Protein Synthesis

• The whole point of all of this is to make proteins.

• DNA (gene) RNA amino acid sequence Protein

***Why are proteins so important?***

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Protein Synthesis

Proteins Synthesis: Transcription

– mRNA

• Why?– Because

mRNA can leave the nucleus.

• What kind of RNA is DNA transcribed into?

A little more detail

• Initiation: begins with Promoter DNA– Identifies which strand will be transcribed– Gives a spot for RNA polymerase to bind

• Elongation : – RNA polymerase adds nucleotides 5’ to 3’ (about 40nts/sec)

• Termination: – Terminator sequence on DNA codes for polymerase to

detach and mRNA to leave nucleus and go to ribosome

Fig. 17-3b-3

(b) Eukaryotic cell

TRANSCRIPTION

Nuclearenvelope

DNA

Pre-mRNARNA PROCESSING

mRNA

TRANSLATION Ribosome

Polypeptide

mRNA modification

• mRNA receives a “cap” and a “tail” to protect it while in transit to the ribosome

• The mRNA includes introns and exons .

– Introns are removed before it leaves nucleus

Protein Synthesis: Transcription

Fig. 17-10

Pre-mRNA

mRNA

Codingsegment

Introns cut out andexons spliced together

5 Cap

Exon Intron5

1 30 31 104

Exon Intron

105

Exon

146

3Poly-A tail

Poly-A tail5 Cap

5 UTR 3 UTR1 146

So what’s the purpose of exons and introns?

• One gene can code for different proteins..

• Depending on which sections of the mRNA template are considered exons during processing.

• This would explain our low number of genes for such a complex organism as a human

Humans have only 1.5x as many genes as a fruit fly

Polypeptide

Translation

Transcription

Gene 1

DNA molecule

DNA strand

Codon

Amino acid

Gene 2

Gene 3

RNA

Proteins Synthesis: Transcription

• Nucleotides are arranged into triplets called codons.– Example: AAC CG T TAC

T TG GCA ATG

– Each codon specifies (codes for) a particular amino acid.

– The sequence of the codons in the DNA will be transferred to the RNA, which will then determine the sequence of amino acids.

Polypeptide

Translation

Transcription

DNA strand

Codon

Amino acid

RNA

Protein Synthesis: Transcription• When RNA is being synthesized by enzymes, they pair the bases just like they would

during DNA replication.

• There is one BIG difference, though.

– RNA does not have the base Thymine.

– Instead, if the enzymes come across an Adenine base, they pair it with Uracil.

– Can you transcribe the following DNA strand?

DNA: ATC GGA TAC GGG CCAmRNA:

Protein Synthesis: Transcription• But let’s remember the goal here…

Proteins! Proteins! Proteins!

• Now that we have our mRNA with the DNA code, the mRNA can leave the nucleus and head to the ribosome!

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