Transcription and Translation

Transcription and Translation

1. What is transcription? Explain what happens during transcription.

2. What is translation? Explain what happens during translation.

3. Explain how transcription and translation are related to DNA replication.

4. If you begin with a parent DNA strand of A A T G C A G T, what will the complementary mRNA strand be? (Think about it before you answer.)

Compared structures of DNA and RNA

A. DNA-Deoxyribonucleic acid1. Bases-cytosine, guanine, adenine and

thymine

2. Double stranded

3. Function-store genetic information

B. RNA-Ribonucleic acid1. Base-Cytosine, guanine, adenine and uracil2. Single stranded3. Functions-

a. rRNA-ribosomal RNA (makes up about 60% of ribosomal structure

b. mRNA-messenger RNA (record information from DNA and carry it to ribosomes)

c. tRNA-transfer RNA (delivers amino acids to proteins at the ribosome to extend the chains)

Compared structures of DNA and RNA

RNA nucleotide

DNA nucleotide

Comparison of RNA and DNA sugars

Deoxyribose

Ribose

Compared structures of DNA and RNA

RNA single strand with hairpin loop

Although it looks like a double helix, it is one strand wrapped around itself. It is similar to a twisted bobby pin.

Reading Quiz (Orange book-Chapter 6)1. Transcription is the process of making:

a. RNA b. tRNA c. mRNA d. rRNA2. An intron is found in:

a. DNA b. RNA c. mRNA d. tRNA3. The enzyme used in transcription is:

a. RNA primase b. RNA polymerasec. DNA polymerase d. a and b are both correct

4. How many bases make a codon?a. 1 b. 2 c. 3 d. 4

5. Translation occurs in the:a. nucleus b. cytoplasm c. both

Transcription

A. Transcription=the synthesis of mRNA from a DNA template

B. Occurs in the 5’→3’ direction (if you don’t know what this means go back and look it up!!)

C. Involves RNA polymerase

D. mRNA, tRNA and rRNA must all be transcribed for protein synthesis to take place

E. mRNA-the sequence of mRNA nucleotides determine the primary sequence of the polypeptides

F. tRNA-carries the amino acids to mRNA-tRNA folds in on itself-see p. 305 figure 17.12

G. rRNA-major components of ribosomes

Transcription (three types of RNA)

Transcription (Initiation)

A. RNA polymerase binds to the promoter siteB. Promoter =region of DNA where RNA

polymerase attaches and initiates transcription-Determines which strand of DNA will serve as the

template

C. RNA polymerase -hooks together RNA nucleotides as they base pair along the DNA template

D. Transcription unit -area of DNA that will be transcribed

E. Transcription initiation complex -the area where transcription factors and RNA polymerase are bound to the promoter

F. TATA box -promoter DNA sequence

-the actual sequence is 5'-TATAAA-3'

-RNA polymerase binding site

G. After polymerase is bound to the promoter DNA, the two DNA strands unwind and the enzyme starts transcribing the template strand

Transcription (Initiation)

Transcription (RNA strand elongation)

A. RNA polymerase moves along DNA template

B. It unwinds 10-20 DNA bases at a time

C. RNA polymerase adds nucleotides in the 5’→3’ direction

D. As RNA polymerase moves along, the DNA double helix reforms

E. The new section of RNA ‘peels away’ as the double helix reforms

Transcription (termination)

A. Transcription stops when RNA polymerase reaches a section of DNA called the terminator

B. Terminator sequence = AAUAAAC. Next, the RNA strand is released and

RNA polymerase dissociates from the DNA

D. The RNA strand will go through more processing

Sense vs. Antisense DNA strands

A. The DNA double helix has two strands

B. Only one of them is transcribed

C. The transcribed strand is the antisense strand

D. The non transcribed strand is the sense strand

E. mRNA is complementary to the anitsense strand

F. The 5’ end of the RNA nucleotides are added to the 3’ end of the growing chain

G. RNA nucleotides are linked together in the same fashion as DNA molecules

Sense vs. Antisense DNA strands

RNA splicing (in eukaryotes)

A. In eukaryotes RNA transcripts have long non-coding stretches of nucleotides

-these regions will not be translated

B. The non-coding sections are dispersed between coding sections

C. Introns-non-coding sections of nucleic acid found between coding regions

D. Exons-coding regions of nucleic acids (eventually these are expressed as

amino acids)

E. RNA polymerase transcribes introns and exons,

-this is pre-mRNA

F. Pre-mRNA never leaves the cell’s nucleus

G. The introns are excised and exons are joined together to form mRNA

H. pre-mRNA

I. Mature mRNA

RNA splicing (in eukaryotes)

Translation

A. Translation-forming of a polypeptide

-uses mRNA as a template for a.a. sequence

-4 steps (initiation, elongation, translocation and termination)

-begins after mRNA enters cytoplasm

-uses tRNA (the interpreter of mRNA)

B. Ribosomes-made of proteins and rRNA-each has a large and small subunit-each has three binding sites for tRNA on its surface-each has one binding site for mRNA-facilitates codon and anticodon bonding-components of ribosomes are made in the nucleus and exported to the cytoplasm where they join to form one functional unit

Translation

B. Ribosomes (continued)

-the three tRNA binding sites are:

1. A site=holds tRNA that is carrying the next amino acid to be added

2. P site= holds tRNA that is carrying the growing polypeptide chain

3. E site= where discharged tRNAs leave the ribosome

#8. Translation

Ribosomal structure

EP A

Large subunit

Peptidyl-tRNA binding site

Aminoacyl-tRNA binding site

mRNA

5’

Exit site

Small subunit

3’

C. The genetic code– Four RNA nucleotides are arranged 20

different ways to make 20 different amino acids

– Nucleotide bases exist in triplets– Triplets of bases are the smallest units that

can code for an a.a. – 3 bases = 1 codon = 1 a.a.– There are 64 possible codes (64=43)

Translation

C. The genetic code– Most of the 20 a.a. have between 2 and 4

possible codes– The mRNA base triplets are codons– In translation the codons are decoded into

amino acids that make a polypeptide chain– It takes 300 nucleotides to code for a

polypeptide made of 100 amino acids (Why?)

Translation

C. The genetic code (continued)– 61 of 64 codons code for a.a.– Codon AUG has two functions

-codes for amino acid methionine (Met)-functions as a start codon

– mRNA codon AUG starts translation– The three ‘unaccounted for’ codons act as

stop codons (end translation)

Translation

D. How it works

DNA (antisense)

A C C A A A C C G

mRNA (transcription)

U G G U U U G G C

polypeptide (translation)

Trp - Phe - Gly-

Translation

E. More on tRNA– tRNA is transcribed in the nucleus and must enter the

cytoplasm– tRNA molecules are used repeatedly– Each tRNA molecule links to a particular mRNA

codon with a particular amino acid– When tRNA arrives at the ribosome it has a specific

amino acid on one end and an anticodon on the other – Anticodons (tRNA) bond to codons (mRNA)

p. 304 (red book)

Translation

Where the a.a. attaches

Hydrogen bonds

Anticodon

=

tRNA diagrams

Although we draw tRNA in a clover shape it’s true 3-D conformation is L-shaped.

Translation (Initiation)

A. Initiation1. Brings together mRNA, tRNA (w/ 1st a.a.) and ribosomal subunits2. Small ribosomal subunit binds to mRNA and an initiator tRNA

-start codon= AUG-start anticodon-UAC-small ribosomal subunit attaches to

5’ end of mRNA

B. Initiation2. (continued)

-downstream from the 5’ end is the start codon AUG (mRNA)

-the anticodon UAC carries the a.a. Methionine

3.After the union of mRNA, tRNA and small subunit, the large ribosomal subunit attaches4. Initiation is complete

#9. Translation (Initiation)

B. Initiation

5. The intitiator tRNA and a.a. will sit in the P site of the large ribosomal subunit

6. The A site will remain vacant and ready for the aminoacyl-tRNA

Translation (Initiation)

Translation (Initiation)

Translation (Elongation)A. Amino acids are added one by one to the first

amino acid (remember, the goal is to make a polypeptide)

B. Step 1- Codon recognitiona. mRNA codon in the A site forms hydrogen

bonds with the tRNA anitcodonC. Step 2- Peptide bond formation

a. The ribosome catalyzes the formation of the peptide bonds between the amino acids (the one already in place and the one being added)

b. The polypeptide extending from the P site moves to the A site to attach to the new a.a.

A. The tRNA w/ the polypeptide chain in the A site is translocated to the P site

B. tRNA at the P site moves to the E site and leaves the ribosome

C. The ribosome moves down the mRNA in the 5’→3’ direction

Translation (Translocation)

A. Happens at the stop codon

B. Stop codons are UAA, UAG and UGA-they do not code for a.a.

C. The polypeptide is freed from the ribosome and the rest of the translation assembly comes apart

D. Animation (you move it)

E. Animation (you watch it)

F. Animation (McGraw-Hill)

Translation (Termination)

Gene expression

A. Jacob and Monad (1961)-studied control of protein synthesis in E. coli and lactose digesting enzymes-found that E. coli do not produce lactose digesting enzymes when grown in a medium without lactose-when bacteria were placed in a lactose environment, enzymes were found within minutes

B. Genes can be switched on or off as necessary

-a gene that is ‘on’ will be transcribed

-in E.coli, the enzyme lactase will be produced if the gene is ‘on’

-if the gene is ‘off’ mRNA will not be created and translation can not occur

Gene expression

C. The operon model

-proposed by Jacob and Monad

-explains how genes switch on and off

-operon=promoter, operator and structural genes

-lac operon is found in E.coli

Gene expression

D. The lac operon

Gene expression

D. The lac operon (no lactose)

-lactose is absent, repressor is active, operon is off, no mRNA is produced, RNA polyermase cannot bind because it is blocked by the repressor that has bound to the operator

Gene expression

D. The lac operon (lactose is present)

-lactose is present, repressor is inactive, operon is on, mRNA is transcribed, RNA polymerase binds to operator

-an isomer of lactose binds to the repressor and changes its shape

-this prevents it from binding to the operator

-lactase is produced

Gene expression

#11. Compare Transcription in Eukaryotes and Prokaryotes

Link