Chapter 12- Transcription and translation. 

Describe the flow of genetic information and recognize that sometimes it ends with RNA molecules that have specialized jobs (not every RNA is translated into a polypeptide).

DNA → RNA → Protein

• DNA stores genetic information.

• RNA carries the instructions.

• Proteins are made based on these instructions and perform various functions in the cell.

But Not All RNA Becomes Protein!

Some RNA molecules have specialized jobs and are not translated into proteins. Examples include:

• Transfer RNA (tRNA) – Helps bring the right amino acids to the ribosome during translation.

• Ribosomal RNA (rRNA) – Helps form the structure of ribosomes, which build proteins.

• MicroRNA (miRNA) & Small Interfering RNA (siRNA) – Help control gene expression by blocking or destroying mRNA.

Identify the molecular differences between DNA and RNA.

• RNA is chemically similar to DNA but RNA has a ribose sugar

• RNA has the base uracil (U) instead of thymine (T)

• RNA is usually single stranded

Know which strand of a gene RNA Polymerase reads and in what orientation it synthesizes RNA.

Reading Direction: RNA polymerase moves along the template strand in the 3' to 5' direction.

Synthesizing Direction: It builds the RNA strand in the 5' to 3' direction (adding nucleotides to the 3' end).

Know how to transcribe a DNA sequence into RNA.

RNA sequence is complementary to the template strand identical to coding strand

ATGCGCCATG

UACGCGGAUG

Distinguish a coding strand from a template strand and how it resembles an RNA.

The coding strand looks like the RNA (except for T/U swap).

The template strand is what RNA polymerase reads.

RNA is synthesized 5' to 3', while the template strand is read 3' to 5'.

Define the function and limitations of RNA polymerase – who helps it get started with transcription?

RNA polymerase is synthesized by mRNA polymerase

RNA polymerase binds to a DNA promoter sequence to start transcription

Describe the steps of RNA processing and know why they happen.

5' Cap Addition

• A modified guanine (G) nucleotide is added to the 5' end of the pre-mRNA.

• Why?

  • Protects mRNA from degradation.

  • Helps ribosomes recognize the mRNA for translation.

Poly(A) Tail Addition

• A chain of adenine (A) nucleotides (about 50-250) is added to the 3' end.

• Why?

  • Increases mRNA stability.

  • Helps with nuclear export and translation efficiency.

Splicing (Removing Introns)

• Introns (non-coding regions) are removed, and exons (coding regions) are joined together.

• This is done by a spliceosome, a complex of proteins and small RNAs.

• Why?

  • Only exons contain instructions for proteins.

  • Allows for alternative splicing, where different proteins can be made from one gene.

Be able to explain how translation occurs, the steps, the components involved and what it produces.

1. initiation: brings together mRNA, a tRNA with the first amino acid and two ribosomal units 

2. elongation (5´to 3´): amino acids are added one by one to the previous aminoacid at the C terminus of the growing chain

   1. each addition involves proteins called elongation factors 

   2. occur in 3 steps 

      1. codon recognition 

      2. peptide bond formation 

      3. translocation (moves from P site to E site and leaves)

3. termination: when a stop codon in the mRNA reaches the A site of the ribosome 

   1. A site accepts a protein called release factor 

   2. release factor causes the addition of water instead of amino acid. 

transcript is generated through complementation with the template strand (3´to 5´)

mRNA (messenger RNA) – Carries the instructions (codons) for making a protein.

tRNA (transfer RNA) – Brings the correct amino acids to the ribosome.

Ribosome – The "machine" that assembles proteins, made of rRNA and proteins.

Amino acids – The building blocks of proteins, linked together by peptide bonds.

Enzymes & Factors – Help with initiation, elongation, and termination

Explain why rRNAs and tRNAs are necessary for translation.

• rRNA builds the ribosome and catalyzes peptide bond formation.

• tRNA ensures the correct amino acids are added in the right sequence.

• Together, they allow the cell to accurately and efficiently produce proteins!

Re: the genetic code: know how many amino acids (20), how many different codons (64), and how are codons read (non-overlapping, 3-nucleotide words indicating speci+ic amino acids); though multiple codons = the same amino acid (code is redundant for the sake of mutations); no codon = more than 1 amino acid.

Start codon is AUG (methionine, M); stop (amber) codons are UAA, UAG, UGA. 

Know how to TRANSLATE mRNA into protein by reading the codon chart/table.

Know ribosome parts and function; P, A, and E sites and what happens at each site (how initiation happens, the mechanism of elongation, and what promotes termination).

Ribosome Parts & Functions

• P site: Holds the growing polypeptide chain.

• A site: Where new tRNA with an amino acid enters.

• E site: Where empty tRNA exits.

Translation Mechanism

1. Initiation: Small subunit binds mRNA, initiator tRNA (Met/fMet) binds the start codon at the P site, and the large subunit joins.

2. Elongation: New tRNA enters A site → Peptide bond forms → Ribosome shifts → Empty tRNA exits via E site.

3. Termination: Stop codon in A site → Release factor binds → Ribosome disassembles, releasing the protein.