DNA

mRNA

Transcription

Chapter 8The Central Dogma

of Molecular Biology

Cell

Polypeptide(protein)

TranslationRibosome

DNA (DEOXYRIBONUCLEIC ACID)• Nucleic acid that composes

chromosomes and carries genetic information.

CHROMOSOME ORGANIZATION1. A chromosome is an enormous strand of

super coiled DNA.2. Sections of DNA on the chromosome that

code for proteins are called genes.3. Noncoding sections of DNA are called

“junk DNA” (regulatory or unknown function)

BUILDING BLOCKS OF DNAComposed of nucleotides• Nucleotides contain three parts:1. 5-Carbon Sugar (deoxyribose)2. Phosphate Group3. Nitrogen Base (four types, adenine,

guanine, thymine and cytosine)

• Adenine and Guanine are purines (composed of two rings of nitrogen atoms)

• Thymine and Cytosine are pyrimidines (composed of one ring of nitrogen atoms)

STRUCTURE OF DNA• Consists of two strands of nucleotides that

form a twisted ladder (double helix)• Sugar and phosphate alternate along the

sides of the ladder (linked by strong covalent bonds)

• Pairs of nitrogen bases form the rungs of the ladder (linked by weak hydrogen bonds).

• Specific base pairing arrangement (Chargaff’s Rule)A-T : 2 hydrogen bondsC-G : 3 hydrogen bonds

• Nitrogen bases attach to the sugar portion of the side (NOT the phosphate)

• Strands run in opposite directions

FUNCTION OF DNA• DNA codes for proteins (structural

proteins, enzymes, and hormones)• information for building proteins is carried

in the sequence of nitrogen bases• proteins determine physical and metabolic

traits and regulate growth and development.

DNA REPLICATIONProcess in which DNA is copied.PURPOSE OF DNA REPLICATIONGives daughter cells produced by cell division

a complete set of genetic information identical to the parent cell.

WHERE REPLICATION OCCURSNucleusWHEN DURING THE CELL CYCLE

REPLICATION OCCURSInterphase (S)

HOW REPLICATION OCCURS1. Helicase enzymes unzip the parent

strand by separating the nitrogen base pairs.

2. DNA polymerase pairs free DNA nucleotides with the exposed bases on both strands following the base pair rules.

• each strand from the parent molecule serve as a template

3. Hydrogen bonds reform spontaneously sealing the two strands of each DNA molecule together.

RESULTS OF REPLICATION• Two molecules of DNA that are identical• Each is half old (strand from parent) and

half new (strand synthesized by DNA polymerase)

RNA (RIBONUCLEIC ACID)Nucleic acid involved in the synthesis

of proteins

RNA STRUCTUREComposed of nucleotides, but differs from

DNA in three ways.

1. Single strand of nucleotides instead of double stranded

2. Has uracil instead of thymine3. Contains ribose instead of deoxyribose

DNA RNA

Double strand Single strand

A---Thymine A---Uracil

Deoxyribose Ribose

RNA FUNCTIONThree forms of RNA involved in protein

synthesis1. mRNA (messenger): copies instructions

in DNA and carries these to the ribosome.

2. tRNA (transfer): carries amino acids to the ribosome.

3. rRNA (ribosomal): composes the proteins (ribosome).

PROTEIN SYNTHESISCells build proteins following instructions

coded in genes (DNA).

• Consists of two parts, transcription and translation

TRANSCRIPTIONDNA is copied into a complementary strand

of mRNA.

WHY? • DNA cannot leave the nucleus. Proteins

are made in the cytoplasm. mRNA serves as a “messenger” and carries the protein building instructions to the ribosomes in the cytoplasm.

LOCATION OF TRANSCRIPTIONNucleus

HOW TRANSCRIPTION OCCURS1. RNA polymerase untwists and unzips a

section of DNA (usually a single gene) from a chromosome.

2. RNA polymerase pairs free RNA nucleotides to the exposed bases of one of the DNA strands following base pair rules.

• Uracil replaces thymine• Only 1 strand of DNA serves as a template,

the other “hangs out”

3. Newly synthesized mRNA separates from template DNA and DNA zips back up.

Protein Synthesis: Transcription

RESULT OF TRANSCRIPTIONmRNA strand with instructions for building a

protein that leaves the nucleus and goes to the cytoplasm.

TRANSCRIPTION EXAMPLE• Transcribe the following DNA Sequence in

mRNA

TAC CGG ATC CTA GGA TCA AUG GCC UAG GAU CCU AGU

PROTEINSStructural and functional components of

organisms.• Composed of amino acids (monomers).• order of nucleotides in DNA determines

order of amino acids in a protein• One gene codes for one protein

GENETIC CODEThe “language” that translates the sequence

of nitrogen bases in DNA (mRNA) into the amino acids of a protein.

• Codon = three nucleotides on DNA or mRNA

• One codon specifies one amino acid• Some codons are redundant (code for the

same amino acid)• The genetic code is universal to all

organisms

TRANSLATIONInstructions in mRNA are used to build a

protein

PROCESS OF TRANSLATION1. mRNA binds to the ribosome.2. Ribosome searches for start codon (AUG)3. tRNA brings correct amino acid

(methionine) to the ribosome.• Each tRNA carries one type of amino acid. • The anticodon (three nitrogen bases on

tRNA) must complement codon for amino acid to be added to protein chain

4. ribosome reads next codon5. tRNA’s continue lining up amino acids

according to codons6. peptide bonds link amino acids together7. ribosome reaches STOP codon• Amino acid chain is released

LOCATION OF TRANSLATIONribosome (in the cytoplasm)

RESULT OF TRANSLATIONA Protein

Protein Synthesis: Translation

SU

GA

R-P

HO

SP

HA

TE B

AC

KB

ON

E

B A

S E

S

H

P OO

HO

O

O

CH2NH2N

NHN

N

HOH

P

O

O

HO

O

O

CH2

NH2

N

NN

N

H

P

O

OH

HO

O

O

CH2

NH2

N

NN

N

O

A Codon

Guanine

Adenine

Adenine

Arginine

Protein Synthesis: Translation

Protein Synthesis: Translation

AELarge

subunit

P

Small subunit

Translation - Initiation

fMet

UACGAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA5’mRNA

3’

AERibosom

e

PUCU

Arg

Aminoacyl tRNAPhe

Leu

Met

SerGly

Polypeptide

CCA

Translation - Elongation

GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA5’mRNA

3’

AERibosom

e

P

PheLeu

Met

SerGly

Polypeptide

Arg

Aminoacyl tRNA

UCUCCA

Translation - Elongation

GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA5’mRNA

3’

DNA: TAC CTT GTG CAT GGG ATCmRNA AUG GAA CAC GUA CCC UAGA.A MET G.A HIS VAL PRO STOP

IMPORTANT CODONS• AUG = start translation (Met)• UAA, UAG, UGA= stop translation