Transcription and Translation

Topic 3.5

Assessment Statements

3.5.1 Compare the structure of RNA and DNA

3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase

3.5.3 Describe the genetic code in terms of codons composed of triplets of bases

3.5.4 Explain the process of translation, leading to polypeptide formation

3.5.5 Discuss the relationship between one gene and one polypeptide

Control center?

• DNA controls proteins produced in cell

• Enzymes control biochemistry of cell

• Protein synthesis occurs b/c of transcription and translation

• Both processes produce or require RNA

Structure comparison of DNA & RNA

DNA RNA

5-carbon sugar deoxyribose

5-carbon sugar ribose

Contains 4 bases: C, G, A, T

Contains 4 bases: C, G, A, U (uracil)

Double stranded

Single stranded

Transcription

• Produces RNA molecules• A section of DNA that

codes for polypeptides are called genes (inside nucleus)

• Messenger RNA (mRNA) carries message of DNA to the cytoplasm where protein synthesis takes place

Process of transcription1. RNA polymerase unzips DNA at one

gene2. RNA polymerase creates a

complimentary strand of RNA nucleotides to one DNA strand

3. Base pairing the same, except uracil binds to adenine

4. mRNA detaches from the single-strand DNA template and floats free in the nucleoplasm

5. Floats through nuclear pores and then to cytoplasm

• Information used to make one polypeptide (amino acids covalently bonded together in a specific sequence)

• Written in triplets (every 3 bases=one amino acid) and is called a codon

Types of RNA

• mRNA – messenger RNA, complementary copy of a DNA gene and codes for a single polypeptide

• rRNA – ribosomal RNA, each ribosome is composed of rRNA and ribosomal protein

• tRNA – transfer RNA, each type of tRNA transfers 1 of the 20 amino acids to the ribosome for polypeptide formation

Process of translation1. mRNA locates a ribosome2. Aligns so that the first two codon triplets are within

boundaries of the ribosome3. A specific tRNA floats in, its tRNA anticodon

complementary to the first codon triplet of the mRNA molecule, with an amino acid attached

4. While the first tRNA sits in the ribosome, a second tRNA molecule brings a second amino acid

5. An enzyme catalyzes a condensation reaction between the two amino acids and the resulting covalent bond between them is called a peptide bond

6. Bond is broken between the first tRNA molecule and the amino acid that it transferred to the chain

7. First tRNA floats away to “reload”8. Process continues until last codon triplet which does

not code for an amino acid signals “stop”9. Entire polypeptide breaks away from the final tRNA

molecule and becomes a free floating polypeptide

One gene/one polypeptide hypothesis• Early work led to hypothesis

that every gene produced one enzyme

• Later discovered that many proteins are composed of more than one polypeptide

• Latest, one gene may lead to a single mRNA molecule, but the mRNA molecule may then be modified

Transcription

Topic 7.3

Assessment Statements

7.3.1 State that transcription is carried out in a 5’ to 3’ direction

7.3.2 Distinguish between the sense and antisense strands of DNA

7.3.3 Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator

7.3.4 State that eukaryotic RNA needs the removal of introns to form mature mRNA

Transcription carried out in a 5’ to 3’ direction• RNA polymerase

combines with region of DNA called a promoter (not transcribed)

• RNA polymerase unzips DNA

• RNA polymerase initiates synthesis of an RNA molecule in a 5’ to 3’direction

Sense and antisense strands

• Sense strand – coding strand (same sequence as RNA strand)

• Antisense strand – template strand (copied during transcription)

The terminator

• Sequence of nucleotides that causes the RNA polymerase to detach from the DNA

• NTPs pair with antisense strand and polymerization of the mRNA occurs

• Portion of transcription known as elongation

Post-transcription processing

• Within eukaryotic DNA protein-coding regions there are non-coding regions

• Exons – coding regions• Introns – non-coding regions• Introns have to be removed to

make a functional mRNA strand• Prokaryotic mRNA does not

require processing because no introns are present

Translation

Topic 7.4

Assessment Statements7.4.1 Explain that each tRNA molecule is recognized

by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy

7.4.2 Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites

7.4.3 State that translation consists of initiation, elongation, translocation, and termination

7.4.4 State that translation occurs in a 5’ to 3’ direction

7.4.5 Draw and label the structure of a peptide bond between two amino acids

7.4.6 Explain the process of translation, including ribosomes, polysomes, start codons, and stop codons

7.4.7 State that free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes synthesize proteins primarily for secretion or for lysosomes

Translation

• Process that changes the language of DNA to the language of protein

• Occurs at the ribosome

Ribosomes• Each consists of a large

subunit and a small subunit composed of rRNA proteins and many distinct proteins

• rRNA proteins are small and are near core

• 2/3 of ribosome mass is rRNA • Made in nucleolus• Prokaryotic ribosomes

smaller than eukaryotic ribosomes

• Decoding of mRNA occurs in space between subunits

Binding sites between subunitsSite

Function

A site

Holds the tRNA carrying the next amino acid to be added to the polypeptide chain

P site

Holds the tRNA carrying the growing polypeptide chain

E site

Site from which tRNA that has lost its amino acid is discharged

• tRNA moves sequentially through the binding sites: from the A site, to the P site, to the E site

• Growing polypeptide chain exits the ribosome through a tunnel in the large subunit core

• Polysome - cluster of ribosomes, bound to a mRNA molecule

Background information• What are codons?• 3 nucleotides that together carry

genetic code from DNA to ribosomes via mRNA

• Start codon (AUG) is on the 5’ end of all mRNAs

• Each codon, other than the 3 stop codons, attaches to a particular tRNA

• 3’ end of tRNA has base sequence CCA which is site of amino acid attachment

• One loop of tRNA contains an exposed anticodon which pairs with a specific codon of mRNA

• Each of the 20 amino acids binds to the appropriate tRNA due to the action of a particular enzyme

• Active site of each enzyme allows a fit only between a specific amino acid and the specific tRNA

• Requires energy which is supplied by ATP

• Ready for delivery!!

RNA → protein

1. Initiation2. Elongation3. Translocation4. Termination

Initiation1. Activated amino acid (methionine

attached to tRNA with the anticodon UAC) combines with mRNA strand and small ribosomal subunit

2. Small subunit moves down the mRNA until it contacts the start codon AUG

3. Contact starts translation4. Hydrogen bonds form between the

initiator tRNA and the start codon5. Large ribosomal subunit combines

to form translation initiation complex and uses energy from guanosine triphosphate (GTP)

Elongation

1. tRNAs bring amino acids to the mRNA-ribosomal complex in the order specified by codons

2. Proteins called elongation factors assist in binding the tRNAs to the exposed mRNA codons at the A site

3. Initiator tRNA then moves to the P site

4. Ribosome catalyzes formation of peptide bonds between adjacent amino acids brought to the polypeptide assembling area

Translocation

1. Occurs during elongation2. Transferring of

polypeptide chain to the new tRNA that moves into the now exposed A site

3. Now empty tRNA is transferred to the E site where it is released

Termination

1. Begins when one of the three stop codons appears at the open A site (UAA, UAG, UGA)

2. Protein fills the A site3. Frees the polypeptide

from the ribosome4. Ribosome separates

from the mRNA and splits into two subunits

Newly synthesized proteins

• If produced by free ribosomes:– Used within cell

• If produced by bound ribosomes:– Secreted from the cell– Used in lysosomes