Chapter 9

DNA – The Genetic Material

EARLY DISCOVERIES… Fred Griffith – trying to develop a

vaccine for pneumonia

…..TRANSFORMATION

Oswald Avery….

Discovered that the substance absorbed by the harmless bacteria was DNA…

DNA is the Transformation factor!

Hershey – Chase Experimentstudies bacteriophages…

Discovering DNA:

Erwin Chargaff found that the amount of

Adenine always equals the amount of Thymine !....AND that Guanine amounts always equaled Cytosine.

Rosaline Franklin and Maurice Wilkins –X-Ray diffraction shows DNA is a coiled helix

Discovered the 3-dimensional structure of DNA……. Showing base pairing in a double helical

staircase…..Nobel prize 1950

Francis Crick and James Watson

Structure of DNA I. DNA: (Deoxyribonucleic Acid) Building blocks called… Nucleotides: (3 parts): 1.Deoxyribose (simple sugar) 2.Phosphate Group 3.Nitrogen Base(A, C, T or G)

ADENINE CYTOSINE THYMINE GUANINE

BASE PAIRING

PURINES – ADENINE AND GUANINE

PYRIMIDINES- THYMINE AND CYTOSINE

ADENINE – HYDROGEN BOND – THYMINEGUANINE – HYDROGEN BOND - CYTOSINE

THE SIDES OF THE DNA MOLECULE

DEOXYRIBOSE SUGAR

PHOSPHATE

DEOXYRIBOSE SUGAR

PHOSPHATE

PHOSPHATE

DNA =

2 strands of nucleotides joined by hydrogen bonds between the bases forms a “DOUBLE HELIX” or twisted ladder

A. DNA Replication:

Each organism has its unique nucleotide sequence in its DNA --->from parents

DNA must be copied for cell division (Replication)

STEPS IN DNA REPLICATION:

Enzymes called DNA Helicases break bonds between bases - DNA

“unzips”

Free nucleotides in pair up with exposed bases guided by DNA Polymerases.

2 new strands formed and rewind

(identical to original DNA)

Free nucleotides

…….How proteins are made !!

II. PROTEIN SYNTHESIS:20 Different amino acids3 Nitrogen bases code for 1 amino acid = CODON“Triplet code” - 64 different codons (3 stop codons)

---> 20 amino acidsOrder of bases in DNA determines type and order of amino acids in proteinCode is universal among all organisms = common ancestor

A. TRANSCRIPTION: Getting DNA to make a m RNA ….which carries the sequence for a particular polypeptide chain!

DNA IS IN THE NUCLEUS Transcription takes place in the

nucleus! RNA structure differs from DNA: Single strand of nucleotides Ribose - simple sugar Bases - uracil instead of thymine U -- A

A, U, C, G

STEPS OF TRANSCRIPTION:

1. DNA makes complimentary copy of mRNA

(using U in place of T)

2. mRNA carries code out of nucleus into

cytoplasm where ribosomes are located!

B. TRANSLATION: Interpreting the code from the mRNA - on the ribosomes.

tRNA (transfer) carries amino acids and bring them to ribosomes to be assembled into proteins

The order of amino acids determines the type of polypeptide assembled. (protein)

tRNA has anticodon (3 bases that code for a specific amino acid)

tRNA matches its anticodon with the mRNA codon and drops off its amino acid to bond with the next amino acid

Continues until a “stop codon” is reached