THE GENETIC CODE AND TRANSCRIPTION

Genetic code

use to read DNA (or RNA) and make proteins

Transcription

process where DNA is used to make RNA, specifically mRNA (messenger RNA), which carries the instructions to build a protein

Translation

process where the cell reads the mRNA and builds a protein by linking together amino acids in the correct order.

Each Genetic code is made up of:

Codons

Condon

Sequence of three RNA bases (nucleotides) that codes for one amino acid or a stop signal during protein synthesis (translation

Start codon

AUG (translation begins (also codes for methionine)

methionine

Amino Acid

special role in protein synthesis:

THE GENETIC CODE IS

unambiguous

degenerate

commaless

nonoverlapping

universal

unambiguous

Each codon codes for only one amino acid

degenerate

More than one codon can code for the same amino acid

commaless

Read in one flow, no spaces or breaks

nonoverlapping

single ribonucleotide at a specific location within the mRNA is part of only one triplet

Universal

Used by almost all living things, from bacteria to humans

Proteins are composed

amino acids

amino acids linked by

Peptide bonds

Peptide bonds

chemical bond that links two amino acids together to form a protein chain

amino acids

Building blocks of a protein

stop codons

UGA, UAA, UAG

All can have Multiple codons
for most amino acids, except
for

methionine (AUG) and
tryptophan (UGG)

Open Reading Frame (ORF)

continuous stretch of nucleotides in DNA or mRNA that can be translated into a protein without any stop codons interrupting it

What are Frameshift Mutations?

type of genetic mutation where nucleotides (DNA or RNA bases) are inserted or deleted from the genetic sequence in numbers not divisible by three

Insertion of Three Nucleotides (3 bases)

-No frameshift

Adds one amino acid, rest unchanged

TRANSCRIPTION IN PROKARYOTES does not need a

Primer

RNA polymerase from E. coli contains

α, β, β’, ω and σ

Key Features of Prokaryotic Transcription:

Initiation:
• Elongation
• Termination

Initiation:
(P)

-RNA bind to promoter region of DNA

-O sigma factor helps RNA polymerase recognize promoter

 -DNA unwinds, exposing “Template strand” 3-5

-RNA is built in 5-3, starting at +1 site

Two important DNA sequences in promoters:

-35 region (TTGACG)

-10 region (TATAAT)

Elongation

(P)

• 8-9 bases made so sigma factor leaves

• Core RNA polymerase enzyme (A,B,B’,W) continues to build RNA

• RNA polmerase adds ribosomes (A,U,G,C) that are complementary to DNA template

Transcription Termination in Bacteria

-RNA polymearse stops transcription when it reaches a termination signal

Termination Signal

Tells RNA polymerase to stop

σ (sigma) factor

protein in prokaryotes that helps RNA polymerase find and bind to the correct promoter on DNA to start transcription.

Two types of Termination:

1. rho independent

2. Rho dependent

Rho Independent

Forms Hairpin loop followed by string if Uracils (U)

Causes RNA polmearse to pause and fall off DNA

Rho dependent

Rho factor binds to RNA at a rut site and moves toward RNA polymearse

When Rho catches up, Causes RNA polymearse to stop and release RNA

Rho factor

Helps stop transcription

Rut site 

newly made RNA where the Rho factor protein binds during Rho-dependent termination in bacteria

Polycistronic mRNAs

one RNA message that tells the cell how to make several proteins

TRANSCRIPTION IN EUKARYOTES

• Happens in the nucleus.

• More complex than in prokaryotes due to:

  • Larger genome

  • Chromatin (DNA wrapped around proteins)

  • Cell specialization (different genes needed in different cells)

RNA polymerase II (RNAPII)

main enzyme in eukaryotic cells that makes messenger RNA (mRNA) — the RNA that carries instructions from DNA to make proteins.

RNA polymerase II (RNAPII) needs help to start transcription by two sources:

1.Cis-acting sequence elements

2.Trans acting factors

Cis-acting sequence elements

DNA sequences located on the same DNA molecule as the gene being transcribed.

Examples of Cis acting elements:

Core promoter (includes TATA box) = where RNAPII binds to start transcription.

• Enhancers = boost transcription

• Silencers = reduce transcription

Trans acting factors

• Help RNAPII find and bind DNA.

• Two types:

  1. General TFs (e.g., TFIID binds the TATA box)

  2. Activators/Repressors – bind enhancers or silencers to regulate expression of specific genes.

Two types of trans acting factors:

1. General TFs

2. Activators/Repressors

General TFs

• These are helper proteins needed for every gene.

• They help start the copying process (transcription).

• One of them, TFIID, finds a special spot on DNA called the TATA box.

• Together, these helpers bring in RNA Polymerase II (RNAPII) to begin making RNA.

Activators/Repressors

• These are special switches that control how much a gene is used.

• Activators = turn the gene up.

• Repressors = turn the gene down or off.

• They work by attaching to special DNA spots called enhancers (turn up) or silencers (turn down)

Posttranscriptional Modifications

After the pre-mRNA is transcribed from DNA, it undergoes several important modifications before it becomes a mature mRNA ready for translation

1 5’ capping

2.’ Polyadenylation (Poly(A) Tail Addition)

3. RNA Splicing

4. RNA Editing (Less common)

5’ capping

• (7-methylguanosine)

• A special “cap” is added to the front (5’ end) of the RNA.

• This cap protects the RNA and helps it get recognized by the machinery that makes proteins.

 3’ Polyadenylation (Poly(A) Tail Addition)

• A string of “A” nucleotides (called a poly-A tail) is added to the end (3’ end).

• This tail protects the RNA from breaking down and helps it leave the nucleus.

RNA Splicing

The pre-mRNA has parts called introns (non-coding) and exons (coding).

• Introns are cut out, and exons are joined together.

• This step creates the correct message that will be translated into protei

Alternative splicing

• cell can join exons in different ways.

• This allows one gene to make different versions of mRNA — and therefore different proteins!