Protein Synthesis
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Protein Synthesis. Protein Production A. Background Info. 1. DNA (in the nucleus) is the blueprint for creating proteins. 2. Ribosomes (in the cytoplasm) are where ALL proteins are initially produced: - proteins staying inside cell will be completed here

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Protein Synthesis

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Protein Synthesis

  • Protein Production

  • A. Background Info

1. DNA (in the nucleus) is the blueprint for

creating proteins

2. Ribosomes (in the cytoplasm) are where

ALL proteins are initially produced:

- proteins staying inside cell will be

completed here

- proteins to be exported or that become

lysosomes are transferred to ribosomes

on RER and synthesis is finished there

3. RNA carries out P.S. by acting as a

messenger b/n DNA & ribosomes.


1. RNA is single-stranded

2. RNA is found in the nucleus & the cytoplasm

3. RNA is also a nucleic acid made up of

nucleotides consisting of 3 parts:

a. phosphate group

b. pentose sugar

c. nitrogenous base


Ribose DEOXYribose

new base pairing


A = U

C = G

4. Three main kinds of RNA:

a. mRNA – carries info from DNA to ribosomes

b. tRNA – carries amino acid from cytoplasm to


c. rRNA – help build ribosomes; binds mRNA

and tRNA together to make polypeptide chain


D. Protein Synthesis

1. Transcription - Takes place in the Nucleus


  • Taking info found in the DNA and turning

    it into a molecule of mRNA

  • As in replication, the DNA must unwind;

    however, ONLY ONE strand of DNA

  • is used as a template, the other remains

  • untranscribed

  • *** DNA IS TRANSCRIBED 3’ to 5’


  • Initiation

  • 1.RNA Polymerase – the enzyme that binds

  • to DNA and transcribes it into mRNA.

2. Promoter - a specific sequence of nucleotides

on the DNA that tells RNA poly “bind here”

 this sequence is known as the

“TATA box” (~25 bases upstream from

the gene to be transcribed)

 RNA polymerase will orient itself here

  • ~20 base pairs of DNA are unwound and

    then RNA poly reaches start site and

    begins transciption

b. Elongation

 RNA Polymerase moves along and “reads”

the DNA adding complimentary RNA

nucleotides (Chargoff’s base-pairing rules)

c. Termination (Fig. 1)

 RNA Poly continues until it reaches a “stop signal”


 RNA Poly will detach and release the new mRNA

 This new mRNA (i.e. transcript) now carries the

instructions for making proteins

mRNA leaves the nucleus and goes where?

To a ribosome in the cytoplasm

IMPORTANT: Before the new mRNA moves to

the cytoplasm, SPLICING occurs



Non-coding regions of DNA

 segments of DNA that are cut out b/c they

do not code for any part of a protein

Coding regions of DNA

 segments of DNA that are EXpressed

 code for proteins

mRNA Processing (i.e splicing)

Promoter region

2. Translation

 translating the info on mRNA into amino

acids (i.e. polypeptide chain  protein)

 mRNA is translated 5’ to 3’

 based on the Genetic Code…

CODON - a series of 3 mRNA nucleotides that specify:

1. a particular amino acid

2. a “start signal” (only one)

3. a “stop signal” (3 different)

(64 possible codons)


If a gene is 99 DNA base pairs long ~~~~~~~~~ (99)

The mRNA is also 99 base pairs long ------------------ (99)

The protein will have 33 amino acids @@@@@@ (33)

99/3 = 33

  • Other things needed for translation:

    1. Ribosomes: contain two subunits,

    large (heavy) and small (light)

Interesting fact:

- Ribosomes have 3 sections you APE:

A = Acceptor site: where tRNA enters

P = Peptidyl site: where one amino acid is

bonded to another in the polypeptide


E = Exit site: where tRNA molecule leaves

after dropping off its amino acid

2. tRNA’s: carry the amino acids

- they contain a region called the ANTICODON:

a sequence of 3 nucleotides that is

complimentary to the codon on the mRNA

- where tRNA binds to mRNA

  • Initiation

  •  Ribosome subunits recognize and bind to

  • a recognition sequence on mRNA

  •  Ribosome then begins moving along mRNA

  • in a 5’ to 3’ direction

  • - translation initiates when ribosome reaches

  • the “start” codon (AUG)

  •  the first amino acid (METHIONINE) enters

  • the P site, the ONLY amino acid to do that.

b. Elongation

 The ribosome moves along the mRNA and

new amino acids (carried by tRNA) are

added forming a polypeptide chain

 Amino acids are linked by peptide bonds

c. Termination (Fig. 2)

 The ribosome reaches one of three “stop”

codons (i.e. there is no complimentary

tRNA anticodon)

 No more amino acids can be added so the

ribosome detaches & releases new protein

Signal sequence will determine what proteins are

finished being synthesized on the ribosomes of

the RER

II. Mutations – Changes in the nucleotide

sequence of DNA. Two general categories:

  • Point Mutations: mutations of single genes

  • 1. base-substitutions (one base for another)

  •  two kinds:

  • a. transition:

  • purine for purine (AG)

  • pyrimidine for pyrimidine (CT)

  • b. transversion:

  • pyrimidine for purine (CA)

  • or vice versa

2. Frameshift Mutations

 involve the insertion or deletion of one or

more nucleotides from DNA

 causes a shift in the reading frame

 almost always lead to non-fxning protein


deletion of C


 base-substitutions & frameshifts are either:

1. silent – code for the same amino acid

2. missense – code for a different amino acid

3. nonsense – code for a stop codon

(can lead to nonfxning protein)

 Silent mutation

 Missense mutation

 Nonsense mutation

B. Chromosomal Mutations

 chromosomes may break during replication

and rejoin in abnormal ways

 4 specific types (examine during genetics)


Changes to the amino acid sequence probably changes

the three-dimensional shape of the protein. Since

protein function if highly dependent on shape, this will

lead to impaired fxn or possibly complete nonfxn of the


If an individual inherits mutated genes for a single

protein, and if that protein is essential for life, the

individual may have seriously impaired health or may

even die

Examples: hemophilia, sickle-cell anemia, cystic fibrosis,


III. Gene Expression

A. Gene Regulation, WHY?

--Why don’t organisms just express every gene in

their genome all the time?

--Bacteria (E. coli), for example, live in a wide

range of env’tal conditions and it is more

efficient to express only those genes that are

necessary for survival

--Remember, a high amount of NRG is involved

in gene expression (i.e. TXN & TLN)

--THUS, genes need to be regulated

B. How are genes regulated?

(Prokaryotic Mechanisms)

1. Genes responsible for a given cellular fxn

are organized into operons

2. These operons may be turned on (inducible)

or turned off (repressible) depending on the


3. EXAMPLE: Lactose Metabolism (Inducible)

Tryptophan sythesis (repressible)

The Lac Operon: An Inducible System

Operator –

Promoter –

Repressor –

Structural Genes -

the On/Off switch of a particular gene

where RNA Polymerase binds to DNA to

begin transcription (1 per set of genes)

Protein that blocks RNA Polymerase from

binding, thus NO txn & NO gene expression

(comes from repressor gene)

Indicate the primary structure of a

proteins (i.e specific a.a. sequence)


Situation #1 – Lactose IS present

 When Lactose is digested, it is broken down

as follows:


Lactose ---------------------- glucose & galactose (major)

  • Lactose is known as an inducer – a compound

    that evokes synthesis of an enzyme

  • - in this example, lactose will induce the

  • production of enzymes Z, Y, and A

1. lactose (inducer) will bind to repressor &

cause a shape change

2. Repressor can no longer recognize the

operator binding site; switch is turned ON

3. RNA Polymerse can

bind to the operon’s


4. RNA Polymerase

begins to transcribe

the genes & genes

are then translated

5. The genes produce

enzymes (Z, Y, A)

that help break down


Situation #2: Lactose is NOT present

  • In the absence of lactose, there is no lactose to

  • bind to the Repressor enzyme & block it from

  • binding to operator

  • 2. Thus, Repressor does bind to the operator and

  • the switch is left OFF

3. RNA Polymerase is now blocked from binding

to the lacPromoter

4. Thus NO TRANSCRIPTION of the genes

The Trp Operon: A Repressible System

  • Sometimes tryptophan is present in high

    concentrations (uh, THANKSGIVING, YUM!!!!!!)

    so it is advantageous to stop making enzymes

    for tryptophan synthesis

 These enzymes are said to be repressible

Situation #1: Tryptophan is NOT present

  • Repressor gene produces an inactive repressor

  • which cannot bind to operator (so stays ON)

2. RNA polymerase can bind to operator,

transcribe genes  enzymes will make Trp.

Situation #2: Tryptophan IS present

  • Tryptophan (i.e. co-repressor) will bind to

  • repressor protein and activates the repressor

2. Repressor binds to operator

3. RNA polymerase can’t bind to operator, thus

genes are not transcribed, no Trp made

SO, What is the difference b/n Inducible & Repressible

Systems? Summarize.

In inducible systems, a substance in the env’t

(i.e. the inducer) interacts with the repressor

making it incapable of binding to operator and

blocking transcription. (enzyme will be produced)

In Repressible systems, a substance in env’t

(i.e the corepressor) binds to repressor to make

it capable of binding to operator and blocking


C. Eukaryotic Gene Regulation/Expression

--Eukaryotes do not have a universal

mechanism (i.e operons) that controls the

activity of coding genes

--Rather, regulation is possible at any point in

the pathway b/n gene to functional protein

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