DNAThe Code of Life

Topics Covered

• The DNA Molecule

• DNA Replication

• How DNA works

• Transcription

• Translation

DNADeoxyribonucleic acid

Discovered by Watson and Crick

Made up of nucleotides

Double helix• 2 coiled strands (twisted

ladder)• Strands are

complimentary

Nucleotide

THREE PARTSSugar

• Deoxyribose (in DNA)

• Ribose (in RNA)Phosphate groupNitrogen Base

S

P

NB

The Four Nucleotides

C•Cytosine (C)

T•Thymine (T)

A•Adenine (A)

G

•Guanine (G)PURINES

PYRIMIDINES

A

Two Complimentary Strands

BASE PAIRING(Chargaff’s Rule)

• A – T or T – A

• G – C or C - G

AT

C

G

C

G

T

The Double Helix

DNA Replication• Why does DNA need to replicate (copy)

itself?– MITOSIS

• Replication preserves the sequence of bases in an organism’s DNA

DNA Replication: Step 1

• The DNA molecule unzips

TACGATTAA

ATGCTAATT

TACGATTAA

ATGCTAATT

Replication: Steps 2 & 3• DNA

polymerase adds the correct complimentary nucleotide to each exposed strand

• A complimentary strand is created for each original strand

TACGATTAA

ATGCTAATT

TAATT

ATTAA

A TT AG CC G

DNA, Genes & Chromosomes

• Chromosomes – long strands of DNA– How many

chromosomes do humans have?

• Genes – part of a chromosome– Each gene codes for a

different protein

ProteinsGene Expression

• The use of genetic material to make a protein

• Protein ultimately determines a trait

Amino Acids• Building blocks of

proteins

How does DNA code for a protein?

• Nucleotide bases “spell out” the message

• ORDER is important– OIGLOBY– BIOLOGY

• DNA alphabet has 4 letter– G A T C

Protein Synthesis

• Transcription– An RNA copy of DNA is made

• Translation– RNA is translated in to a protein

DNA RNAtranscription

Proteintranslation

RNA• Ribonucleic acid• Messenger for

DNA

• Single stranded

• No thymine (T) has uracil (U) instead

A

C

G

U

TranscriptionAn RNA copy of DNA is made

•Called mRNA• Only one side of DNA is copied

Occurs in the Nucleus

Once mRNA is made it leaves the nucleus (through a pore)

TRC: Step 1

• The DNA molecule unzips

TACGATTAA

ATGCTAATT

TACGATTAA

ATGCTAATT

TRC: Step 2• RNA

polymerase binds to one strand of DNA at the promoter site

TACGATTAA

ATGCTAATT

TRC: Step 3• RNA pol. Pairs

each nucleotide base with its complimentary base

• One strand Only!!

TACGATTAA

ATGCTAATT

AU

CUAAUU

G

TRC: Step 4

• New messenger RNA (mRNA) leaves the nucleus through a nuclear pore

A U G C U A A U U

nucleuscytoplasm

Translation

• RNA translated into a protein• Two types of RNA work together

– mRNA – messenger– tRNA – transfer

• Occurs in the cytoplasm• Genetic Code is the translator• Each “word” is 3 letters

– GCA = alanine (see genetic code)

Genetic Code

• CODON – three letter “word” in mRNA– Specify a particular amino acid

• ANTICODON– Three leter “word” in tRNA– Complimentary to the codon

The Genetic Code

TRL: Step 1

• mRNA “start” codon binds to a ribosome– AUG = start– AUG =

methionineA U G C U A A U U

ribosome

TRL: Steps 2 & 3

• tRNA picks up an amino acid in the cytoplasm and carries to ribosome

• tRNA anticodon binds to complimentary mRNA codon

A U G C U A A U U

U A C

met

U A C

met

TRL: Steps 4 & 5

• Amino acid detaches from the tRNA and attaches to a growing protein chain

• tRNA leaves to find another amino acid A U G C U A A U U

U A C

metU A C

met

A U G C U A A U U

met

A U G C U A A U U

met

TRL: Step 6

• Continues as mRNA passes through ribosome and one tRNA after another is selected to match the mRNA codons

G A U

leu

G A Uleu

met

G A U

leuG A U

leu

TRL: Step 7

• Ends when a “stop” codon is reached and the newly assembled protein is released into the cell

A U G C U A A U U

leu

met

stop

AT

C

G C

A

T

G

C

G

A

T