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CHS
H Biology
Chapter 12 DNA
A: The Genetic Code
Genetic Code – the way in which cells store the program that they seem to
pass from one generation of an organism to the next generation
Evidence that DNA is the Genetic Material
1928 – Fred Griffith studied pneumonia caused by bacteria. He worked with 2
strains of bacteria, each containing different genetic information.
 S - strain – virulent produced capsule
 R- strain – nonvirulant – did not have capsule
Conclusion: Some of the genetic material from the dead, virulent bacteria (S), had
entered the living, nonvirulent bacteria (R) changing them to the virulent form.
This is called BACTERIAL TRANSFORMATION (one strain of bacteria had been
transformed into another)
Oswald Avery, Maclyn McCarty, and Colin MacLeod wanted to know – What
factor had transformed the bacteria?
1944 - Made “juice” from heat killed bacteria and treated “juice” with enzymes to
destroy lipids, proteins, carbs, and RNA  transformation still occurred BUT when
the treated the “juice” with enzymes to destroy DNA  transformation did not
occur therefore, DNA was the TRANSFORMING FACTOR
Scientists were still skeptical about the genetic material of higher organisms.
Hershey-Chase 1952 – hypothesized that bacteriophages (made of DNA and a
protein coat) don’t enter bacteria intact, but the phages protein coat attaches to
the bacterial cell wall and the phage then injects its DNA into the bacterial cell.
Conclusion: DNA and not protein entered the bacteria – strong evidence that the
genetic material of bacteriophages is DNA. DNA was the molecule that carried
the genetic code http://highered.mcgraw-hill.com/olc/dl/120076/bio21.swf
The Role of DNA
o Storing Information
o Copying Information
o Transmitting Information
B: The Structure of DNA
1. Rosiland Frankiln (& Maurice Wilkins) (early 1950’s)– produced photographs
(using X-ray diffraction) showing DNA is twisted into a spiral or HELIX with
the bases perpendicular to the length of the molecule. Picture also showed that
DNA must be composed of more than one strand and that sugar-phosphate
backbone is on the outside of the helix
2. Erwin Chargaff –Chargaff’s Rule – number of nucleotides containing A
(adenine) equals the number of nucleotides containing T (thymine) and that the
number of G (guanine) equals the number of C (cytosine) purine with a
pyrimidine
3. James Watson and Francis Crick 1953 – used Franklin and Wilkins x-ray
crystallography picture of DNA and information from Chargaff to make a model
of DNA (still used today)
C: The Double Helix
DNA is made of nucleotide subunits
 5 C-sugar – DEOXYRIBOSE
 One or more phosphate groups
 One of 4 possible nitrogen-containing bases
Sugar
Phosphate
Base
Model suggests that there are 2 strands of DNA and that the 2 strands are
arranged like a ladder.
Sides of the ladder = sugar and phosphate backbone (phosphodiester bonds)
Rungs of the ladder = nitrogen bases – each rungs consists of 2 nitrogen bases
COMPLEMENTARY Base Paired. (A=T) (C=G) (held together by H bonds)
Purine – Adenine & Guanine- Double Ring (It’s 2x’s as good to be pure)
Pyrmidine – Thymine & Cytosine- single ring
D: DNA Replication
Semi-conservative model was suggested by Watson and Crick but proven by
Matthew Meselson and Franklin Stahl (1958) – each parent strand is a template
for a new complimentary strand – end result is two identical DNA molecules each
consisting of one old side “conserved” from parent and one new side
Takes place in the nucleus in eukaryotes
Four Easy steps to remember:
1. Unwind
2. Unzip
3. Add new parts
4. 2 new molecules of DNA rewind
DNA Replication
UNWIND/UNZIP
1. DNA helicase separates parental DNA and exposes bases (unwinds/unzips)
2. Single Stranded Binding Proteins (SSBP) hold strands apart, preventing
them from recoiling
Adding New Parts/Elongation
3. RNA primase lays down short segments of RNA (RNA primer) to which new
strands of DNA can be made
4. DNA polymerase- attaches to the RNA primer and begins to elongate
(attach free nucleotides to exposed bases) the strands. Done continuously
on the leading strand, in short pieces (Okazaki fragments) on the lagging
strand.
Why is there a Leading and a Lagging Strand?????
The 2 strands of DNA are antiparallel
DNA polymerase can only add nucleotides to the 3’ end (5’  3’)
therefore both strands can not be made continuously
5. DNA Polymerase replaces RNA primer with DNA nucleotides, it also
proofreads new strand for base-pair errors (lagging strand requires many RNA
primers)
6. DNA ligase joins sugar-phosphate backbone (“glues”) of Okazaki fragments
(phosphodiester bonds link fragments)
2 New IDENTICAL Molecules of DNA REWIND
*Telomeres – tips of chromosomes are difficult to copy, the enzyme telomerase
adds short repeated DNA sequences to telomeres, lengthening the chromosomes –
this makes it less likely that important gene sequences will be lost during
replication
The Big Picture
o Two strands unwind and unzip (HELICASE) splits H bonds between bases
o Add new parts – new nucleotides are added to the exposed strands by DNA
POLYMERASE (RNA PRIMASE – first adds RNA nucleotides)
o DNA LIGASE “glues”
o 2 new identical molecules of DNA rewind
Prokaryotic Cells vs. Eukaryotic Cells: DNA Replication
PROKARYOTIC
EUKARYOTIC
Single Chromosome (circular)
Many Chromosomes (up to 1000 times more
DNA)
1 Origin of Replication – Regulatory protein
Many (dozens or hundreds) Origin of
binds to a single starting point and triggers
Replications proceed in both
beginning of S phase - proceeds in two
directions(shorten time for replication)
directions until entire chromosome is copied
Okazaki Fragments in lagging strand
Chapter 13 RNA and Protein Synthesis
A. Comparison of DNA and RNA
Nucleic Acid
Monomer
Sugar
DNA
Nucleotide
P,S,B
Nucleotide
P,S,B
Deoxyribose
Nitrogen
Bases
A, C, T, G
Ribose
A, C, U, G
RNA
Type of RNA
mRNA
messenger
tRNA
transfer
rRNA
ribosomal
Function
Structure
Contains hereditary
information
DNA’s helper
Double Helix
Function
Provides the instructions for assembling
amino acids into a polypeptide chain
Delivers amino acid to a ribosome for
their addition into a growing polypeptide
chain
Combines with proteins to form ribosomes
Varies due to
type (single
strand)
Structure
Linear
Clover leaf shaped
Globular
B. Gene Expression
1. Transcription – synthesis of RNA strand from a DNA template
a. Genes nucleotide sequence is written from DNA to complementary
sequences in RNA
b. mRNA carries the transcript of protein building instructions to
ribosome
2. Translation – synthesis of a polypeptide
a. Sequences of bases in mRNA translated into sequence of amino acids –
polypeptide
b. tRNA - translator
Central Dogma: DNA  RNA  Protein
A gene is a linear sequence of many nucleotides. 3 Types:
1. Structural genes: have info to make proteins
2. Regulatory genes: are on/off switches for genes
3. Genes that code for tRNA, rRNA, histones
C.Transcription (DNARNA)
Steps: (nucleus of eukaryotes)
1. Initiation – RNA polymerase splits H bonds in DNA (unzips) and attaches to
promoter (sequence on DNA that signals the beginning of transcription)
2. Elongation – RNA polymerase assembles RNA nucleotides using one strand of
DNA (non-coding) as the template; complementary base pair (A=U, C=G)
3. Termination – ends transcription – special sequence of nucleotides is
recognized (terminator sequence) mRNA (or any RNA) is released and DNA
winds/rezips back together
mRNA Processing (only in eukaryotes)
RNA Processing – Before mRNA leaves the nucleus it undergoes 2 alterations
 A “cap” is added to the 5’ end of mRNA and a “tail” is added to the 3’end of
the mRNA – both give mRNA stability(help bind), protect & regulate gene
expression

RNA splicing – cutting and pasting
Exons are Expressed – express a code for a polypeptide
Introns are INTervening sequences that are non-coding
mRNA can now leave the nucleus (through nuclear pore) and enter the cytoplasm
cut out introns and splice exons together
B. Genetic Code
1. Genetic Code -the language in which instructions for proteins are
written in the base sequences
Codon – combination of three nucleotides on the mRNA that signifies a
particular amino acid
must be 3 nucleotides (1 or 2 not enough to represent all 20 aa)
genetic code has redundancy (more than one codon for each amino
acid) but no ambiguity (codons only represent one amino acid each)
Universal for all organisms
all polypeptide chains start with the codon AUG (methonine)
C. Translation – process by which mRNA is translated into a polypeptide
chain (tRNA is the translator) occurs in cytoplasm at ribosome
Steps:
1. Initiation- mRNA binds to ribosomal subunit
a. 1st CODON (AUG) base-pairs with ANTICODON (UAC) on tRNA
carrying amino acid methionine. (*AUG is ALWAYS the start codon
and the codon is found on the mRNA)
2. Elongation – begins when next tRNA brings amino acid from cytoplasm to
ribosome (codon and anticodon complementary base pair and place amino
acids in the correct order)
Ribosome: 2 protein subunits and ribosomal RNA
•
allows aa’s to attach by making peptide bonds
•
•
travels along mRNA strip, tRNA’s join and bring correct amino acids
3 sites on ribosome:
•
A site – where new tRNA’s and amino acids join
•
P site – where protein is growing
•
E site – where empty tRNA’s exit ribosome
Translocation: as ribosome moves, tRNA’s move from A site  P site  E
site. “A” site is now open for new tRNA with attached amino acid to join
3. Termination - Polypeptide chain continues growing until a STOP codon is
reached (UAA, UAG, UGA- termination codons)