Molecular genetics - Test Flashcards
Which nitrogenous bases are purines? What is their structure?
Adenine and guanine. They are double ringed structures.
Which bases are pyrimidines? What is their structure?
Thymine, cytosine and uracil in RNA. They are single ringed structures
What is Chargaff’s rule?
That the percent composition of a purine, is the same as its pyrimidine counterpart.
What are the purine-pyrimidine combos?
A-T or U and G-C
Initiation (DNA)
DNA unzips into the leading strand and the lagging strand. Helicase facilitates it, and SSBPs stabilize the DNA
What is elongation
The leading and lagging strands are synthesized. DNA polymerase III synthesizes the leading strand, DNA polymerase I synthesizes the lagging strand with the help of primase.
What is termination?
The two new DNA molecules wind into their helices. The replication machine is disassembled, and DNA polymerase II proofreads the sequencing
messenger RNA (mRNA)
RNA synthesized from the DNA of genes. It contains the genetic information for protein synthesis and carries it to the protein synthesis machinery. It determines the amino acid sequence of a protein.
Triplet hypothesis
Hypothesis stating that the genetic code is read three nucleotide bases at a time
What is the start amino acid? What is its codon?
Methionine. Its codon is AUG
What are the stop codons?
UGA, UAA, UAG
What are the steps to protein synthesis?
Transcription and Translation
Transcription
The synthesis of RNA from a DNA template.
Three differences between RNA and DNA
RNA uses ribose sugar, DNA uses deoxyribose sugar. RNA uses Uracil instead of Thymine. RNA is single stranded, DNA is double stranded.
Translation
Synthesis of a protein from an mRNA template
Three steps of translation
Initiation, elongation, termination
Initiation (transcription)
RNA polymerase binds tightly to a promoter region on DNA. This unzips the DNA into two strands: antisense strand and the sense strand.
Antisense strand
The template strand. This one is the strand that RNA polymerase uses to create the mRNA
Sense strand
The coding strand. Except for the Thymine, it is identical to the mRNA strand. This strand is useless in protein synthesis.
Promoter region
A sequence of nucleotides in DNA that tells RNA polymerase where to bind and in what orientation
Elongation (transcripton)
RNA polymerase complex works along the DNA molecule to make the mRNA strand. It also works in the 5’ to 3’ direction. As soon as the RNA polymerase starts to move, another polymerase can bind and make another mRNA strand. Hundreds can be made from one gene at the same time. No proofreading
Termination (transcription)
Specific sequences in DNA tells the RNA polymerase where to stop. RNA polymerase detaches and the DNA double helix reforms. This is precursor mRNA or primary transcript
pre-mRNA (primary transcript)
unprocessed mRNA
mRNA processing
Eukaryotes only. A 5’ cap is added, which is a modified G nucleotide. Added to the 5’ end. This is what makes it recognizable to the protein synthesis machine. A 3’ poly-A tail is added. This is a string of about 200 A’s that makes the strand more stable and able to last in the cytoplasm.
Introns
Non-coding regions. Removed.
Exons
Coding regions. When Introns are removed, these are joined together to make mature mRNA
snRNPs and snRNA
Bind to the places where Introns and Exons meet. snRNPs interact with other proteins to make large splicesome complexes and remove introns. Many exons may be used and this allows more proteins to be made from a single gene.
tRNA
Contains an anticodon that complements the mRNA. It has the corresponding amino acid attached to it. Cloverleaf shape. Formed by anticodon group, acceptor stem and aminoacyl.
Ribosomes
Made of rRNA and involved in protein synthesis. They are structures that allow a place for the mRNA and tRNAs to interact
Translation factors
proteins that act as accessory factors and are needed at every stage
anticodon loops
triplet of bases at the bottom end of tRNA. Recognizes and pairs with a codon on mRNA
acceptor stem
3’ end of tRNA molecule that is the side of attachment for the amino acid. Based on anticodon.
aminoacyl-tRNA synthase
attaches the appropriate amino acid to the tRNA. Based on anticodon. One different enzyme for each of the 20 amino acids.
How are codons and anticodons written
Codons: 5’ to 3’
Anticodons: 3’ to 5’
Polyribosome
Structure when there are multiple ribosomes translating a single strand of mRNA
Three steps to translation
Initiation, elongation, termination
Initiation (translation)
Small subunit covers 2 codons to ensure the correct reading frame.
tRNA’s UAC (methionine) bonds to AUG of mRNA. tRNA held in P-site.
Larger subunit binds to smaller subunit and A-site is ready to accept another tRNA and amino acid.
Large subunit
P-site: Stores the growing protein
A-site: Holds the next tRNA with its amino acid.
Large subunit
P-site (peptidly site: Stores the growing protein
A-site (aminoacyl site): Holds the next tRNA with its amino acid.
Movement of ribosomes
Moves 5’-3’ along the mRNA. 3bases at a time and joins amino acids to each other according to the codons
Elongation (translation)
tRNA and it’s amino acid enters A-site and pairs to mRNA strand.
Peptide bond formed between a.a in P-site and a.a in A-site.
Ribosomal unit moves along and releases tRNA without the a.a. Aminoacyl tRNA goes from A-site to P-site.
P-site ejects aminoacyl tRNA and A-site can receive more tRNA with a.a
Termination (translation)
At stop codons.
No tRNA to enter A-site anymore. Protein release factors bind to the stop codon, water molecule added at the end of the chain instead of a.a.
Ribosomal subunits separate and peptide chain released. All of this powered by GTP
Single-gene mutations
Involve changes to the nucleotide sequence of one gene.
Chromosomal mutations
Involve a whole chromosome. May involve multiple genes.
Point mutation
Mutation caused by deletion, substitution or addition of a base PAIR. Deletion and addition shifts the whole thing. Substitution may have a very mild effect.
Frameshift mutation
When the number of nucleotides added or deleted is not divisible by three, it causes the whole thing to shift and changes the reading frame
