PCB 3063 Lecture 4 Genetic Code and Translation

Where does eukaryotic mRNA modification occur before leaving the nucleus?

In the nucleus.

What is added to the 5' end of eukaryotic mRNA during processing?

A 5' cap.

When is the 5' cap added to the growing mRNA?

Very soon after the start of transcription.

What specific nucleotide is added to the absolute 5' end of mRNA?

A guanine.

What type of linkage connects the 5' cap to the first nucleotide?

A 5'-5' linkage.

How does a 5'-5' linkage differ from normal RNA linkages?

Normal linkages are 5'-3' phosphodiester bonds.

What chemical modification happens to the 5' guanine and first few nucleotides?

They are methylated.

What proteins attach to the completed 5' cap?

Cap-binding proteins.

What is one function of cap-binding proteins regarding mRNA protection?

They protect mRNA from RNases in the cytoplasm.

How do cap-binding proteins assist in translation initiation?

They indirectly allow mRNA to attach to the small ribosomal subunit.

What is added to the 3' end of eukaryotic mRNA during processing?

A poly-A tail.

Why must the 3' end of mRNA be cut before adding the poly-A tail?

Most genes are transcribed beyond the coding sequence.

Which enzyme detects the consensus sequence near the 3' end of mRNA?

Poly(A) polymerase.

What is the consensus sequence recognized by poly(A) polymerase?

AAUAAA.

Where does poly(A) polymerase cut the mRNA relative to the consensus sequence?

Approximately 25 nucleotides downstream.

How many adenine nucleotides does poly(A) polymerase add to the cut end?

50 to 200 adenines.

What is one protective function of the poly-A tail?

It protects the 3' end of mRNA from RNases.

How does the length of the poly-A tail affect eukaryotic mRNA?

Longer tails increase mRNA lifespan and stability.

How does the poly-A tail assist in initiating translation?

It helps in mRNA-ribosome binding.

What are the noncoding sequences in eukaryotic genes called?

Introns.

What are the coding sequences of eukaryotic genes called?

Exons.

What are two possible evolutionary origins of introns?

Retroviruses, transposons, or mutated former exons.

How does intron abundance compare between prokaryotes and eukaryotes?

Introns are common in eukaryotes but rare in prokaryotes.

What is the relationship between organism complexity and intron abundance?

More complex organisms generally have more complex and abundant introns.

True or False: All eukaryotic genes contain the same number of introns.

False. Intron abundance and size vary per gene.

What are the two major classifications of introns based on removal mechanism?

Group I/II self-splicing introns and nuclear pre-mRNA introns.

Where are Group I and Group II introns typically found?

In rRNA genes and a few bacterial genes.

What molecule is used by Group I and II introns to excise themselves?

A molecule of guanosine.

Why are nuclear pre-mRNA introns considered more complex than Group I/II?

They require a spliceosome complex for removal.

What features at intron-exon borders attract the spliceosome?

Consensus sequences.

What process allows one gene to yield multiple different proteins?

Alternative splicing.

What are different protein products generated from the same gene called?

Isoforms.

How does the number of exons in a gene affect potential isoforms?

More exons allow for more potential isoforms.

How many exons and isoforms does the tropomyosin gene have?

14 exons yielding 10 different isoforms.

What does alternative splicing disprove about the "one gene, one protein" hypothesis?

It shows that one gene can code for multiple proteins.

What three features must a processed eukaryotic mRNA have to exit the nucleus?

A 5' cap, a poly-A tail, and no introns.

Through what structures does processed mRNA exit the nucleus?

Nuclear pore complexes.

What specific protein complexes associate with processed mRNA to guide nuclear export?

Messenger ribonucleoproteins (mRNPs).

With which nuclear pore complex do mRNPs interact during export?

The mRNA exporter.

What happens to eukaryotic mRNAs that fail to undergo splicing?

They are retained in the nucleus and not translated.

Which disease is associated with a splicing failure in beta-hemoglobin mRNA?

Thalassemia.

What is the first stage of going from DNA instructions to a protein?

Transcription.

What molecule is synthesized as a complementary copy of DNA during transcription?

Messenger RNA (mRNA).

What is the second stage of gene expression in eukaryotes?

mRNA editing and export.

What is the final stage where ribosomes read mRNA to make proteins?

Translation.

Where does translation take place within the cell?

In the cytoplasm.

What is the set of rules by which ribosomes read mRNA called?

The genetic code.

Why is the genetic code considered to be located in mRNA, not DNA?

Ribosomes read mRNA directly; they do not interact with DNA.

What four ribonucleotide letters make up the alphabet of mRNA?

A, U, C, and G.

In what direction and manner do ribosomes read the mRNA nucleotides?

Linearly, one after another, without skipping.

Who used point mutations to show that ribosomes read nucleotides in groups of three?

Francis Crick and colleagues.

What happens to the reading frame if one nucleotide is inserted?

It shifts completely, altering all subsequent amino acids.

What happens to the reading frame if two nucleotides are inserted?

It shifts completely, altering subsequent amino acids.

What happens to the reading frame if three nucleotides are inserted?

The reading frame is restored after the insertion site.

Why do three-nucleotide insertions restore the reading frame?

Because codons are read in groups of three.

If the genetic code used single-letter words, how many codons would exist?

4.

If the genetic code used two-letter words, how many codons would exist?

16.

Why is a two-letter codon system insufficient for translation?

It only yields 16 codons, but there are 20 amino acids.

How many possible codons exist in a three-letter codon system?

64.

What is the term for a group of three ribonucleotides in mRNA?

A codon.

How are codons spaced along an mRNA strand?

They are continuous with no spaces or breaks.

What does it mean that the genetic code is unambiguous?

Each codon specifies only one single amino acid.

Which amino acid is specified by the codon AUG?

Methionine.

Which amino acid is specified by the codon AAA?

Lysine.

What does it mean that the genetic code is universal?

Almost all species use the exact same codons for the same amino acids.

Which domain of life contains some exceptions to the universal genetic code?

Archaea.

What does it mean that the genetic code is nonoverlapping?

The ribosome reads three bases, then moves down exactly three bases.

How many amino acids change from a single point mutation in a nonoverlapping code?

Only one amino acid.

How many amino acids would change from a single mutation in an overlapping code?

Multiple amino acids.

What does it mean that the genetic code is degenerate or redundant?

Multiple codons can code for the same amino acid.

How many of the 64 codons code for amino acids?

61.

What is "wobble" in the genetic code?

Flexibility allowing the third nucleotide of a codon to change without altering the amino acid.

Which researchers deciphered the genetic code starting in 1961?

Marshall Nirenberg and Heinrich Matthaei.

What did Nirenberg and Matthaei use to synthesize proteins in a test tube?

An in vitro translation system.

What components are required for an in vitro translation system?

Ribosomes, tRNAs, amino acids, translation factors, and mRNA.

What artificial mRNA did Nirenberg and Matthaei use to discover the codon for phenylalanine?

Poly-U.

What amino acid is coded for by the codon UUU?

Phenylalanine.

What artificial mRNA did Nirenberg and Matthaei use to identify the codon for lysine?

Poly-A.

What artificial mRNA did Nirenberg and Matthaei use to identify the codon for proline?

Poly-C.

What is the primary function of transfer RNA (tRNA)?

To transfer specific amino acids to the ribosome.

What shape does a tRNA molecule fold into?

A cloverleaf 3D shape.

What are the two key structural regions of a tRNA molecule?

The amino acid binding site and the anticodon loop.

Where is the amino acid binding site located on a tRNA?

At the single-stranded 3' end.

Which enzymes attach the correct amino acid to its corresponding tRNA?

Aminoacyl-tRNA synthetases.

What is the function of the anticodon loop in tRNA?

To bind complementarily to a specific mRNA codon.

What experimental method was used to assign amino acids to almost all remaining codons?

The triplet binding assay.

What size mRNA is used in a triplet binding assay?

An artificial mRNA of just one codon.

Why was the triplet binding assay able to identify codon-amino acid matches?

The single codon recruits only the tRNA with the complementary anticodon.

How many codons do not code for any amino acid?

Three.

What occurs when a ribosome encounters a stop codon?

Translation terminates and protein synthesis ceases.

What is the universal start codon for translation?

AUG.

What is the first amino acid incorporated into every newly synthesized protein?

Methionine.

What are the two main structural components of a ribosome?

A large subunit and a small subunit.

What two types of macromolecules make up ribosomal subunits?

Proteins and ribosomal RNA.

What is the sedimentation coefficient of a prokaryotic ribosome?

70S.

What are the sizes of the large and small subunits of prokaryotic ribosomes?

50S and 30S.

Which specific rRNA molecule in prokaryotes is commonly used for species identification?

16S rRNA.

What is the sedimentation coefficient of a eukaryotic ribosome?

80S.

What are the sizes of the large and small subunits of eukaryotic ribosomes?

60S and 40S.

How many mRNA binding sites and tRNA binding sites does a ribosome have?

One mRNA binding site and three tRNA binding sites.