Lecture 18: The Genetic Code Flashcards
How to synthesize new DNA?
Add the following in a test tube?
- Triphosphate nucleotides(bases with phosphate)
- DNA polymerase III(enzyme that catalyses replication)
-Template DNA strand
-Ragged Ends
What is a ragged end?
-A stretch of DNA that is partly double stranded and partly single stranded
When does DNA replication occur?
During interphase
How does DNA polymerase III work?
-Find the end of the area of double strandedness and binds to the 3’ hydroxyl group
-It then sees what nucleotide base is on the opposite strand and adds its complementary base
Importance of ragged ends?
-Without ragged ends you would only have one single strand of DNA
-A double strand is needed so that the DNA polymerase III has a 3’ hydroxyl to bind to
Role of helicase?
Enzyme that unwinds the DNA
Role of primase?
-Enzyme that adds a short sequence of complementary RNA(primer), this acts as the ragged end for DNA polymerase III
What are origins of replication? How many do eukaryotes/bacteria have?
Specific spots where replication starts
- Eukaryotes: have multiple origins of replication along a DNA strand
-Bacteria: Have one origin of replication
What is a replication bubble?
Helicases unwind DNA in different directions from the origins of replication
What is replication fork?
-Half of a replication bubble
How does primase work?
1.Pry the double strand of DNA apart(creating a replication bubble)
2.Primase as a little piece of RNA (10-20 base pairs long)( primer)
3.The primer provides the 3 prime hydroxyl that the DNA polymerase III can now use
4.Synthesis then occurs(addition of nucleotide bases)
Does primase require a 3’ hydroxyl to add RNA primer?
NO
Leading Strand vs Lagging Strand
Leading Strand: Run 3’ to 5’ towards the fork and is replicated continuously(Template strand runs 5’ to 3’)
Lagging Strand: Runs 5’ to 3’ towards the fork and is replicated discontinuously(template strand runs 3’ to 5’)
How is the lagging strand transcribe?
- Primase adds and RNA primer
- DNA polymerase III then add DNA nucleotides (Okazaki fragments)
How much primer is on the leading and lagging strand after replication?
Leading strand: Only contains one primer
Lagging strands: Contains multiple primers mixed within DNA
What does DNA polymerase I do ?
1.Rips out the RNA primers between okazaki fragments
2. Then uses the 3’ hydroxyl from the previous okazaki fragment to fill in the gap with DNA nucleotide bases
What still must be done after RNA primers are ripped out and replaced with DNA?
- The Okazaki fragments are missing phosphate bonds between them
What does DNA ligase do?
Catalyzes the formation of the phosphodiester bonds between the 3’ end of one fragment and the 5’ end of another okazaki fragment
What happens if there is a mistake?
-The DNA polymerase all have the ability to proofread the strands (they immediately remove bases that are not complimentary)
What does mismatch repair do and when does it work ?
- Repairs during recombination, when strand are being annealed back together
-Correct newly synthesized DNA and make sure bases are properly matched
What is excision repair?
-Corrects newly synthesized DNA
Epistasis?
-One gene completely masks another gene(baldness, hair colour)
What does the genetic code tell cells to make?
Tells cells how to make proteins
Hypothesis of Beadle and Tatum?
Since genes encode for proteins, if an organism is unable to convert one compound into another, it is presumably missing an enzyme and the mutation is in the gene that encodes for that enzyme
What is an auxotroph?
An organism that is unable to synthesize a needed nutrient due to a gene mutation
What is a prototroph?
An organism than CAN synthesize needed nutrients
True/False: The DNA sequence encodes for the amino acid sequence of proteins?
True
Proteins are made by ribosomes in the cytoplasm and DNA is in the nucleus, How does information get from DNA to cytoplasm?
DNA does not work directly with ribosomes but mRNA acts an intermediary between the two
The central dogma
Idea that genetic information flows from DNA to RNA to Protein
-The flow is unidirectional (never reverse)
Does RNA having a 2’ hydroxyl affect replication?
NO
How is RNA synthesis done?
RNA polymerase binds to DNA at the beginning of a gene.
Then it unwinds the DNA and starts creating a complementary copy of the DNA , it does not stop until it reaches the end of the gene
Does RNA polymerase require a 3’ hydroxyl like DNA polymerase?
No
What is the process of making the RNA strand called?
Transcription
True/False: After transcription the mRNA now contains all of the genetic information necessary to encode for the protein?
True
Where does the mRNA go after transcription?
the cytoplasm to find the ribosomes
What is a codon?
Three letters of DNA(AGU, GAC)
What was the first protein to have its amino acid sequence determined?
Insulin
Stop Codons?
Tell ribosome that the protein is done(stop translating)
Why is the genetic code degenerate?
Several three letter codons code for the same amino acids
Does the amino acid sequence determine the DNA sequence?
No, because there are multiple codons that can encode for the same amino acids
What is tRNA?
-tRNA has a loop of three anticodons that stick out the bottom, these anticodons bind to a sequence on the newly synthesized mRNA
- On the opposite end of the tRNA, there is an amino acid that is encoded by that codon
How many tRNAs are there ?
There are at least one for each amino acid
What causes the tRNAs to bind to the mRNA strand?
Ribosomes match up the tRNA anticodon to the mRNA codon
The process of converting the mRNA into the protein is called?
Translation
How is the amino acid attached to the tRNA?
Each amino acid has a specific enzyme that catalyzes the reaction of the attachment of a specific amino acid to a tRNA
What enzymes attach amino acids to tRNAs?
Aminoacyl tRNA synthases
How do aminoacyl tRNA synthases work?
-They recognize the structure of the tRNA (not the anticodon) and its biding site for the amino acid
How do we know that the ribosome doesn’t know the right amino acid to go with the right codon?
If we add random amino acids to tRNAs the ribosome will still create the protein
How doe we know aminoacyl syntheses only recognize shape?
If you change the anticodon and not the shape of the tRNA, the aminoacyl will still recognize it as the previous amino acid
Ture or False: We all share the same genetic code with other organisms
True, proves genetic code is very old and we all have a common ancestor that had a similar genetic code
