Transcription

Q: What are the three stages of making proteins?

A:
• DNA Replication – makes DNA
• Transcription – DNA makes mRNA
• Translation – RNA makes proteins

Q: What does the One Gene–One Polypeptide Hypothesis state?

• This theory ( by Beadle and Tatum) states that each gene is unique and codes for the synthesis of a single polypeptide

• If any part of a gene is affected, the protein/ polypeptide cannot be produced

General Overview of

Transcription

• Transcription is the process where DNA codes and produces RNA

• In other words, information encoded by DNA is transcribed to make a complementary RNA strand

  • This is necessary as DNA cannot directly make proteins as it cannot leave the nucleus

  • Therefore, RNA acts as a messenger

• This process occurs in the nucleus of the cell

DNA

• Double-stranded
• Adenine pairs with Thymine
• Guanine pairs with Cytosine
• Contains deoxyribose sugar (H on the 2’ carbon)

RNA

• Single-stranded
• Adenine pairs with Uracil
• Guanine pairs with Cytosine
• Contains ribose sugar (OH on the 2’ carbon)

Q: What are the major types of RNA?

A: messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA)

1. Messenger RNA (mRNA)

Characteristics and key functions:
• varies in length, depending on the gene that has been copied
• acts as the intermediary between DNA and the ribosomes
• is translated into protein by ribosomes
• is the RNA version of the gene encoded by DNA

2. Transfer RNA (tRNA)

Characteristics and key functions:
• functions as the delivery system of amino acids to ribosomes as they synthesize proteins
• is very short, only 70 to 90 base pairs long

3. Ribosomal RNA (rRNA)

Characteristics and key functions:

• binds with proteins to form the ribosomes
• varies in length

Q: What are the 3 stages of transcription?

A:
• Initiation – unwinding DNA
• Elongation – building new complementary RNA strand
• Termination – ending transcription

Initiation

• RNA polymerase binds to the promoter region of DNA
 – This is a specialized sequence on one strand of DNA, located upstream from the start of a gene

• More specifically, it attaches to the TATA box in the promoter region
 – This region has a high concentration of A–T bonds
 – This makes it recognizable for RNA polymerase to attach

• A–T bonds only have 2 hydrogen bonds, making it easier for RNA polymerase to break
 – Now DNA is open

Elongation

• RNA polymerase begins adding nucleotides in the 5’ to 3’ direction, using the 3’ to 5’ DNA strand
 – Unlike DNA, no primer is needed

• The other strand of DNA is known as the coding strand
 – Contains the same base pair sequence of RNA, with the exception of Uracil  

  - The coding strand has thymine instead

Elongation (continued)

• While the RNA strand elongates, it temporarily winds with the DNA strand
 – Creates an RNA–DNA helix

• Eventually, the RNA strand unwinds and DNA reforms with the other strand

• There can be multiple RNA polymerases completing this stage at a time

Termination

Transcription ends when the RNA polymerase reaches a termination sequence

• This step can happen in different ways:
 – A protein binds to the mRNA to stop the process (prokaryotes)
 – mRNA binds to itself in a hairpin structure, causing the loop to stop the process (prokaryotes)
 – A string of adenines is reached, causing a protein to stop the process (eukaryotes)

Post-Transcriptional Modifications

• The newly synthesized pre-mRNA strand is vulnerable to enzymes and external conditions of the cell

• As a result, it must undergo modifications to exit the cell

• The modifications include:
 • Poly-A tail
 • 5’ cap

Modifications: Q: What is a poly-A tail and what is its purpose?

• 50–250 adenines added to the 3’ end by poly-A-polymerase.
• Protects mRNA from RNA-eating enzymes.

Modifications: Q: What is the 5’ cap and what is its purpose?

A:
• Seven G’s are added to the 5’ end.
• Allows mRNA to move to translation.

Introns and Exons

• Further modifications are done after tailing and capping

• Eukaryotic DNA consists of both:
 • Introns (non-coding regions)
 • Exons (coding regions)

• Since introns are non-coding, they must be removed
 • If kept in, they can alter the sequence to code for a protein

Splicing Introns

• Ribonucleoprotein (snRNP’s) are able to identify introns

• Form complementary pairings at the junctions of the intron and exons

• Other snRNP’s attach

• Causes intron to loop and bring the exons together

• Spliceosome is formed

• The spliceosome cleaves the introns and attaches the exons together