protein synthesis – turning genes into proteins

Protein Synthesis – Turning genes into proteins

Gene—DNA segment that codes for one polypeptide

2 steps of protein (polypeptide) synthesis:

1. Transcription – genes ‘transcribed’ from DNA to messenger RNA (mRNA)

2. Translation – mRNA ‘translated’ into proteins by ribosomes and transfer RNA (tRNA)

From DNA to Protein

Figure 3.33

DNA

From DNA to Protein

Figure 3.33

DNATranscription

From DNA to Protein

Figure 3.33

DNA

Pre-mRNARNA Processing

Transcription

mRNA

From DNA to Protein

Figure 3.33

DNA

Pre-mRNARNA Processing

Transcription

mRNA

Nuclearenvelope

From DNA to Protein

Figure 3.33

Nuclearenvelope

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide

Translation

RNA Processing

Transcription

Figure 3.34

Transcription: DNA genes mRNA

Transcription factors = proteins that…

Loosen histones from DNA

Bind to promoter = DNA sequence specifying start site of mRNA synthesis

Help bind RNA polymerase to promoter

RNA Polymerase = enzyme that makes mRNA

Unwinds DNA template

DNA bases (ATGC) pair with RNA bases (UACG)

A-U, T-A, C-G, G-C

Joins RNA nucleotides together

Releases at termination signal (specific set of DNA bases at end of gene) to stop transcription

Released mRNA

mRNA

Template strandof DNA

RNA polymeraseNuclear pore

Nuclear membrane

Nucleus

T

A

Figure 3.34

Codingstrand

Templatestrand

PromoterTermination signal

Transcription unit

In a process mediated by a transcriptionfactor, RNA polymerase binds topromoter and unwinds 16–18 basepairs of the DNA template strand

RNApolymerase

Unwound DNA

RNAnucleotides

RNA polymerasebound to promoter

mRNA synthesis begins

RNA polymerase moves down DNA;mRNA elongates

RNAnucleotides

mRNA synthesis is terminatedRNApolymerase

mRNA

DNA

mRNA transcript(a)

RNAnucleotides

RNA polymerase

Unwindingof DNA

Coding strand

Rewinding of DNA

mRNARNA-DNAhybrid region

Template strand

(b)

The mRNA code is then translated into proteins

Codon – 3 bases of mRNA code for 1 amino acid

Processed mRNA leaves nucleus and binds with ribosome

Ribosome coordinates polypeptide construction with tRNA

Translation: mRNA proteins

Figure 3.16

tRNA – brings amino acids to ribosomes

Transfer RNA Molecules Serve as Interpreters During TranslationTransfer RNA Molecules Serve as Interpreters During Translation

tRNA: single strand of RNA about 80 nucleotides in length

One single stranded loop = anticodon (special triplet of bases that pairs with complementary mRNA sequence)

At the other end, aa attachment site

Enzymes add amino acids to tRNA

(one enzyme per amino acid)

Three-dimensional structure

tRNA enzyme

ATP

Figure 3.36

Released mRNA

mRNA


RNA polymerase

Nuclear pore

Nuclear membrane

Nucleus

Figure 3.36

After mRNA processing, mRNAleaves nucleus and attaches toribosome, and translation begins.

Largeribosomalsubunit

Small ribosomalsubunit

Released mRNA

mRNA


RNA polymerase

Nuclear pore

Nuclear membrane

Portion of mRNAalready translated

Direction ofribosome advance

Nucleus

Codon 16Codon 15 Codon 17

1

Figure 3.36


Amino acids

tRNA

Aminoacyl-tRNAsynthetase



Released mRNA

mRNA


RNA polymerase

Nuclear pore

Nuclear membrane



Nucleus


1

Energized by ATP, the correct amino acid is attached to each species of tRNA by aminoacyl-tRNA synthetase enzyme.

Figure 3.36


Amino acids

tRNA


tRNA “head” bearinganticodon



Released mRNA

mRNA


RNA polymerase

Nuclear pore

Nuclear membrane



Nucleus

Incoming aminoacyl-tRNA hydrogen bonds via its anticodon to complementary mRNA sequence (codon) at the A site on the ribosome.



1

2

Figure 3.36


Amino acids

tRNA





Released mRNA

mRNA


RNA polymerase

Nuclear pore

Nuclear membrane



Nucleus


As the ribosomemoves along the mRNA, a new amino acid is added to the growing protein chain and the tRNA in the A site is translocated to the P site.



1

2

3

Figure 3.36


Amino acids

tRNA





Released mRNA

mRNA


RNA polymerase

Nuclear pore

Nuclear membrane



Nucleus

Once its amino acid isreleased, tRNA is ratcheted to the E site and then released to reenter the cytoplasmic pool, ready to be recharged with a new amino acid.


As the ribosomemoves along the mRNA, a new amino acid is added to the growing protein chain and the tRNA in the A site is translocated to the P site.



1

2

34

At the ribosome…At the ribosome…

P site on ribosome holds tRNA that holds growing polypeptide

A site on ribosome receives next tRNA w/ next amino acid

Amino acid is added to polypeptide

tRNA shift positions from A to P after amino acids are added

Amino Acids Added Until Amino Acids Added Until Stop Codon Terminates Stop Codon Terminates

TranslationTranslation

Ribosomes disassemble and start translating another mRNA

Information Transfer from DNA to RNA to polypeptide

Figure 3.38

Cells differentiate by turning on different genes (i.e. making different proteins)

Different genes turned ‘on’ or ‘off’ by chemical signals

Early development likely controlled by simple gradients of CO2 and O2

Signal Mechanism of Protein Synthesis

Figure 3.19

Cytosol

mRNA

ERcisterna

ERmembrane

Signal-recognitionparticle(SRP)

Signalsequence

Receptorsite

1

Ribosomes of rough ER are transient (i.e. not permanently attached)

Signal sequence = short amino acid chain of new protein that binds to an SRP (signal recognition particle)


Figure 3.19

Cytosol

mRNA

ERcisterna

ERmembrane


Signalsequence

Receptorsite Growing

polypeptide

1

2

SRP guides ribosome – polypeptide (protein) complex to ER by binding to receptors on rough ER

SRP removed by enzymes at receptor site


Figure 3.19

Cytosol

Ribosomes

mRNA

ERcisterna

ERmembrane


Signalsequence

Receptorsite

SignalsequenceremovedGrowing

polypeptide

1

2

3

Original signal sequence quickly removed as well

Protein continues forming inside ER (sometimes sugars added)


Figure 3.19

Cytosol

Ribosomes

mRNA

Releasedglycoprotein

ERcisterna

ERmembrane


Signalsequence

Receptorsite


polypeptide

1

2

34

Primary structure completes, ribosomes detach

Folding of proteins often aided by chaperones

Integral proteins remain embedded in membrane


Figure 3.19

Cytosol

Ribosomes

mRNA

Transportvesiclebudding off


ERcisterna

ERmembrane


Signalsequence

Receptorsite

Sugargroup


polypeptide

1

2

34

5

Coatomer-coated transport vesicle breaks away

Coatomer = proteins that aid intracellular vesicle formation


Figure 3.19

Cytosol

Ribosomes

mRNA

Coatomer-coatedtransportvesicle

Transportvesiclebudding off


ERcisterna

ERmembrane


Signalsequence

Receptorsite

Sugargroup


polypeptide

1

2

34

5

Where does this go?

Golgi receives from its cis side, ships from the trans side

Figure 3.21

Secretion by exocytosisExtracellular fluid

Plasma membrane

Vesicle incorporatedinto plasma membrane

Coatomercoat

Lysosomes containing acidhydrolase enzymes

PhagosomeProteins in cisterna

Membrane

Vesicle

Pathway 3

Pathway 2

Secretory vesicles

Proteins

Pathway 1

Golgi apparatus

CisternaRough ER

protein synthesis – turning genes into proteins

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translation mrna

mrna processing

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dna sequence

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bases of mrna code

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