chapter 8 part 4

Transcription and Translation

Transcription: The Process of RNA Synthesis

Transcription is the fundamental process by which cells convert genetic information from DNA into RNA. This process is essential for gene expression and is carried out by the enzyme RNA polymerase.

• Transcription: The synthesis of RNA from a DNA template.

• RNA polymerase: The enzyme responsible for synthesizing RNA; it does not require a primer, unlike DNA polymerase.

• Promoter: A specific DNA sequence where RNA polymerase binds to initiate transcription.

• Terminator: A DNA sequence signaling the end of transcription.

• Directionality: RNA is synthesized in the 5' → 3' direction.

Types of RNA:

• mRNA (messenger RNA): Carries genetic information from DNA to the ribosome for protein synthesis.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

• rRNA (ribosomal RNA): Structural and catalytic component of the ribosome.

Example: In Escherichia coli, RNA polymerase binds the promoter region of a gene, synthesizes mRNA, and stops at the terminator sequence.

RNA Processing in Eukaryotes

In eukaryotic cells, newly synthesized mRNA undergoes processing before it can be translated. This includes the removal of noncoding regions and the joining of coding regions.

• Exons: Coding regions that remain in mature mRNA after processing.

• Introns: Noncoding regions that are removed from the pre-mRNA.

• RNA processing: Modifications to eukaryotic mRNA, including splicing, before translation.

• Ribozymes: RNA molecules that catalyze the removal of introns (splicing).

Comparison Table: RNA Processing in Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Introns	Absent	Present; removed before translation
RNA Processing	Minimal	Extensive (splicing, modification)
Ribozymes	Rare	Used for splicing

Example: In human cells, introns are removed from pre-mRNA by ribozymes before the mRNA exits the nucleus.

Translation: Protein Synthesis from mRNA

Translation is the process by which ribosomes synthesize proteins using the information encoded in mRNA. This process involves decoding the mRNA sequence into a chain of amino acids.

• Translation: The synthesis of a protein from mRNA.

• Codon: A sequence of three nucleotides on mRNA that specifies an amino acid.

• Start codon: AUG (codes for methionine; signals the beginning of translation).

• Stop codons: UAA, UAG, UGA (signal the end of translation).

Example: The mRNA sequence AUG-GGC-UAA is translated into a short peptide starting with methionine and ending at the stop codon.

Translation Process: Steps and Ribosome Function

The translation process involves the assembly of ribosomal subunits, reading of codons, and the sequential addition of amino acids to the growing polypeptide chain.

1. The small and large ribosomal subunits assemble into an active ribosome.

2. The ribosome reads the first codon (AUG).

3. A tRNA with the complementary anticodon (UAC) enters, carrying the first amino acid (methionine).

4. The ribosome reads the next codon; another tRNA enters with its amino acid.

5. A peptide bond forms between the amino acids.

6. The first tRNA leaves; the ribosome moves to the next codon.

7. The protein grows one amino acid at a time.

8. When a stop codon (e.g., UGA) is reached, the completed protein is released, and the ribosome either dissociates or binds another mRNA.

Ribosome Sites:

• A Site: Entry point for incoming tRNA with amino acid.

• P Site: Holds the growing peptide chain.

Example: During translation, the ribosome moves along the mRNA, facilitating peptide bond formation at the P site while new tRNAs enter at the A site.

Simultaneous Transcription and Translation in Prokaryotes

Prokaryotic cells can perform transcription and translation simultaneously due to the absence of a nucleus. This allows rapid gene expression and protein synthesis.

• Prokaryotes: Transcription and translation occur at the same time; ribosomes can attach to mRNA as it is being synthesized.

• Eukaryotes: Transcription occurs in the nucleus; translation occurs in the cytoplasm after mRNA is exported.

Comparison Table: Transcription and Translation in Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Simultaneous Transcription & Translation	Yes	No
Nucleus	Absent	Present
Location of Transcription	Cytoplasm	Nucleus
Location of Translation	Cytoplasm	Cytoplasm

Example: In Escherichia coli, ribosomes begin translating mRNA while it is still being transcribed by RNA polymerase.

Key Terms and Definitions

• Transcription

• RNA polymerase

• Promoter

• Terminator

• mRNA

• tRNA

• rRNA

• RNA processing

• Exons

• Introns

• Ribozymes

• Translation

• Codon

• Start codon (AUG)

• Stop codons (UAA, UAG, UGA)

• Ribosome

• Large ribosomal subunit

• Small ribosomal subunit

• Anticodon

• Amino acid

• Peptide bond

• A site

• P site

• Simultaneous transcription and translation (prokaryotes)

• Nucleus prevents simultaneous transcription and translation (eukaryotes)

Additional info:

• Transcription and translation are central to gene expression and are covered in detail in Microbial Genetics (Ch. 8).

• RNA processing is a distinguishing feature of eukaryotic gene expression.

• Understanding codons and ribosome function is essential for interpreting genetic code and protein synthesis.