Genetic Code and Genome Flashcards
Transcription and Translation Definition
Transcription: involves copying the code for a gene into mRNA
Translation: involves interpreting the code into a sequence of amino acids
mRNA function
The template for protein synthesis (aka translation): codes for proteins (codons)
difference between mRNA in prokaryotes and eukaryotes?
mRNA in prokaryotes: can be produced for each gene or group of genes (aka operon) that is to be expressed in E. Coli
- operon: a group of genes that can be transcribed together into a single mRNA
- multiple genes –> one mRNA
mRNA in eukaryotes: a distinct mRNA is produced for EACH gene
- one gene –> one mRNA
- Has structural features (ie. Stem-loop structure) that regulate the efficiency of translation and the lifetime of the mRNA
tRNA function
Serves as adapters between mRNA and amino acids during protein synthesis
- Carries amino acids in an activated form to the ribosome for the peptide-bond formation in a sequence dictated by the mRNA template
- Reacts with specific amino acids in a reaction catalyzed by aminoacyl-tRNA synthetases
- there is at least 1 tRNA molecule for each amino acid
rRNA (ribosomal)
forms the core of the ribosome’s structure and catalyze protein synthesis (the major component of ribosomes)
What are 7 general characteristics of tRNA molecules?
- Each tRNA molecule is a single strand of RNA between 73-93 ribonucleotides long
- The 3D structure of the molecule is L-shaped
- tRNA molecules contain UNUSUAL BASES (ie. inosine) or bases that have been modified
- In a 2D representation, all tRNA molecules appear as a CLOVERLEAF PATTERN that is stabilized by many hydrogen bonds
- The 5’ end is phosphorylated; the 5’ terminal residue is usually pG (guanine)
- The amino acid is attached to a hydroxyl group (OH) of ADENOSINE in the CCA region of the acceptor stem
- The ANTICODON is in a loop near the center of the sequence
What is the general 3D structure and 2D representation of a tRNA molecule
The 3D structure of the molecule is L-shaped
In a 2D representation, all tRNA molecules appear as a CLOVERLEAF PATTERN that is stabilized by many hydrogen bonds
Where is the anticodon located in a general tRNA molecule?
The ANTICODON is in a loop near the center of the sequence
anticodon
a template recognition site in tRNA molecules that consists of 3 bases and recognize a complementary 3-base sequence in the mRNA (aka the codon)
anticodons base pair with codons
What can Inosine pair with?
Inosine can pair with cytosine, uracil, or adenine
if the template DNA strand is 3’-ACC-5’, what is the coding DNA strand, mRNA codon, and tRNA anti-codon sequences?
DNA coding strand: 5’-TGG-3’
mRNA codon: 5’-UGG-3’
tRNA anticodon: 3’-ACC-5’
aminoacyl-tRNA synthetases function
synthesize aminoacyl-tRNAs
- aka catalyzes the attachment of the appropriate amino acid to the tRNA molecule to produce a charged tRNA (aka aminoacyl-tRNAs)
aminoacyl-tRNAs
the amino acid substrates for protein synthesis
aka the charged tRNA formed after an ester linkage is formed between the carboxyl group of the amino acid and either 1. the 2’ or 2. 3’ hydroxyl group of the terminal ADENOSINE of the TRNA
what is the general structure of an aminoacyl-tRNA?
- The amino acid is attached at the 3’ end of the RNA; the anticodon is the template-recognition site
- tRNA has a cloverleaf structure with many HYDROGEN bonds (shown as green dots) between bases
how is the mRNA decoded in protein synthesis? (translation)
mRNA is decoded in the 5’ to 3’ direction
decoded one codon at a time
how do aminoacyl tRNA synthetases read the genetic code?
aminoacyl-tRNA synthetases catalyze the activation of amino acids
In order to be incorporated into proteins, amino acids must be ACTIVATED
–> Amino acids are activated by the formation of an ester linkage between…
The carboxyl group of the amino acid and…
Either the 2’ or 3’ hydroxyl (OH) group of the terminal adenosine of the tRNA
→ the ester linkage forms an aminoacyl tRNA (aka charged tRNA)
Each aminoacyl-tRNA synthetase is specific for a particular amino acid
What are the 2 chemical reactions/steps for the activation of amino acids?
- The formation of aminoacyl adenylate (aka aminoacyl-AMP)
Amino acid + ATP → aminoacyl-AMP + PPi - The aminoacyl group is transferred to a specific tRNA recognized by the synthetase
aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP
→ the sum of the two reactions:
amino acid + ATP + tRNA → aminoacyl-tRNA + AMP + PPi
Charging
Charging: the process where each aminoacyl tRNA synthetases couples a particular amino acid to the proper tRNA
- Each aminoacyl-tRNA synthetase is specific for a particular amino acid
What and how is the bond formed to activate amino acids?
–> Amino acids are activated by the formation of an ester linkage between…
The carboxyl group of the amino acid and either the 2’ or 3’ hydroxyl (OH) group of the terminal adenosine of the tRNA
→ the ester linkage forms an aminoacyl tRNA (aka charged tRNA) that can be incorporated into proteins
what is aminoacyl adenylate?
aminoacyl adenylate is the ACTIVATED form of the amino acid (aka the form that will work thermodynamically in peptide-bond synthesis reactions)
- Acyl adenylate intermediates are also prominent in fatty acid activation for fatty acid synthesis reactions
what is threonyl-tRNA synthetase and what does it contain in its active site?
an example of an aminoacyl tRNA synthetase: that is responsible for attached the amino acid threonine to its corresponding tRNA during translation
- threonyl-tRNA synthetase contains a ZINC ION at the active site that interacts with the hydroxyl (OH) group of threonine
How do amino acids valine and serine interact with threonyl-tRNA synthetase?
Valine: valine is similar in overall structure to threonine but LACKS the OH group → valine is NOT joined to tRNA
Serine: although serine is smaller than threonine, it is occasionally linked to tRNA because of the presence of the hydroxyl OH group
What is the “code” between DNA and amino acids?
There are FOUR nucleotides (A, G, C, T or U) but only TWENTY amino acids (21 counting selenocysteine)
the mRNA is read in groups of 3 (aka codons)
what does the genetic code link?
The genetic code LINKS the nucleic acid sequence info and the amino acid sequence info
why are there only 20 amino acids, but 64 different codons?
- genetic code is degenerate: most amino acids are specified by more than one codon
- among the 64 codons, 3 codons are stop codons
- wobbling: the 3rd position in a codon is less critical for defining the amino acid –> allows tRNA molecule with an anticodon that matches 2 or more codons to still bind effectively
ie. the codons GGU, GGC, GGA, and GGG all code for Glycine and a single tRNA with the anticodon CCI (inosine) can pair with all 4 Glycine codons
inosine
The base in inosine is hypoxanthine, which is structurally similar to adenine but lacks an amino group (NH2)
Formation: Inosine is created by a post-transcriptional modification in tRNA, where an enzyme deaminates adenosine (converts the amino group on adenine to a keto group) at specific positions in the anticodon loop of the tRNA molecule
Nobel prize in physiology or medicine 1968
Awarded to Robert W. Holley, Har Gobind Khorana, and Marshall W. Nirenberg for “their interpretation of the genetic code and its function in protein synthesis”
what are the 5 characteristics of the genetic code?
- 3 nucleotides (aka a codon) encode an amino acid
- The genetic code is NON OVERLAPPING
- A base in a mRNA is NOT used for 2 different codons (aka 3 bases of one codon are DISTINCT from the bases of adjacent codons)
- Ie. a sequence UGGAUCGAU is read UGG AUC GAU, and not as UGG GGA GAU, etc… - The genetic code as no punctuation: its representation is continuous and there is no punctuation mark to separate two codes
- The code has DIRECTIONALITY: the code is read from the 5’ end to the 3’ end of the mRNA
- The code is DEGENERATE: some amino acids are encoded by MORE than one codon
- main idea: 444 = 64 codons > 20 amino acids
what are the benefits of the genetic code being degenerate?
→ minimizes the deleterious effects of mutations
This redundancy provides flexibility and error tolerance in the genetic code. Mutations that change one base in a codon may still result in the same amino acid being produced, helping reduce the likelihood of harmful effects from single-point mutations.
what are synonyms in respect to codons?
Synonyms: codons that specify the SAME amino acid (ie. CAU and CAC are synonyms for histidine)
The genetic code is nearly universal among most organisms but some organisms have slight modifications. give two examples of variations in genetic code.
Ie. in ciliated protozoa: codons that are stop signals in most organisms DO encode amino acids
Ie. Mitochondria also uses variations of the genetic code
wobble
a phenomenon where the recognition of the THIRD (3’-most) base in the codon by the ANTICODON is sometimes less discriminating
- 3rd position in the codon <==> 1st position in the anticodon
Wobble base pairing explains why many alternative codons differ in the 3rd molecule
why is wobble tolerated in the 3rd position of the codon?
because although ribosomal RNA monitors fidelity of the base pairing in the 1st two positions of the codon-anticodon complex, ribosomal RNA does NOT evaluate the correctness of the 3rd position
2 Generalizations of the codon-anticodon interactions are…
- Codons that differ in either the 1st or 2nd position nucleotides MUST be recognized by DIFFERENT tRNAS
- The 1st (5’-most) base of the ANTICODON determines the degree of wobble
if the 1st base is INOSINE, the anticodon can recognize THREE codons: inosine can pair with C, U, or A
Allowed pairings at the 3rd base of the codon according to the wobble hypothesis (given first base of anticodon, what can the third base of the codon be?)
C –>
A –>
U –>
G –>
I –>
C –> G
A –> U
U –> A or G
G –> U or C
I –> U, C or A
How do mRNA, tRNA, and rRNA differ in terms of abundance in cell, composition, stability, and structure?
abundance
- mRNA: lowest
- tRNA: slightly higher than mRNA
- rRNA: most abundance
composition
- mRNA: AG CU
- tRNA: AG CU and about 40 unusual bases
- rRNA: AG CU
stability
- mRNA: unstable
- tRNA: very stable due to intrastrand pairing
- rRNA: very stable due to lots of intrastrand pairing
structure
- mRNA: mostly single stranded
- tRNA: 50% double stranded, 50% single-stranded (2D cloverleaf structure; 3D L-shaped structure)
- rRNA: mostly double stranded (the precursor sequences form extended double-stranded regions at the termini of each rRNA species –> these regions are important structural determinants for various processing events)
what are the 3 stop codons?
TAA - UAA
TAG - UAG
TGA - UGA
what is the start codon
AUG
how do the 3 eukaryotic RNA polymerases differ in terms of genes transcribed and relative abundance of total mass in cells?
RNA polymerase I: most rRNA genes and around 98% abundance
RNA polymerase II: all protein-coding genes (mRNA), miRNA genes, and genes for other noncoding RNAs (ie. those of the spliceosome) and < 1% abundance
RNA polymerase III: tRNA genes, 5S rRNA genes, genes for many other small RNAs and around 1% abundance
where on the tRNA molecule is the amino acid attached to?
The amino acid is attached to a hydroxyl group (OH) of ADENOSINE in the CCA region of the acceptor stem
what are the 6 components of a regular tRNA molecule?
- 3’ terminus –> CCA region: (OH) amino acid attachment site
- phosphorylated 5’ terminus
- DHU loop (on the left)
- TψC loop
- anticodon loop
- “extra arm” (variable)
how are the CCA region and anticodon loop located relative to each other on the tRNA molecule?
The CCA region is at the end of one arm and the anticodon loop is at the end of the other arm
codon wobble explains what?
Wobble base pairing explains why many alternative codons differ in the 3rd molecule: have more than 20 tRNA (about 31 tRNA but varies between species) for 20 amino acids to their 61 codons
TψC loop of tRNAs function
interacts with rRNA and proteins within the ribosome to position the tRNA correctly during translation
plays a role in recognition by aminoacyl-tRNA synthetases which attach the correct amino acid to the tRNA
helps maintain the L shaped 3D structure of tRNA
