micro lecture Protein Synthesis; Transcription unit 2

Protein synthesis

gene expression

how DNA instructions become a protein.

DNA writes it → RNA reads it → Protein builds it

gene

sequence of DNA that codes for protein

• a piece of DNA

• that codes for a protein

2 steps to gene expression

transcription and translation

transcription

DNA information copied into RNA form
RNA 1st product of transcription

DNA ➜RNA

3 forms of RNA-transcription

messenger RNA, ribosomal RNA, transfer RNA

Transcription ( DNA → RNA)

What happens

The DNA instructions are copied into RNA .

The first product made is RNA.
Transcription = DNA makes a working copy

mRNA

copy of DNA sequence of gene

• carries instructions to ribosome

message

tRNA

brings amino acid to ribosome

transporter

rRNA

part of ribosome

• helps build protein

the builder

Where Transcription Happens

Eukaryotes ( cells with nucleus)

Transcription happens in the nucleus .

Prokaryotes ( bacteria)

No nucleus.

So transcription and translation happen at the same time.

🧠 Phrase:
No nucleus = no waiting

transcription

occurs in nucleus in eukaryotes

translation

RNA used to assemble amino acids into proteins
occurs in cytoplasm at the ribosome in eukaryotes

RNA ➜ Protein

transcription and translation

occur at the same time in prokaryotes
no nuclear membrane in prokaryotes to separate the two steps

4 RNA nucleotides

each nucleotide composed of 3 parts

• ribose sugar (5 carbon sugar)
  phosphate group
  nitrogen base

nitrogen bases

4 RNA nucleotides distinguished from each other by ….

no thymine in RNA
RNA single stranded

adenine (A), guanine (G), cytosine (C), uracil (U; binds A in DNA)-

RNA has U instead of T

DNA double, RNA single

RNA polymerase enzyme

controls transcription

• This enzyme controls transcription .
  recognizes start and stop sequences of genes
  binds and separates DNA strands in double helix
  transcribes only one DNA strand into RNA

• finds the start of the gene

• opens DNA

• builds RNA

the copier machine

Transcription-phase

DNA copied into RNA form
3 phases- initiation, elongation and termination

initiation

RNA polymerase recognizes, binds promotor sequence at beginning (start) of gene

Start copying

elongation

• RNA polymerase adds RNA bases complementary to DNA bases in gene
  -

• RNA polymerase forms RNA copy of DNA

RNA polymerase adds RNA bases matching DNA bases .

Example pairing:

• DNA A → RNA U

• DNA T → RNA A

• DNA C → RNA G

• DNA G → RNA C

🧠 Phrase:
RNA strand grows

termination

RNA polymerase recognizes stop sequence at end of gene- ends transcription
self complimentary bases and poly A sequence in E. coli DNA
forms hair pin loop followed by poly U tail in mRNA

termination-chat

RNA polymerase reaches stop sequence and stops transcription .

In bacteria:

• hairpin loop forms

• poly- U tail appears

🧠 Phrase:
Stop copying

Eukaryotic RNA

processed in nucleus after transcription

mRNA

contains exons and introns
exons- coding sequences for gene
introns- non coding sequences between exons

mRNA Processing ( Eukaryotes Only)

After transcription, the RNA must be edited .

Exons

Coding sequences

Introns

Non- coding sequences

🧠 Phrase:
Exons = expressed

Splicing

Special enzymes remove introns and join exons.

🧠 Phrase:
Cut introns, keep exons

ribozymes remove

introns and splice exons together
5’ cap and poly A tail added to mRNA
allows movement of mRNA from nucleus through nuclear membrane to cytoplasm
protects mRNA from enzymes in cytoplasm

mRNA feature

5' cap

• protects mRNA

Poly A tail

• helps mRNA leave nucleus

• protects from enzymes

🧠 Phrase:
Cap the front, tail the back

prokaryotes

no mRNA processing in…

Translation

RNA used to assemble amino acids into proteins

• requires mRNA, ribosomes and tRNA

using RNA instructions to build a protein.

translation requires

1. mRNA

2. Ribosome

3. tRNA

🧠 Catchy phrase:
Message → Machine → Materials

• mRNA = message

• Ribosome = machine

• tRNA = materials carrier

mRNA carries DNA information

for amino acid sequence of protein
codon

The mRNA carries instructions for the protein.

The instructions are read in codons .

codon

3 base grouping on mRNA that determines each amino acid for protein
UUU phenylalanine, AAA lysine, GGG glycine, CCC proline

A codon

3 bases on mRNA that determines an amino acid.

• UUU → phenylalanine

• AAA → lysine

• GGG → glycine

• CCC → proline

🧠 Catchy phrase:
3 letters = 1 amino acid

64 codons

61 code for 20 different amino acids
most (18 of 20) amino acids coded for by more than 1 codon
makes the genetic code redundant

• first 2 bases in codon usually most important
  -

• 3rd base usually least important
  -

• 4 different codons code for proline
  -

• 6 codons for serine, arginine, leucine
  -

• only methionine and tryptophan have only 1 codon for each

Number of Codons

There are 64 codons total .

• 61 codons → amino acids

• 3 codons → stop signals

🧠 Catchy phrase:
61 build, 3 stops

The third base wobbles

3 termination codons

UAA, UAG, UGA
stop translation at end of gene- do not code for amino acid

They do NOT code for amino acids .

U Are Away → stop

ribosomes

site of protein synthesis
composed of rRNA and proteins

where proteins are made .

protein factory

Special Amino Acids

Only two amino acids have one codon .

• Methionine

• Tryptophan

🧠 Catchy phrase:
Two loners: MET and TRP

2 mRNA codon sites in ribosome

P site
growing peptide chain
A site
incoming new amino acid

Ribosome Sites

There are two important sites .

P Site

• holds the growing protein chain

A Site

• holds the incoming amino acid

🧠 Catchy phrase:
P = Peptide chain
A = Arrival site

---

same time

2 tRNA with amino acids can bind in ribosome at ….

tRNA

…. brings amino acid designated by codon to ribosome

• all …. are single stranded with cloverleaf shapes
  …. distinguished by anticodons

tRNA ( Transfer RNA)

tRNA brings amino acids to the ribosome .

Each tRNA has:

• an anticodon

• an amino acid

Anticodon

3 bases that match the mRNA codon .

🧠 Catchy phrase:
Codon and anticodon lock like puzzle pieces

Example:
mRNA codon → AAA
tRNA anticodon → UUU

mRNA codon

3 base complement of …

wobble

45 different tRNA’s
most respond to more than 1 codon
….

brings same amino acid

1st 2 bases most important

• 3rd base variable….

aminoacyl tRNA synthetase

joins amino acid to its specific tRNA

This enzyme attaches the correct amino acid to tRNA .

🧠 Catchy phrase:
The matchmaker enzyme

3 steps to translation

initiation, elongation and termination

initiation of translation

• starts protein synthesis
  -

• mRNA binds ribosome
  -

• initiation codon (AUG) on mRNA in left side of ribosome
  AUG designates start for translation of gene
  AUG codes for methionine (MET)
  -

• methionine always first amino acid in new protein

1⃣ Initiation ( Start) (translation)

This starts protein synthesis .

Steps:

1. mRNA attaches to ribosome

2. AUG start codon is recognized

3. tRNA with methionine ( MET) binds

Important:

• AUG = start codon

• codes for methionine

🧠 Catchy phrase:
AUG = Always Use Methionine

elongation of protein-translation

tRNA bring amino acid specified by codon in mRNA to ribosome
connects amino acids together to form protein

tRNA brings amino acids to ribosome.

The ribosome:

• connects amino acids

• builds the protein chain

🧠 Catchy phrase:
protein gets longer

termination of translation

stops protein synthesis
termination codon on mRNA enters ribosome- UAG, UGA or UAA

A stop codon enters ribosome

then:

• protein is released

• ribosome detachments

🧠 Catchy phrase:
Stop codon = protein done

translation ends

completed protein and mRNA released from ribosome
some proteins function immediately in cytoplasm
others modified before functioning

What Happens After Protein Is Made

Some proteins:

• work immediately in cytoplasm

Others:

• must be modified first .

initiation

AUG start codon
tRNA with MET in P site