Chapter 17

From Gene to Protein

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DNA contains the genes that make us who we are.

The characteristics we have are the result of the proteins our cells produce during the process of transcription and translation.

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The Bridge Between DNA and Protein

Main Questions:Somehow the information content in DNA--

--the specific sequence of nucleotides along the DNA--strands needs to be turned into protein. How does this information determine the organism’s

appearance? How is the information in the DNA sequence

translated by a cell into a specific trait?

The Bridge Between DNA and Protein

RNA is the single stranded compound that carries the message from the DNA to the ribosome for translation into protein. Recall, DNA = A,T,C,G; RNA= A,U,C,G

The order of these bases carries the code for the protein which is constructed from any or all of the 20 amino acids.

RNARNA is used because it is a way to protect

the DNA from possible damage.Many copies of RNA can be made from one

gene, thus, it allows many copies of a protein to be made simultaneously.

mRNA and RNA PolymerasemRNA is the “messenger” or vehicle that

carries the genetic information from the DNA to the protein synthesizing machinery.

RNA polymerase pries apart the DNA and joins RNA nucleotides together in the 5’-->3’ direction (adding, again, to the free 3’ end).

RNA polymerase is just like DNA polymerase, but it doesn’t need a primer.

Transcription and TranslationThe process of going from gene to mRNA is

called transcription.Translation is the process that occurs when

the mRNA reaches the ribosome and protein synthesis occurs.

The mRNA produced during transcription is read by the ribosome and results in the production of a polypeptide.

The polypeptide is comprised of amino acids.

The specific sequence of amino acids is determined by the genetic code on the DNA.

Transcription and Translation

TranscriptionThe gene determines the sequence of

bases along the length of the mRNA molecule.

One of the two regions of the DNA serves as the template.

The DNA is read 3’-->5’ so the mRNA can be synthesized 5’-->3’

Not all regions of DNA codes for protein.

There are numerous segments of DNA to which transcription factors bind.

These govern the synthesis of mRNA and regulate gene expression. Promoter sequence Termination sequence Enhancers

Transcription

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Other Functions of Non-Coding DNAOther regions of non-coding DNA are

involved in regulating gene expression, coding for tRNA molecules, and ensuring that the DNA maintains its length (telomeres).

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tRNA Structure and Function tRNA, like mRNA, is made

in the nucleus and is used over and over again.

tRNA binds an aa at one end and has an anticodon at the other end.

The anticodon acts to base pair with the complementary code on the mRNA molecule, and delivers an aa to the ribosome.

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Additionally, in eukaryotes, once genes get transcribed, the RNA that is produced is often modified before getting translated.

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Transcription and Translation

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Post Transcriptional ModificationIn eukaryotes, once the primary transcript is

made, it is spliced and modified before getting translated into protein.

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mRNA Modification The initial transcript (~8000

bp) is reduced (to ~1200 on average).

The large, non-encoding regions of the DNA that get transcribed are spliced out. Introns--intervening

regions are removed. Exons--expressed

regions are kept.

mRNA ModificationSome untranslated regions of the exons are

saved because they have important functions such as ribosome binding.

TranslationmRNA triplets are called codons.Codons are written 5’-->3’Codons are read 5’-->3’ along the mRNA

and the appropriate aa is incorporated into the protein according to the codon on the mRNA molecule.

As this is done, the protein begins to take shape.

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Protein SynthesisMany copies of protein can be made

simultaneously within a cell using a single mRNA molecule.

This is an efficient way for the cell to make large amounts of protein in times of need.

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Polyribosome Here you can see an

mRNA transcript being translated into many copies of protein by multiple ribosomes in a eukaryote.

This is a way in which the cell can efficiently make numerous copies of protein.

Polyribosome Here it is again in a

prokaryote. The process essentially

the same between prokaryotes and eukaryotes.

The main exception is where it occurs.

One Main DifferenceBetween prokaryotes and eukaryotes, there

is one main difference between transcription and translation. The two processes can occur simultaneously in prokaryotes because they lack a nucleus.

In eukaryotes, the two processes occur at different times. Transcription occurs in the nucleus, translation occurs in the cytoplasm.

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TranslationSo how, exactly, does the cell translate

genetic code into protein?

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The Genetic CodeScientists began wondering how the genetic

information contained within DNA instructed the formation of proteins.

How could 4 different base pairs code for 20 different amino acids?

1:1 obviously didn’t work; a 2 letter code didn’t work either; but a 3 letter code would give you more than enough needed.

The Genetic Code Codons are composed

of triplets of bases. 61 of the 64 codons

code for amino acids. 3 of the codons code

for stop codons and signal an end to translation.

AUG--start codon

Genetic CodeThe genetic code is said to be redundant.More than one triplet codes for the same

amino acid.One triplet only codes for one amino acid.The reading frame is important because any

error in the reading frame codes for gibberish.

RibosomesrRNA genes are found on chromosomal

DNA and are transcribed and processed in the nucleolus.

They are assembled and transferred to the cytoplasm as individual subunits.

The large and small subunits form one large subunit when they are attached to the mRNA.

Ribosomes The structure of ribosomes fit their

function. They have an mRNA binding site,

a P-site, an A-site and an E-site. A-site (aminnoacyl-tRNA) holds the

tRNA carrying the next aa to be added to the chain.

P-site (peptidyl-tRNA) holds the tRNA carrying the growing peptide chain.

E-site is the exit site where the tRNAs leave the ribosome.

Each of these are binding sites for the mRNA.

The 3 Stages of Protein Building1. Initiation2. Elongation3. TerminationAll three stages require factors to help them

“go” and GTP to power them.

1. Initiation Initiation brings together

mRNA, tRNA and the 2 ribosomal subunits.

Initiation factors are required for these things to come together.

GTP is the energy source that brings the initiation complex together.

1. Initiation Initiation brings together

mRNA, tRNA and the 2 ribosomal subunits.

Initiation factors are required for these things to come together.

GTP is the energy source that brings the initiation complex together.

2. Elongation The elongation stage is

where aa’s are added one by one to the growing polypeptide chain.

Elongation factors are involved in the addition of the aa’s.

GTP energy is also spent in this stage.

3. Termination Termination occurs when a stop codon on the mRNA

reaches the “A-site” within the ribosome. Release factor then binds to the stop codon in the “A-site”

causing the addition of water to the peptide instead of an aa. This signals the end of translation.

Polypeptide SynthesisAs the polypeptide is being synthesized, it

usually folds and takes on its 3D structure.Post-translational modifications are often

required to make the protein function. Adding fats, sugars, phosphate groups, etc. Removal of certain proteins to make the protein

functional. Separately synthesized polypeptides may need to

come together to form a functional protein.