Fig. 16-16b6

Template strand

5

53

3

RNA primer 3 5

5

3

1

1

3

35

5

Okazaki fragment

12

3

3

5

5

12

3

3

5

5

12

5

5

3

3

Overall direction of replication

The Central Dogma of Molecular Biology

DNA RNA Protein Trait

RNA vs. DNAStudy the images of the two molecules and observe their

similarities & differences.

Basic Principles of Transcription and Translation

• RNA is the intermediate between genes and the proteins for which they code

• Transcription is the synthesis of RNA under the direction of DNA

• Transcription produces messenger RNA (mRNA)

• Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA

• Ribosomes are the sites of translation

The Central Dogma of Molecular Biology

DNA RNA Protein TraitTransferring genetic information into protein.

Transcription: Rewriting DNA into mRNA

•Enzymes add nucleotides to mRNA from 5’ to 3’

•Transcription is initiated at a of DNA that promoter, a sequence signals the start of a gene

Enzymes Modify the “pre-mRNA”

• Enzymes attach a “cap” to the mRNA which binds the mRNA to the ribosome

• Enzymes attach a tail of nucleotides to the mRNA, this controls the “lifespan” of the mRNA

mRNA is Spliced

• Introns: Non-coding regions of DNA

• Exons: Coding regions of DNA

The Functional and Evolutionary Importance of Introns

• Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during RNA splicing

• The number of different proteins an organism can produce is much greater than its number of genes

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The Genetic Code

• How are the instructions for assembling amino acids into proteins encoded into DNA?

• There are 20 amino acids, but there are only four nucleotide bases in DNA

• How many bases correspond to an amino acid?

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Codons: Triplets of Bases

• The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words

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• During transcription, one of the two DNA strands called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript

• During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction

• Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide

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• Codons along an mRNA molecule are read by translation machinery in the 5 to 3 direction

• Each codon specifies the addition of one of 20 amino acids

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Fig. 17-4

DNAmolecule

Gene 1

Gene 2

Gene 3

DNAtemplatestrand

TRANSCRIPTION

TRANSLATION

mRNA

Protein

Codon

Amino acid

The Genetic Code

Translation: The RNA directed synthesis of a polypeptide

• Molecules of tRNA are not identical:– Each carries a specific amino acid on one end– Each has an anticodon on the other end; the

anticodon base-pairs with a complementary codon on mRNA

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Translation

mRNA codons are translated into a polypeptide chain

Ribosomes

• Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis

• The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)

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• A ribosome has three binding sites for tRNA:– The P site holds the

tRNA that carries the growing polypeptide chain

– The A site holds the tRNA that carries the next amino acid to be added to the chain

– The E site is the exit site, where discharged tRNAs leave the ribosome

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Fig. 17-18-4

Amino endof polypeptide

mRNA

5

3E

Psite

Asite

GTP

GDP

E

P A

E

P A

GDPGTP

Ribosome ready fornext aminoacyl tRNA

E

P A

• During and after synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape

• Proteins may also require post-translational modifications before doing their job

• Some polypeptides are activated by enzymes that cleave them

• Other polypeptides come together to form the subunits of a protein

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Fig. 17-25

TRANSCRIPTION

RNA PROCESSING

DNA

RNAtranscript

3

5RNApolymerase

Poly-A

Poly-A

RNA transcript(pre-mRNA)

Intron

Exon

NUCLEUS

Aminoacyl-tRNAsynthetase

AMINO ACID ACTIVATIONAminoacid

tRNACYTOPLASM

Poly-A

Growingpolypeptide

3

Activatedamino acid

mRNA

TRANSLATION

Cap

Ribosomalsubunits

Cap

5

E

P

A

AAnticodon

Ribosome

Codon

E