8.4 Transcription• Proteins are made in the ribosomes outsideoutside the nucleus.

• DNA is copied (replicated) in the nucleus but cannot leave the nucleus.

THEREFORE…….• A message must be sent to the

ribosomes in the cytoplasm telling them what proteins to make.

• This message is carried by a nucleic acid called messenger (mRNA ).

8.4 TranscriptionRNA Structure

1. RNA is a single strand

2. RNA has the sugar ribose

3. Adenine bonds with Uracil (NOT thymine).

4. Can leave the nucleus

RNA differs from DNA in 4 ways:

8.4 Transcription

KEY CONCEPT Transcription converts a gene into a single-stranded RNA molecule.

8.4 TranscriptionRNA carries DNA’s instructions.

• The central dogma states that information flows in one direction from DNA to RNA to proteins.

8.4 Transcription

• The central dogma includes three processes.1. Replication

2. Transcription

3. Translation

• RNA is a link between DNA and proteins.

replication

transcription

translation

8.4 Transcription

Transcription copies DNA to make a strand of RNA.

DNA

8.4 Transcription

start site

nucleotides

transcription complex

Transcription occurs in two major steps.1. DNA unzips & unwinds at the start of a gene.

8.4 Transcription

DNA

• Transcription occurs in two major steps.2. Nucleotides pair with their complementary bases

on one strand of the DNA.– RNA polymerase bonds the nucleotides together.– The DNA helix winds again as the gene is transcribed.

RNA polymerase moves along the DNA

8.4 Transcription

The RNA strand detaches from the DNA once the gene is transcribed. Then it leaves the nucleus and heads to a ribosome.

RNA

8.4 Transcription

Transcription makes three types of RNA.

1. Messenger RNA (mRNA) - carries the message that will be translated to form a protein.

2. Ribosomal RNA (rRNA) - forms part of ribosomes where proteins are made.

3. Transfer RNA (tRNA) - brings amino acids from the cytoplasm to a ribosome.

8.4 TranscriptionKEY CONCEPT

Translation converts an mRNA message into a polypeptide, or protein.

8.4 TranscriptionAmino acids are coded by mRNA base sequences.

codon formethionine (Met)

codon forleucine (Leu)

• Translation converts mRNA messages into polypeptides.

• A codon is a sequence of three nucleotides that codes for an amino acid.

8.4 TranscriptionThe language of amino acids is based on codons

1 codon = 3 mRNA nucleotides

1 codon = 1 amino acid

A U A U A U G C C C G C

How many codons are in this sequence of mRNA?

4

How many Amino Acids does this gene code for?

4

8.4 Transcription• The genetic code matches each codon to its amino

acid or function.

• three stop codons

• one start codon, codes for methionine

13.1 Ecologists Study Relationships

How to read a codon table

Use in protein synthesis for translating the mRNA code into amino acid

sequence

8.4 Transcription

You need a sequence of mRNA

• Where does this come from?

From the transcription

of DNA --> mRNA

DNA sequence: TAC GGA CAT AAC ACC TGC ATC

mRNA sequence: AUG CCU GUA UUG UGG ACG UAG

Example:

8.4 TranscriptionTranscription

• mRNA sequence leaves the nucleus and travels to the cytoplasm to a free floating ribosome or to the rough ER.

• It will attach to the ribosome and begin the second step of protein synthesis, translation.

8.4 TranscriptionTranslation

• mRNA is read as a series of codons (three letters) within the ribosome.

• tRNA molecules have an anticodon sequence of letters that are complements to the mRNA

ex: mRNA CGA UCC (codon) tRNA GCU AGG (anticodon)

8.4 TranscriptionSo now we get to the codon table!

• Locate the first letter of your codon using the left side of the table.

• Ex. AUG• look for the A

8.4 Transcription

• Now move to the second letter of your codon which is ‘U’

• Look at the top of the table where you see the title ‘2nd letter’

• Find the letter ‘U’ and follow it down until it intersects with the letter ‘A’ from the left side.

• You should see four amino acids (isoleucine, isoleucine, isoleucine, and (start) methionine.

8.4 Transcription• Down to the last letter

of the codon!• Look to the right hand

side for the third letter. Find the letter ‘G’ which will intersect with the box that had our four choices.

• Move your finger from the ‘G’ on the left over to the left and you should land on ….. Methionine (start)

• Yes you did it!!!• Now try another codon

8.4 TranscriptionTry the codon CAC

Don’t peek until you come up with your answer!

Did you get the amino acid ‘histidine’?

8.4 TranscriptionUsing this chart, you can determine which amino acid the codon “codes” for!

Which amino acid is encoded in the codon CAC?

8.4 Transcription

Notice there is one start codon AUG. Transcription begins at that codon!

8.4 Transcription

Notice there are three stop codons. Transcription stops when these codons are encountered.

8.4 TranscriptionWhat do these codons have to do with proteins?

• Each codon represents an amino acid that will eventually form a protein that is used within a cell.

• Proteins are made up of hundreds of amino acids in a specific sequence.

• When they get “out of order’ a mutation occurs.

Long string of amino acids will form

8.4 Transcription• A change in the order in which codons are read

changes the resulting protein.

• Regardless of the organism, codons code for the same amino acid.

8.4 TranscriptionAmino acids are linked to become a protein.

• An anticodon is a set of three nucleotides that is complementary to an mRNA codon.

• An anticodon is carried by a tRNA.

8.4 Transcription• Ribosomes consist of two subunits.

– The large subunit has three binding sites for tRNA.

– The small subunit binds to mRNA.

8.4 Transcription• For translation to begin, tRNA binds to a start codon

and signals the ribosome to assemble.– A complementary tRNA molecule binds to the

exposed codon, bringing its amino acid close to the first amino acid.

8.4 Transcription

8.4 Transcription

• The ribosome helps form a peptide bond between the amino acids.

8.4 Transcription

– The now empty tRNA molecule exits the ribosome.

• Once the stop codon is reached, the ribosome releases the protein and disassembles.

8.4 Transcription

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mRNA

From gene to protein

DNA

transcription

nucleus cytoplasm

protein

translation

trait

U C CCCCCA A U G U G A A A A AG G G GU Uribosome

tRNA

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8.4 Transcriptionprotein

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transcription

cytoplasm

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8.4 Transcription

From gene to protein

transcriptiontranscription

translationtranslation

proteinprotein