translated.
What did Archibald Garrod suggest of genes? What guided his suggestion in relation to inherited disease? Which disease specifically?
Suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions.
The thought was that symptoms of inheritable diseases are the result of an inability to synthesize a certain enzyme.
alkaptonuria - a disease where urine turns dark when exposed to air due to presence of alkapton. Most people have an enxyme that breaks down alkapton, however, some people inherit an inability to make that enzyme.
What must we understand in order to link genes to their enzymes?
Metabolic pathways - the synthesis and degradation of molecules in a series of steps. A series of catabolic and anabolic reactions.
What did Beadle and Tatums experiment result in? Describe the experiment. What was their ultimate proposal?
They took bread mold (Neurospora Crassa) and exposed it to xrays to form mutants.
They were then placed in minimal medium and the ones that did not grow were identified as nutritional mutants.
The nutritional mutant colony cells were then placed in a separate minimal mediums, each with one additional nutrient to see which grow.
The results show that they only grew in the medium with added arginine, concluding that the mutants were missing enzyme responsible for synthesizing argenine.
One gene-one enzyme hypothesis
Why was Neurospora crossa a good organism for scientists to use for their genetics experiments?
They are haploid organisms, so the mutants are always expressed since there is no other gene to compensate like in a diploid organism.
This allows for the true effect of the mutation to be seen.
Also easy to grow
Describe the experiment performed by Srb and Horowitz with Neurospora crassa. What was their conclusion?
Using the premise that arginine production required a precursor and 2 other intermediate molecules in its metabolic pathway.
The nutritional mutants deficient in arginine were exposed to minimal medium with one of the two intermediate molecules, or arginine.
The mutants were then able to be divided into three classes based on their growth patterns in each medium as a collective.
See picture for more detail.
This led them to support the one gene-one enzyme hypothesis proposed by Beadle and Tatum
What were each of the classes of mutants found in Srb and Horowitz experiments? What were they deficient in?
Wild type - grow in minimal medium
Class I - Don’t grow in minimal medium but in all other added mediums, means enzyme that converts precursor is missing.
Class II - Don’t grown unless citruline or arginine are added. means enzyme that converts ornithine to citruline is missing.
Class III - Only grow with arginine added, means that enzyme that converts citruline to arginine is missing.
What were the two eventual modifications made to Beadle and Tatums one gene-one enzyme hypothesis?
one gene-one protein: this is dues to the fact that not all proteins are enzymes
and after finding that many proteins are mad eof polypeptides, which have their own gene if was changed to…
one gene-one polypeptide: which it remains today.
What are the 2 caveats to the one gene-one polypeptide hypothesis?
1 - Some genes code for RNA that have functions in the cell other than protein synthesis.
What is transcription? Translation?
Transcription - synthesis of mRNA from the direction of DNA.
Translation - the synthesis of a polypeptide under the direction of mRNA
What roles do ribosomes play in protein synthesis?
Translation occurs in the ribosomes.
What are some of the differences in the process of gene expression between eukaryotes and prokaryotes?
Prokaryotes - mRNA is not modified and can be immediately translated, since they lack a nucleus transcription and translation can occur simultaneously.
Eukaryotes - RNA processing occurs to modify RNA to its finished mRNA product, trnascription and transltion are separated by the nuclear envelope.
What was the “central dogma” proposed by Francis Crick?
This is the cellular chain of commnd.
DNA—>RNA—>protein (polypeptide)
What is RNA processing?
This is the modification of the mRNA transcript before it exits the nucleus to be translated.
Where does translation take place?
The cytoplasm
How was it deduced that 3 bases correspond to an amino acid? Describe the math involved.
There are 20 amino acids, and there are 4 DNA bases.
If 2 bases correspond to an amino acid, the 4^2=16 amiono acids, so this isnt right.
If 3, then 4^3 = 64, this can account for the 20 essential amino acids in our body.
What is a triplet code?
This is a series of nonoverlapping, three-nucleotide words that serves as the information for polypeptide formation.
During transcription, there is a template strand of DNA and a non-template coding strand of DNA, what is the different between these two?
Template strand - this is the template for the mRNA to be produced, the RNA is synthesized antiparrallel to theis strand in a 5’ to 3’ direction.
Coding strand - this strand has essentially the same base sequence except the thymine must be replaced with uracil. looking at the coding strand in the 5’ to 3’ direction will be the same as the mRNA, just remember U instead of T.
What is a codon? What direction do you read them in?
These are the triplets of mRNA bases, each triplet is a codon.
They are read in a 5’ to 3’ direction.
What does a codon tell us?
Tells us the amino acid placed at that corresponding position along the polypeptide chain.
After the 64 codons were deciphered, what was concluded about the genetic code? Elaborate on this.
The genetic code is redundant, but not ambiguous.
This means that more than one codon may specify the same amino acid, but one codon does not specify for more than one amino acid.
Is the 64 codon genetic code nearly universal to all organisms? What is significant about this?
YES
This means that genes can be transcribed and translated after being trasplanted from one species to another species. Like the tobacco plant that was given firefly DNA that made the plant glow.
This shared language must have been operating very early in the history of life and has been evolved.
What are the three steps of transcription? Describe each.
- Initiation - after RNA polymerase has bound to promoter the DNA strands unwind and RNA synthesis begins at the start point of the promoter of template strand.
- Elongation - the polymerase moves downstream (3’ to 5’ along template), unwinding DNA and elongating RNA transcritp in 5’ to 3’ direction. DNA rewinds after RNA polymerase passes.
- Termination - RNA transcript is released and RNA polymerase is detached.
Describe the extra steps involved in the initiation of transcription in eukaryotes.
- TATA box promoter located about 25 nucleotides upstream of the start point.
- Transcription factor recognizes and binds to TATA box promoter of DNA.
- Additional transcription factors bind to the DNA in this region as well as RNA polymerase to form the transcription initiation complex. DNA unwinds and transcription begins at the start point.
In RNA processing, what are the two things that happen to the ENDS of the pre-mRNA molecule? What is the purpose of these modification (3)?
- The 5’ end receives a modified nucleotide cap made of modified guanine.
- The 3’ end receives a poly-A tail
- Facilitates export of mRNA from nucleus
- Protects mRNA from hydrolytic enzymes
- Helps ribosomes attach to 5’ end of mRNA
What is RNA splicing that is performed in RNA processing?
The introns are removed and exons are joined together to form one continuous coding sequence of mRNA.
One way RNA splicing is performed is via spliceosomes, describe the 3 steps of splicing using a spliceosome.
- Small nuclear ribonucleoproteins (snRNPs) and other proteins form the spliceosome complex on pre-mRNA containing introns and exons.
- small nuclear RNA (snRNA) base pairs with nucleotides at specific sites of the intron.
- The RNA transcript is cut, releasing the intron and splicing the exons together at the same time. The spliceosome comes apart releasing the spliced RNA that now only has exons.
What is the architecture of proteins described as having? How do exons relate to this?
Described as having modular architecture with discrete regions called domains.
Different exons tend to code for different domains in a protein.
