Protein Synthesis (DP IB Biology)

The four bases in RNA are:

• Adenine

• Uracil

• Cytosine

• Guanine

A polypeptide is a chain of amino acids covalently bonded together with peptide bonds.

A codon is a sequence of three mRNA bases that codes for a specific amino acid.

False.

Translation occurs in the cytoplasm, transcription occurs in the nucleus.

tRNA binds with specific amino acids and brings them to the mRNA on the ribosome.

Ribosomal RNA (rRNA) is part of the ribosome structure and helps catalyse the formation of peptide bonds in proteins.

An anticodon is a sequence of three bases on the tRNA molecule that are complementary to a codon on mRNA.

False.

The codon GAC on mRNA would pair with the anticodon CUG on tRNA.

A stop codon is a specific sequence of mRNA bases that signals the end of translation.

After leaving the nucleus, the mRNA attaches to a ribosome in the cytoplasm for translation.

The ribosome facilitates the binding of tRNA to mRNA and catalyses the formation of peptide bonds to synthesize a polypeptide.

A triplet code is a sequence of three DNA nucleotide bases that codes for one amino acid.

False.

Each amino acid can be coded by multiple triplets, making the genetic code degenerate.

A codon is a sequence of three mRNA bases that codes for a specific amino acid.

A degenerate code means that multiple codons can code for the same amino acid.

The start codon TAC codes for methionine and signals the beginning of a gene in protein synthesis.

True.

The genetic code is universal, meaning the same triplets code for the same amino acids in all living organisms.

A stop codon signals the end of translation, indicating where the polypeptide chain should stop being synthesized.

The genetic code being non-overlapping means each base is read only once in a single codon, preventing overlap in the sequence interpretation.

ATP provides the energy required for forming peptide bonds between amino acids during translation.

Binding with two tRNA molecules at a time allows the formation of a peptide bond between adjacent amino acids to accurately build up the amino acid sequence in the polypeptide chain.

A gene mutation is a change in the sequence of bases in a DNA molecule that may result in a new allele.

False.

Most mutations are harmful or neutral (have no effect).

A point mutation is a mutation where one base in the DNA sequence is altered, potentially changing the amino acid at that location.

False.

Mutations in body cells are not inherited; only mutations in gametes are passed on to offspring.

An allele is a variant form of a gene that may result from a mutation and can lead to different traits or conditions.

As the genetic code is degenerate, a change in nucleotide base sequence may still code for the same amino acid despite being a different codon. This means that some mutations will have no impact on the amino acid sequence or the structure of the polypeptide that is produced.

The three stages of mRNA synthesis are initiation, elongation, and termination.

RNA polymerase adds the 5' end of free RNA nucleotides to the 3' end of the growing mRNA molecule, occurring in the 5' to 3' direction.

Transcription is terminated when RNA polymerase reaches a terminator sequence, which causes the polymerase enzyme and mRNA strand to detach.

mRNA is read in the 5' to 3' direction during translation at the ribosome.

The promoter is a DNA sequence that acts as the binding site for RNA polymerase to initiate transcription.

General transcription factors in eukaryotes bind directly to the promoter to help initiate transcription by allowing RNA polymerase to attach.

Non-coding DNA sequences like enhancers and silencers are involved in the control of gene expression by enhancing or suppressing transcription.

Telomeres, the repeated sequences at the ends of chromosomes, facilitate the binding of an RNA primer to allow for the synthesis of the final Okazaki fragment during replication.

Post-transcriptional modification helps prevent mRNA degradation and increases the efficiency of protein synthesis, as well as expanding the complexity of the proteome.

Prokaryotic mRNA does not require significant post-transcriptional modification since translation can occur immediately after transcription, while eukaryotic mRNA undergoes several modifications before translation.

The three post-transcriptional events are:

1. Adding a methylated cap to the 5' end

2. Adding a poly-A tail to the 3' end

3. Removing non-coding introns and joining the coding exons

Exons are the coding sequences that are translated into proteins, while introns are the non-coding sequences that are removed during splicing.

During splicing, the introns (non-coding sections) are removed and the exons (coding sections) are joined together to form the mature mRNA molecule.

Alternative splicing allows different combinations of exons to be included in the final mature mRNA, enabling a single gene to code for multiple protein variants.

Alternative splicing is one reason why the proteome, the set of proteins expressed by a genome, is much larger than the genome itself.

Pre-mRNA is the initial transcript that includes both exons and introns, while mRNA is the mature form that only contains the joined exons after intron removal.

The methylated cap is added to the 5' end to protect against degradation by exonucleases.

The poly-A tail (long chain of adenine nucleotides) is added to the 3' end for further protection and to help the transcript exit the nucleus.

The three tRNA binding sites on a ribosome are the E (exit), P (peptidyl), and A (aminoacyl) sites.

At the A site the mRNA codon joins with the tRNA anticodon.

At the P site the amino acids attached to the tRNA are joined by peptide bonds.

At the E site the tRNA exits the ribosome.

The polypeptide must be modified post-translation in order to be transformed from the just the primary structure into a functional protein.

Protein folding is an example of post-translation modification where the primary protein structure is transformed into the secondary, tertiary and quaternary structures. This includes the formation of disulfide bonds in the tertiary and quaternary stages.

Folding can require molecular chaperones that help to prevent incorrect folding.

Proteolysis is the process of breaking down proteins into their constituent amino acids using enzymes called proteases.

The proteasome is an organelle that contains proteases and acts as the location for proteolysis in the cell.

Tagging proteins with ubiquitin begins the process of them being broken down in the proteasome.