Protein SynthesisProtein Synthesis

Transcription and Translation

AP BiologyUnit 2

Flow of Genetic Information

• All living organisms use DNA to synthesize RNA to make proteins

• Same two-step process:Transcription

Translation • Some antibiotics inhibit

protein synthesis in bacteria.– Ex. Neomycin (the antibiotic

in Neosporin) interferes with the process of translation)

Genes and Chromosomes• DNA is organized into

chromosomes– Humans have 46 chromosomes in

each cell.

• Genes are “coding” regions of DNA– Each gene is the code for how to

make a specific protein. • Human chromosomes are made up

of – DNA– Histone proteins that DNA is

wound around

Structure of DNA

• The carbons in the 5C sugar each have a number– Start to the right of the oxygen

and go around clockwise PO

phosphate

Sugar Base

1

23

4

5

• This gives the nucleotide 2 distinct ends

–5’ end (closer to carbon 5) –3’ end (closer to carbon 3)

A way to remember it: Human Nucleotide

4 C5 C

3C C 2

C 1

Phosphate

Base

Nucleic Acid Structure

• DNA is double stranded

• Hydrogen bonds between bases– A pairs with T– C pairs with G

Image taken without permission from http://bcs.whfreeman.com/thelifewire

• The strands are antiparallel–One strand runs 5’-3’–The other runs 3’-5’

Question…

• Why can’t the DNA strands be parallel (both running 5’-3’)?– This wouldn’t allow the bases to be near each

other to hydrogen bond.

Transcription

• DNA is transcribed into 3 kinds of RNA– mRNA = messenger RNA (the RNA code used

to make protein)– tRNA = transfer RNA (participates in

translation)– rRNA = ribosomal RNA (part of ribosomes)

• RNA Polymerase is the enzyme that transcribes the DNA into RNA

Initiation• How transcription starts• RNA Polymerase recognizes a

promoter sequence on the DNA• RNA Polymerase binds to the

promoter• DNA is unwound to start

transcription– What kind of bonds are being

broken to unwind/separate the strands of DNA?

– Hydrogen bonds

Promoter Sequences

• In prokaryotes, RNA Polymerase must find these sequences:

• + 1 is the first base in the RNA (where the actual transcription of DNA starts from)

5’ 3’ 5’ 3’

Eukaryotic Promoter Sequences

• In eukaryotes, the RNA polymerase must find the following sequences:

• Eukaryotic genes can also have enhancer sequences to help RNA polymerase bind– We’ll talk about these a little later– don’t worry

about them right now

What do you think this diagram shows about transcription?

Bases changed

to…

Promoters

• In order for RNA Polymerase to recognize it, the promoter sequences– Must be the correct sequence of bases (small

changes OK)– Must be correctly spaced apart

• If these conditions aren’t met, RNA Polymerase can’t bind to the DNA and no transcription occurs.

Elongation

• How the RNA strand is built• RNA Polymerase matches the

appropriate (complementary) nucleotides to the DNA template strand– Template strand = the actual

strand RNA Polymerase uses to build RNA

– Coding (Nontemplate) strand = not used for building RNA, but has the same sequence as the RNA.

Building the RNA• The RNA Polymer grows in a 5’-3’ direction

• RNA Polymerase only adds new nucleotides on to the 3’ end.

• Considering this, in what direction must the template strand of DNA be running?

– 3’-5’ (since it is building its complement)P

OPP

PPP

OPP

P

OPP

P

O

5’

3’

5’

3’

5’

3’

5’

3’

Question …

• In terms of the sequence, how will the RNA differ from the sequence of the coding strand in the DNA?– T’s are replaced with U’s

Termination • How transcription of RNA ends• RNA Polymerase recognizes a

termination signal on the DNA template– Usually a long string of A’s or a

series of A’s and T’s

• RNA Polymerase falls off the DNA template

• Stability of mRNA is minutes hours (depends on type of cell and RNA)

Question…

• How do the specific chemical properties of the termination sequence cause termination to occur?– There are only 2 hydrogen bonds between A

and T/U– With a string of A’s and U’s, there are much

fewer bonds to hold the DNA template and RNA together they separate transcription ends

TranslationTranslation

• Using the mRNA code to create the appropriate protein.

• Occurs in the cytoplasm/on the rough ER

• Sequence of 3 nucleotides codes for a particular amino acid = codon

• 64 different codons

Question…

• Why can’t 1 or 2 nucleotides code for an amino acid?– Not enough combinations to code for all 20

amino acids– With 1 nucleotide only 4 possibilities (A, C,

G, U)– With 2 nucleotides only 4 x 4 = 16

possibilities (AA, AU, AC, AG, CC, …)

The codon table

tRNA

• tRNA brings the correct amino acid to match with the mRNA codon

• Each tRNA holds a specific amino acid and has a particular anticodon.

• Aminoacyl tRNA synthetases are enzymes that attach the correct amino acids to the tRNA

Amino acid attached

here

Question…

• For the anticodon shown in the diagram, what would the complementary codon on the mRNA be?– 5’ UUC 3’

• Which amino acid is attached to this tRNA?– Phe

Ribosomes

• Made up of 2 subunits• Composed of rRNA and protein• Not specific to any particular

protein– can be used to translate any RNA into protein

• Workbench for translation – holds mRNAs and tRNAs in the correct positions to assemble protein.

Ribosomes

• 3 sites on the ribosome– A site = where tRNA

first binds to mRNA – P site = where the amino

acid is added on to the polypeptide chain

– E site = exit site

Translation

• Begins with the Start codon = AUG– Codes for methionine (Met)– Not the same thing as +1

Translation

• Ribosome moves along mRNA in a 5’->3’ direction, catalyzing the translation of the mRNA into protein – breaks bond between

tRNA and amino acid – creates a new peptide

bond to link it to polypeptide chain

Question…

• How does the mRNA know if it is correctly matched to the tRNA?– Hydrogen bonding between the bases is correct

Stopping Translation

• Ribosome is released when a stop codon is reached– UAA, UAG, UGA = stop codons (don’t code for any

tRNA anticodons)– A release factor binds to the mRNA instead– Ribosome breaks apart, mRNA and protein are

released

Summary of Protein Synthesis

• In Eukaryotes

• How is it different in prokaryotes?

Why is this important?

1. Changes in the DNA sequence will lead to changes in the transcribed _________.

2. This results in a different codon which may code for a different ______________.

3. A different ___________ means a different R group.4. A different R group may have different chemical

properties.5. These different chemical properties may lead to a

different protein ____________.6. A different protein structure may affect its _________!7. See how this is all starting to connect! Exciting!!!

Why is this important?

1. Changes in the DNA sequence will lead to changes in the transcribed RNA.

2. This results in a different codon which may code for a different amino acid.

3. A different amino acid means a different R group.4. A different R group may have different chemical properties.5. These different chemical properties may lead to a different

protein structure.6. A different protein structure may affect its function!

7. See how this is all starting to connect! Exciting!!!

microRNAs and RNAi

• Small, single stranded RNA molecules (miRNAs and siRNAs)– microRNA = miRNA– Small interfering RNA = siRNA

• Bind to complementary sequences in mRNA molecules

• Can control the expression of (translation of) specific RNA molecules

Question…

• How will microRNAs disrupt translation?– Block translation by creating a physical road block– RNA-RNA binding also marks the mRNA for

degradation

• Called RNAi = RNA interference