Unit 5 Protein Synthesis

Central Dogma

Describes the flow of genetic information in a cell: DNA → RNA → Protein. DNA stores genetic information, RNA is the intermediate message, and proteins are the final functional products that do work in the cell.

Two Main Processes of Protein Synthesis

(1) Transcription – making an RNA copy of a DNA segment to form mRNA in the nucleus; (2) Translation – using the mRNA sequence at a ribosome in the cytoplasm to build a polypeptide (protein).

Why mRNA is Necessary

DNA cannot leave the nucleus, but ribosomes that build proteins are in the cytoplasm. mRNA is needed to carry genetic instructions from DNA in the nucleus to ribosomes in the cytoplasm, where the instructions are used to make proteins.

Transcription (Definition)

The process of using a DNA template to make a complementary RNA strand, usually mRNA, which carries genetic information out of the nucleus to the ribosome.

Four Stages of Transcription

(1) Initiation – RNA polymerase binds to DNA; (2) Elongation – RNA polymerase builds the RNA strand; (3) Termination – transcription stops at a termination signal; (4) Post-termination processing – the RNA is modified to become mature mRNA.

First Stage of Transcription

Initiation

Second Stage of Transcription

Elongation

Third Stage of Transcription

Termination

Fourth Stage of Transcription

Post-termination processing

Transcription Initiation

RNA polymerase binds to a promoter region on the DNA. The promoter is a specific DNA sequence that signals the starting point for transcription and allows RNA polymerase to begin RNA synthesis.

Transcription Elongation

RNA polymerase moves along the DNA template strand, synthesizing a complementary RNA strand by adding RNA nucleotides to the 3′ end. As it moves, it briefly untwists the DNA double helix, exposing 10–20 bases at a time.

Transcription Termination

Transcription ends when RNA polymerase reaches a termination signal in the DNA. This signal causes RNA polymerase to stop, release the newly formed RNA molecule, and detach from the DNA.

Purpose of Post-Transcriptional Processing

Modifies the initial RNA transcript (pre-mRNA) to form mature mRNA that is protected from damage, stable in the cytoplasm, and properly edited so it contains only coding regions (exons).

5′ Cap Addition

A protective cap is added to the 5′ end of the RNA. The 5′ cap protects mRNA from degradation and helps the ribosome recognize and bind to the mRNA during translation.

Poly-A Tail Addition

This consists of many adenine nucleotides added to the 3′ end of the RNA. Helps stabilize the mRNA and assists in its export from the nucleus to the cytoplasm.

RNA Splicing

RNA splicing removes introns and joins exons.

Introns

Noncoding “junk” sequences that are cut out of pre-mRNA.

Exons

Coding sequences that remain and are spliced together to form the final, continuous coding sequence in mature mRNA.

Translation (Definition)

RNA-directed synthesis of a polypeptide (protein). The ribosome reads mRNA codons, tRNA brings specific amino acids, and the amino acids are linked together to form a polypeptide chain.

Requirements for Translation

mRNA (carries the codons), ribosomes (made of rRNA and proteins, the site of protein synthesis), tRNA (transfers specific amino acids to the ribosome), and the genetic code (rules that match codons to amino acids).

Codon (Definition)

A sequence of three RNA nucleotides on mRNA that codes for a specific amino acid or a stop signal. There are 64 possible codons, but only 20 amino acids, so many amino acids have multiple codons.

Start Codon and Stop Codons

The start codon is AUG, which codes for Methionine (Met) and marks the start of translation. Stop codons are UAG, UAA, and UGA; they do not code for amino acids and signal the end of translation and the protein.

mRNA (Messenger RNA)

A single-stranded RNA molecule complementary to a DNA template strand. It carries genetic instructions from DNA in the nucleus to ribosomes in the cytoplasm, where its codons are read to assemble a protein.

tRNA (Transfer RNA)

A small RNA molecule (about 80 nucleotides) with two important regions: an amino acid attachment site and an anticodon. It carries specific amino acids to the ribosome and uses its anticodon to pair with matching codons on mRNA.

Anticodon (Definition)

A sequence of three bases on tRNA that is complementary to a specific mRNA codon. It allows tRNA to recognize and bind to the codon and deliver the correct amino acid during translation.

rRNA (Ribosomal RNA)

Together with proteins, they make up ribosomes. Ribosomes have two subunits and provide the site where mRNA and tRNA interact, helping match codons with anticodons and catalyze peptide bond formation between amino acids.

Three Stages of Translation

(1) Initiation – assembly of the ribosome, mRNA, and initiator tRNA at the start codon; (2) Elongation – addition of amino acids one by one to the growing polypeptide; (3) Termination – the process stops at a stop codon and the polypeptide is released.

First stage of translation

Initiation

Second stage of translation

Elongation

Third stage of translation

Termination

Translation Initiation

During initiation, the ribosome binds to the mRNA and scans until it finds the start codon AUG. A tRNA carrying Methionine binds to AUG, and the large ribosomal subunit joins, forming the complete ribosome to begin translation.

Translation Elongation

During elongation, the ribosome moves along the mRNA codon by codon. At each codon, a tRNA with the complementary anticodon brings its amino acid. The ribosome forms peptide bonds, adding each amino acid to the growing polypeptide chain.

Translation Termination

Termination occurs when the ribosome reaches a stop codon (UAG, UAA, or UGA). A release factor binds to the stop codon, causing the ribosome to release the completed polypeptide and detach from the mRNA.

Ribosome A, P, and E Sites

The ribosome has three tRNA binding sites: A site (aminoacyl site) where new tRNA with an amino acid first binds; P site (peptidyl site) holding the tRNA with the growing polypeptide; E site (exit site) where empty tRNA leaves the ribosome.

Mutation (General Definition)

A change in the DNA sequence. It can alter the order of bases, which may change the resulting protein’s amino acid sequence and possibly its structure and function.

Mutation Frequency and Impact

Genes typically incur base substitutions about once in 10,000 to 1,000,000 cells. With about 6 billion DNA bases per human cell, each cell likely carries several mutations. Most mutations are neutral, only mutations in gamete-forming cells are passed to offspring, and somatic mutations mainly cause problems when they contribute to cancer or similar diseases.

Mutagens

Chemical or physical agents that increase the rate of mutations. They interact with DNA and can cause base changes, insertions, deletions, or breaks; examples include certain chemicals and different types of radiation.

Point Mutation (Definition)

A change in a single gene, usually involving the substitution of one base pair for another. It can significantly affect the protein encoded by that gene and may occur spontaneously or due to DNA damage from mutagens.

Base-Pair Substitution

A point mutation where one nucleotide and its complementary partner are replaced by another pair. It can result in silent, missense, or nonsense mutations depending on how the codon and resulting amino acid are affected.

Silent Mutation

A base-pair substitution that changes a codon but does not change the amino acid it codes for, due to redundancy in the genetic code. It usually does not affect the protein’s structure or function.

Missense Mutation

A base-pair substitution that changes a codon so that it codes for a different amino acid. This can alter the protein’s structure and function, sometimes slightly and sometimes severely.

Nonsense Mutation

A base-pair substitution that changes a codon for an amino acid into a stop codon. This causes premature termination of translation and usually produces a shortened, nonfunctional protein.

Frameshift Mutation (Definition)

This is caused by insertions or deletions of nucleotides that are not in multiples of three. This shifts the reading frame of the mRNA, changing all downstream codons and almost always resulting in a severely altered, nonfunctional protein.

Insertions and Deletions

Add one or more nucleotides to the DNA sequence; deletions remove one or more nucleotides. When they are not in multiples of three, they cause frameshift mutations, altering the grouping of codons and drastically changing the resulting protein.