Transcription and Translation: from DNA to trait

How Are Different Types of Cells Created and Maintained?

Differential gene expression allows cells to do different things.

The same genetic information is in all 100 trillion cells of any one person. Different cells use the same blueprint in different ways. How do they do this?

In essence, the control of gene expression occurs by regulating the flow of information from DNA to protein.

DNA

Transcription Translation

TraitRNA Protein

The “Central Dogma” of Molecular Genetics

RNA processing

The Central Dogma

Review

■ DNA ■ RNA ■ ProteinsMade up of 4

different nucleotides

Made up of 4 different

nucleotidesMade up of 20 different amino acids

RNA

RNA is a nucleic acid polymer that uses a slightly different sugar than DNA and the base uracil (U) in place of thymine (T).

RNA Is Single-Stranded

Gene Expression in Prokaryotes vs. Eukaryotes

■ Transcription and translation may occur simultaneously

■ Transcription and translation are separated in time and space

The Genetic Code■ Problem: How do only 4 different nucleotides

code for the 20 amino acids that make up proteins?

■ Solution: Each group of 3 nucleotides codes for a different amino acid. These 3 nucleotide units are called codons.

DNA RNA Amino AcidCAT

GUA

Valine

Reading Frame■ The 3-nucleotide

units (codons) must be read in the correct reading frame

■ Start codons determine the reading frame

The Genetic Code Dictionary■ There are

multiple codons for each amino acid

■ AUG is always the start codon

■ UAA, UGA, and UAG are stop codons

Transcription■ DNA is used to make a strand of RNA

called the primary transcript (pre-mRNA)

■ The pre-mRNA is further processed to create the finished mRNA

■ mRNA exits the nucleus to be translated

Transcription

3 main steps

1. Initiation

2. Elongation

3. Termination

Transcription Initiation■ RNA polymerase binds to DNA at a region

called the promoter

■ RNA polymerase unwinds the DNA and adds nucleotides in the 5’ → 3’ direction

Transcription Elongation■ RNA polymerase moves along the DNA strand,

adding 60 nucleotides/sec

■ DNA strands rejoin after polymerase passes by

Transcription Termination■ Polymerase stops

when it reaches a DNA sequence called the terminator

■ The mRNA has been completely transcribed

■ In eukaryotes, this is pre-mRNA and must be further processed

mRNA Processing■ In eukaryotes, pre-mRNA must be further processed to

mRNA before it leaves the nucleus

■ GTP is added to 5’ end, forming the 5’ cap

■ 100’s of adenines are added to 3’ end, forming the poly-A tail

■ Non-coding regions of RNA are spliced out

■ Introns (non-coding sequences) are cut out by spliceosomes. Leaving only Exons (Coding sequences) making up the mRNA that leaves the nucleus.

■ In some genes more than 90% of the pre-mRNA is destroyed, never to appear in the mRNA.

■ Alternative splicing patterns means one gene can make more than one protein

■ This has the consequence that the count of our genes (~20,000) is far lower than the count of our different proteins.

Translation■ The process in which mRNA is used to make

proteins■ Occurs in the cytoplasm using ribosomes

■ Requires tRNA bound to amino acid

■ 3 steps: initiation, elongation, termination

What Translation Accomplishes

In translation, information present in the mRNA is read by the ribosome to synthesize a polypeptide.

The sequence of amino acids determines the structure, and therefore the function, of a protein.

Amino Acids – What the Genetic Code Specifies

Two examples

There are 20 different amino acids

tRNA■ A clover-shaped RNA

molecule

Structure of tRNA

■ 3’ end of tRNA binds to specific amino acids

■ Anti-codon on tRNA complements mRNA codon

tRNA Synthesis■ RNA is made in the

nucleus■ Amino acids float

free in the cytoplasm

■ Aminoacyl-tRNA synthase joins each amino acid to the appropriate tRNA

Ribosomes■ 2 subunits of each

ribosome (large and small)

■ Composed of proteins and rRNA

■ 3 tRNA binding sites

Translation: Initiation

■ mRNA, tRNA and small ribosomal subunit bind with the P site at the start codon

■ Large subunit binds using energy from GTP

Translation: Elongation

•mRNA is read 3 nucleotides at a time (codons)•tRNA brings corresponding amino acid into the A site of the ribosome

■ Ribosome catalyzes dehydration synthesis reaction between amino acids in P site and A site

■ Growing polypetpide now attached to tRNA in A site

■ Ribosome moves forward one codon

■ Free tRNA in P site exits out the back of ribosome

■ tRNA (with polypeptide) moves into P site

Translation: Termination

■ Elongation continues until reaching a stop codon

■ Release factor binds and hydrolyzes the bond between the last tRNA and its amino acid, freeing the new protein

The Genetic Code is Biology’s Rosetta Stone

These are the words of the genetic language.

Gene Expression: Overview