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Chapter 12:
DNA and RNA
Biology- Kirby
12-1: DNA
• 1928- Frederick Griffith- studied the
bacteria that caused pneumonia. He
found 2 strains of bacteria: diseasecausing and harmless.
• Griffith’s experiment: page 288
• Transformation- one strain of bacteria
is changed by a gene from another
strain of bacteria.
12-1: DNA
• 1944- Oswald Avery- repeated
Griffith’s work- discovered that DNA
stores and transmits the genetic
information from one generation to
the next.
• 1952- Hershey-Chase Experiment– Bacteriophage- virus that infects bacteria.
– Concluded that the genetic material of
the bacteriophage was DNA, not protein.
12-1: DNA
• DNA is made of nucleotides.
• 4 nitrogenous bases in DNA:
–
–
–
–
–
Adenine
Guanine
Cytosine
Thymine
Adenine and guanine are purines (2 ring
structure). Cytosine and thymine are
pyrimidines (1 ring structure).
12-1: DNA
• The backbone of DNA is made from
sugar and phosphate groups.
• Chargaff’s Rules– A=T, and G=C
• 1950’s- Rosalind Franklin- used x-ray
diffraction to get information about
the structure of DNA.
12-1: DNA
• 1950’s- Francis Crick and James
Watson- began making 3-D models of
DNA. After viewing Franklin’s x-rays
Watson and Crick developed the
double helix model for DNA.
• Double helix- 2 twisted strands held
together by base pairs. Base pairing
explains Chargaff’s rules; A bonds to
T, and G bonds to C.
12-2: Chromosomes and
DNA Replication
• DNA is very long and has to be folded
tightly to fit inside cells.
• The nucleus of a human cell has more
than 1 meter of DNA!!
• The composition of the chromosomes
allows for these long stands of DNA to
exist.
12-2: Chromosomes and
DNA Replication
• Chromosome Structure– Made of DNA and proteins
– Chromatin- consists of DNA coiled
tightly around proteins.
– Histone- proteins DNA is coiled
around.
– Nucleosome- DNA and histones in a
beadlike structure. Nucleosomes fold
DNA into tiny spaces in the nucleus.
12-2: Chromosomes and
DNA Replication
• The double helix structure of DNA
explains how it can be replicated, or
copied.
• Each side, or strand, can make another
copy.
• DNA begins replication at one point
and proceeds in both directions until
replication is complete.
12-2: Chromosomes and
DNA Replication
• Replication- process in which the cell
copies its DNA.
– The DNA molecule separates into 2
strands.
– Base pairs are made to complement
the original nitrogenous bases on
the strand.
– Example: TACGTT (original)
–
ATGCAA (copy)
12-2: Chromosomes and
DNA Replication
– Each strand of the double helix of
DNA serves as a template, or model,
for the new strand.
– Results in 2 identical DNA strands.
– Replication is carried out by
enzymes. These enzymes “unzip”
DNA by breaking the hydrogen
bonds between the base pairs.
12-2: Chromosomes and
DNA Replication
– DNA polymerase- enzyme involved
in DNA replication that joins
individual nucleotides to produce a
DNA molecule.
12-3: RNA & Protein Synthesis
• RNA consists of:
– Nucleotides
– Ribose (sugar)
– Single-strand
– Contains uracil instead of thymine.
12-3: RNA & Protein Synthesis
• Types of RNA:
– Messenger RNA (mRNA)- carries copies of
instructions for the assembly of amino
acids into proteins from DNA to the rest
of the cell.
– Ribosomal RNA (rRNA)- makes up
ribosomes.
– Transfer RNA (tRNA)- transfers amino
acids to ribosomes during protein
synthesis.
12-3: RNA & Protein Synthesis
• Transcription- the nucleotide
sequence of DNA is copied into a
complementary sequence of RNA.
– RNA polymerase- enzyme that binds
to DNA and separates the DNA
strands. It uses one strand of DNA
as a template to assemble a strand
of RNA.
12-3: RNA & Protein Synthesis
• RNA polymerase will only bind to
regions of DNA called promoters.
These specific base sequences signal
DNA to start and stop making RNA.
• Codon- 3 nucleotide sequence on
mRNA that codes for amino acids.
– AUG is the “start” codon.
12-3: RNA & Protein Synthesis
• Translation- decoding of mRNA
message into a polypeptide chain.
– Anticodon- complementary to a codon.
– How it works:
12-4: Mutations
• Mutation- changes in the genetic
material.
• Gene mutations- change in the DNA
sequence.
• Chromosomal mutations- change in
the chromosome structure.
• Most mutations are neutral, but
dramatic changes are harmful.