Chapter 15 - Transcription & Translation

Used the ch. 15 wrap-up and slides

Order of events for the central dogma

DNA → RNA → Protein

1. Replication on DNA inside the nucleus

2. DNA is transcribed into RNA inside the nucleus

3. RNA is translated into proteins in the cytoplasm

4. Protein synthesis occurs in ribosomes in the cytoplasm

Beadle and Tatum’s experiment

One enzyme, one gene hypothesis - surmised that each gene would influence a specific step in a metabolic pathway

Knowledge formed based on their findings:

1. Genes encode for proteins other than enzymes

2. Genes may encode a subunit

3. Some genes encode for non-coding RNA’s

4. Many genes have more than one exon & can be processed differently to develop different proteins

Define the term gene

specific sequence of nucleotides on a strand of DNA

• aid i the development of specific traits

• usually result in a protein product

• before protein synthesis… an RNA copy called “transcript,” of the gene must be made (mRNA)

Reading Genetic Code 

A gene is interpreted by reading the codons but… these codons must be read in the correct reading frame

• Which nucleotide starts the first codon of the coding region of the gene

• Always starts with the “start codon” AUG which encodes the amino acid Methionine

Prokaryotic gene expression

1. Happens solely in the cytoplasm

2. Do not require RNA transcript modification- their RNA transcripts can be translated immediately after being transcribed

3. RNA transcript can be transcribed and translated at the same time

• multiple polymerases can transcribe a single gene

• numerous ribosomes can concurrently translate the mRNA transcripts into polypeptides (polyribosomes)

  • This can allow a specific transcript and/or specific protein to rapidly reach high concentration in a cell

Eukaryotic gene expression

transcription and mRNA modification occur in the nucleus

translation occurs in the cytoplasm

Overall process of transcription

The gene sequence determines the sequence of bases along the length of a mRNA molecule. RNA is compromised of A, U, C, G

Occurs in the nucleus of eukaryotic cells

RNA polymerase is THE enzyme (she is THAT girl)

What do promoter sequences do in the initiation phase of transcription?

A sequence of DNA that serves as recognition and recruitment site for transcription factors and RNA polymerase

What do transcription factors do in the initiation phase of transcription?

Proteins that aid in the initiation and regulation of transcription

What do RNA Polymerases do in the initiation phase of transcription?

Synthesize the RNA transcript

Breaks hydrogen bonds between DNA strands and links together RNA nucleotides (same base pairing rules except RNA uses uracil instead of thymine)

RNA polymerase can only add nucleotides onto the 3’ end of the growing RNA transcript

Several types (I, II, III)

What are the steps for initiation in transcription

1. Transcription factors bind to promoter region of the gene needing transcription

2. T-factors recruit RNA polymerase to bind and form an initiation complex

3. RNAP will then recognize this start sequence and begin to synthesize the RNA transcript in a 5’ to 3’ direction

• Reads 3’ to 5’

• Writes 5’ to 3’

What is the importance of the elongation phase in transcription

It is where the production of the RNA transcript occurs

What is the role of RNAP in the elongation phase in transcription

Unwinds DNA to access the template strand (only exposes around 10-20 nucleotides at a time)

Connects the RNA nucleotides using DNA as a template

• produces RNA transcript in a 5’ to 3’ direction

• Temporary attachment

What are the steps for the elongation phase of transcription

1. RNAP unwinds DNA

2. RNAP reads 3’ to 5’ direction the the DNA nucleotides from the template strand and attaches RNA nucleotides

3. RNA nucleotides are joined to the previous one on the 3’ end by a phosphodiester bond along its backbone

What is the role of terminator sequences in the termination phase of transcription

sequences of DNA at the end of a gene that is transcribed and signals the RNA transcript is complete

What are the steps for termination in transcription

1. RNAP reaches and transcribes the termination sequence 

2. RNA transcript is released by RNAP

3. RNAP detaches from the DNA, ending transcription

GOAL of termination phase is to completely disassociate RNAP from DNA by stopping transcription and releasing the new RNA strand

How does the termination phase of transcription vary for prokaryotes versus eukaryotes

Pro-

Euk-