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Protein Synthesis

Unit 6

Goal: Students will be able to describe the

processes of transcription and translation.

Types of RNA

• Messenger RNA (mRNA) makes a copy of

DNA, carries instructions for making

proteins,

• Transfer RNA (tRNA) decodes mRNA and

transfers amino acids to the ribosome

• Ribosomal RNA (rRNA) build the protein

by bonding amino acids together.

Ribosomes are made of rRNA.

Central Dogma

• DNA RNA Protein• DNA is the instructions for making the

protein

• RNA copies the instructions & builds the

protein

• The protein is built by the ribosome and

used either by that cell or transported to

other cells.

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DNA RNA Protein

Nuclear

membrane

Transcription

RNA Processing

Translation

DNA

Pre-mRNA

mRNA

Ribosome

Protein

Eukaryotic Cell

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Codons & Amino Acids

• Codon = 3 base (letter) sequence

on mRNA

• Amino Acid = monomer of protein

•1 codon CODES FOR 1 amino acid

• Some amino acids have multiple codons.

• 64 possible codons; 20 Amino Acids in nature

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Codon Chart•Chart tells which Amino Acid that codon means•Find the correct letter in the 1st

position 2nd

position 3rd

position (where do those meet?)•Example: AUGcodes for Met (Methionine)

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Name the Amino Acids

• GGG?

• UCA?

• CAU?

• GCA?

• AAA?

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Start/Stop Codons

• There will always be a “Start” codon

(AUG) at the beginning. It is like

capitalizing the 1st letter of a sentence.

• There will always be a “Stop” codon

(UGA, UAA, UAG) at the end. It is like a

period at the end of a sentence.

Protein Synthesis: Protein synthesis

uses the information in genes to make

proteins. 2 Steps

1.Transcription

–Copying the DNA into mRNA

2.Translation

–tRNA decodes mRNA & delivers the

amino acids

–rRNA bonds the amino acids

together to build the protein

Transcription: Where?

• mRNA is made in the NUCLEUS

• mRNA will leave the nucleus through the

pores in the nuclear membrane

• (DNA does NOT leave the nucleus)

Transcription: Involved Molecules

• DNA (instructions for making the protein)

• mRNA (copy of the instructions)

• RNA polymerase (enzyme that builds

mRNA)

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RNA Polymerase = The Enzyme responsible for Transcription

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Transcription: Produces?• mRNA (messenger

RNA)

• Long Straight chain of Nucleotides

• Made in the Nucleus

• Copies DNA & leaves through nuclear pores

• Contains the Nitrogen Bases A, G, C, U ( no T )

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Remember the Complementary Bases

On DNA:A=TC=G

On RNA:A=UC=G

DNA RNA

G C C G

T A A U

Transcription: What Happens?Big Idea: DNA is copied into a

complementary sequence of mRNA.

1.RNA polymerase is the enzyme that is responsible for

transcription

2.RNA polymerase reads the gene and adds the correct

complementary RNA base to make a new strand of

RNA.

– DNA = GCCATT

– mRNA = CGGUAA

3. The mRNA gets processed (edited and packaged)

4. mRNA leaves the nucleus and goes to the

cytoplasm.

RNA

DNA

RNApolymerase

Adenine (DNA and RNA)

Cystosine (DNA and RNA)

Guanine(DNA and RNA)

Thymine (DNA only)

Uracil (RNA only)

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Transcription: Goes to?•mRNA leaves the nucleus through the pores in the nuclear membrane, travels through the cytoplasm, and goes to the ribosomes

Translation

Translation: Where? • Ribosome

–Free floating ribosome in cytoplasm

(makes proteins for that cell)

OR

–attached ribosome on the Rough

ER (makes proteins to be exported

to other cells)

Translation: Involved Molecules

• mRNA (copy of the instructions)

• tRNA (translates the instructions)

• rRNA (builds the protein)

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Transfer RNAamino acid

attachment site

U A C

anticodoncopyright cmassengale

• Decodes (translates) the mRNA

• Delivers (transfers) amino acids to the ribosome

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Codons and Anticodons• The 3 bases of an anticodon are

complementary to the 3 bases of a codon

• Ex: Anticodon UGA

• Codon ACU

UGA

ACU

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Ribosomal RNA (rRNA)

•Ribosomes are made of rRNA

•Site of Protein Synthesis

•Forms peptide bonds between amino acids

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Translation: Produces?• Chain of

amino acids

• Polypeptide

(protein)

Translation: What Happens?

• Big Idea: The mRNA is then

TRANSLATED by tRNA at

the RIBOSOME (made of

rRNA). The RIBOSOME then

BUILDS the protein by

bonding amino acids

together.

Translation: What Happens?

1. tRNA translates the mRNA using “anticodons.” (complementary to the codon)

– Each tRNA molecule has an anticodon on one end and an amino acid on the other.

2. tRNA transfers the amino acids to the ribosome in the correct sequence.

3. rRNA forms peptide bonds between the amino acids. This links them together to build the protein.

Translation: Where does it go?

The protein will do one of 2 things

• Stay inside the cell to carry out functions

for that cell

OR

• Be exported to other cells

– Rough ER Golgi Exocytosis

Messenger RNA

Messenger RNA is transcribed in the nucleus.

Transfer RNA

The mRNA then enters the cytoplasm and attaches

to a ribosome. Translation begins at AUG, the start

codon. Each transfer RNA has an anticodon

whose bases are complementary to a codon on the

mRNA strand. The ribosome positions the start

codon to attract its anticodon, which is part of the

tRNA that binds methionine. The ribosome also

binds the next codon and its anticodon.

mRNA Start codon

Ribosome

Methionine

Phenylalanine tRNALysine

Nucleus

mRNA

The Polypeptide “Assembly Line”

The ribosome joins the two amino acids—

methionine and phenylalanine—and breaks the

bond between methionine and its tRNA. The

tRNA floats away, allowing the ribosome to bind

to another tRNA. The ribosome moves along

the mRNA, binding new tRNA molecules and

amino acids.

mRNA

Ribosome

Translation direction

LysinetRNA

tRNA

Ribosome

Growing polypeptide chain

mRNA

Completing the Polypeptide

The process continues until the ribosome reaches

one of the three stop codons. The result is a growing

polypeptide chain.