Biology Chapter 12 DNA and RNA
12.1 DNA
I. Griffith and Transformation (Fig 12-2; pg 288)
A. 1928, Fredrick Griffith, trying to figure out how bacteria made people sick
1. He was specifically studying bacteria that caused pneumonia
a. isolated two slightly different strains (types) of pneumonia
bacteria from mice
b. both strains grew well in culture; only 1 caused pneumonia
i. pneumonia grew smooth culture on plate
ii. harmless grew with rough edges
B. Griffith’s Experiments
1. When mice were injected with disease-causing strain, they developed
pneumonia and died; mice injected with harmless strain were fine
2. Heat-killed disease causing strain; injected mice; they lived
3. Mixed heat-killed strain/live harmless strain; injected mice; many died
a. lungs filled with disease-causing bacteria
4. Somehow heat-killed bacteria passed their disease causing ability to
harmless strain
a. called this transformation
i. harmless strain changed into disease causing strain
b. when mixed, some factor transferred from heat-killed to live cells
c. hypothesized, factor might have a gene with info that could
change harmless bacteria into disease-causing ones
II. Avery and DNA
A. 1944, Avery and company repeated Griffith’s work
1. To determine which molecule was most important in transformation
2. They made an extract from the heat-killed bacteria
a. destroyed proteins, lipids, carbs, RNA and more with enzymes
b. transformation still happened
i. these molecules not responsible for transformation
3. Repeated experiment, but this time used enzymes to break down DNA
a. transformation didn’t happen
b. DNA was the transforming factor
III. Hershey-Chase Experiment (Alfred Hershey; Martha Chase – 1952) (Fig 12-4; pg 290)
A. Bacteriophages – virus that infects and kills bacteria
1. Bacteriophage means bacteria eater
a. composed of DNA or RNA and a protein coat
b. virus attaches to surface of bacterium and injects DNA
c. viral genes act to produce many new bacteriophages
d. gradually destroy bacterium
e. cells splits open; new viruses spill out
B. Radioactive Markers
1. Wanted to figure out if genes were made of DNA or protein
a. grew cultures containing radioactive isotopes
i. 32P and 35S
ii. proteins have almost no P; DNA has no S
b. if one or the other was found in the bacteria, that would be the
one that makes up genes
c. nearly all radioactivity in bacteria was from P (one in DNA)
d. H&C concluded genetic material of bacteriophage was DNA
IV. The Structure of DNA
A. Scientists wanted to know how DNA could do 3 things:
1. Genes had to carry information from one generation to the next
2. They had to put that info to work by determining the heritable
characteristics of organisms
3. Genes had to be easily copied, because all of a cell’s genetic
information is replicated each time it divides
B. DNA is a long molecule made of nucleotides; each made of 3 parts: 5-Carbon
sugar (deoxyribose), phosphate group and nitrogenous base
1. There are 4 types of nitrogenous bases in DNA:
a. adenine and guanine are purines
i. 2 rings in their structure
b. cytosine and thymine and pyrimidines
i. 1 ring in their structure
2. Sugar and phosphate groups form the backbone of nucleotides
a. nitrogenous bases stick out sideways
b. nucleotides can be added in any order; any sequence possible
C. Chargaff’s Rules
1. Percentages of G&C/A&T are about the same in any sample of DNA
a. this is known as Chargaff’s rule
i. no explanation why
D. X-Ray Evidence
1. Rosalind Franklin used X-Ray diffraction to study structure of DNA
a. recorded scattered pattern of X-Rays on film
b. eventually patterns of DNA became clear
c. showed DNA strands are twisted around each other
i. shape known as helix
E. The Double Helix
1. Francis Watson & James Crick were trying to make a 3D model of DNA
a. made of cardboard and wire; early attempts failed
2. Watson saw Franklin’s X-Rays and knew what to do; showed Crick
a. within weeks they had figured out the structure
b. model was a double-helix
i. two strands wound around each other
ii. twisted ladder
3. They discover H-Bonds could form between certain bases
a. held double helix together
4. Base Pairing explained Chargaff’s Rules
a. A pair with T; C paired with G
12.2 Chromosomes and DNA Replication
I. DNA and Chromosomes
A. Prokaryotes
1. DNA is found in the cytoplasm
a. most have a single circular DNA molecule that has almost all of
its genetic info
B. Many Eukaryotes have 1000 times the amount of DNA as prokaryotes
1. Typically found in the nucleus in a number of chromosomes
a. numbers of chromosomes varies by species
C. DNA Length (Fig 12-9; pg 296)
1. Chromosome of e. coli contains 4,639,221 base pairs
a. 1.6 mm; length of e. coli about 1.6 mm too
i. chromosome must be folded many times
D. Chromosome Structure
1. Human cells have almost 1000 times as many base pairs as a bacterium
a. nucleus of a human cell have over 1 m of DNA
2. Eukaryotic chromosomes have both protein and DNA
a. tightly packed together, they form chromatin
3. Chromatin consists of DNA that is tightly coiled around proteins called
histones (Fig 12-10; pg 297)
4. DNA & histone molecules form beadlike structure called a nucleosome
a. these pack with each other to form a thick fiber which is
shortened by a system of loops and coils
5. Interphase-these fibers are dispersed and chromosomes aren’t visible
6. Nucleosomes can fold enormous lengths of DNA into the nucleus
E. DNA Replication
1. W&C discovered structure of DNA they also figured out how its copied
a. because of base pairing, both sides have all the info needed to
make copies
b. strands are complimentary; can use one to make the other
F. Duplicating DNA (Fig 12-11; pg 298)
1. Before cells divide they have copied each chromosome (replication)
a. chromosome separates into two strands; each used as template
2. Prokaryotes
a. DNA begins at one point; goes in both directions until finished
3. Eukaryotes
a. starts at hundreds of places; goes in both directions
i. continues until all chromosomes are copied
b. locations of separation/replication are called replication forks
G. How Replication Occurs
1. Replication carried out by enzymes
a. enzymes unzip molecule of DNA
i. H-bonds between base pairs broken; molecule unwinds
b. TACGTT makes ATGCAA; results are identical
2. Main enzyme is DNA polymerase
a. polymerizes individual nucleotides to produce DNA
b. also “proofreads” each strand
12.3 RNA and Protein Synthesis
I. Double helix doesn’t explain how genes work
A. Genes are coded DNA instructions; control the production of proteins in a cell
1. First step in decoding these genetic messages is to copy part of the
nucleotide sequence from DNA into RNA
a. RNA carry out the process of making proteins
II. The Structure of RNA
A. RNA is also a long chain of nucleotides (5-C sugar, phosphate group, N-base)
1. Differences between DNA and RNA
a. RNA’s sugar is ribose, not deoxyribose
b. RNA is generally single stranded
c. RNA has uracil in place of thymine
2. RNA is like a disposable copy of a segment of DNA
a. RNA is like the working copy of a single gene
b. ability to copy into RNA makes it possible for a single gene to
produce 100s-1000s of RNA molecules
III. Types of RNA
A. RNA molecules have many jobs, but in most cases the job is protein synthesis
1. Assembly of amino acids into proteins is controlled by RNA
2. Three main types of RNA (Fig 12-12; pg 300):
a. messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA
(tRNA)
B. Most genes contain instructions for assembling amino acids into proteins
1. RNA molecules that carry these instructions are mRNA
2. Proteins are assembled on ribosomes
a. ribosomes are made of several dozens of proteins and rRNA
3. During construction of a protein, tRNA transfers each AA to the
ribosome as it is specified by codes in mRNA
IV. Transcription (Fig 12-14; pg 301)
A. RNA molecules are produced by copying part of the nucleotide sequence of
DNA into a complimentary sequence in RNA – transcription
1. Transcription requires RNA polymerase
a. RNA polymerase binds to DNA and separates the DNA strands
b. RNA polymerase uses one strand of DNA as a template
c. nucleotides are assembled into a strand of RNA
2. How does RNA polymerase know where to start and stop copying?
a. RNA only binds to regions of DNA known as promoters
i. these have specific base sequences
b. similar signals tell when to stop
V. RNA Editing (Fig 12-15; pg 302)
A. Many RNA molecules require editing before they are ready for action
1. Some, like some rRNA molecules are produced from larger RNA
molecules that are cut and trimmed to final sizes
a. large pieces are removed from RNA molecules transcribed
before they become functional
b. pieces known as introns (intervening sequences) are cut out
while still in the nucleus
c. remaining pieces known as exons (expressed sequences) are
spliced together to form final mRNA
2. Why make such a large RNA and cut it? Scientists don’t know
a. can help one gene make many forms of RNA
VI. The Genetic Code (Fig 12-16; pg 302)
A. Proteins are made by joining AA into long chains – polypeptides
1. Polypeptides have a combination of any or all 20 different AA
a. properties of proteins determined by order of combination
B. Language of mRNA instructions is called the genetic code
1. Language contains 4 bases (A, U, C and G)
a. code’s language has only 4 letters
2. Genetic code is read 3 letters at a time – codons
a. consists of 3 consecutive nucleotides that specify a single amino
acid to be added to the polypeptide
b. UCGCACGGU is UCG-CAC-GGU or Serine-Histidine-Glycine
3. Because there are four bases there are 64 possible 3-base codons
a. Fig 12-17; pg 303 shows all 64 possible codons
4. Some AA can be specified by more than one codon
a. 6 different codons specify leucine; 6 others arginine
5. AUG can specify methionine or be the start codon
a. there are 3 stop codons
VII. Translation (fig 12-18; pg 304)
A. The sequence of nucleotide bases are the instructions, but something needs
to read them and put them to use – ribosomes
1. Decoding mRNA into polypeptide chain (protein) known as translation
a. takes place on ribosomes; uses info from mRNA to make proteins
2. 4 Steps of Translation
a. mRNA must be transcribed from DNA and released to cytoplasm
b. starts when mRNA attaches to ribosome
i. as each codon moves through ribosome, proper AA is
brought into ribosome and attached to pp chain
ii. tRNA tells which AA matches each codon
iii. each tRNA has an AA attached to one end and a region
of 3 unpaired bases at the other
iv. 3 bases on tRNA are called the anticodon,
complimentary to one of the mRNA codons
v. Eg – anticodon UAC pairs with AUG (methionine)
vi. ribosome has another binding site so this happens again
c. ribosome forms a peptide bond between 1st and 2nd AA
i. at the same time, the ribosome breaks the bond that held
the 1st tRNA to its AA and releases tRNA molecule
ii. ribosome then moves to the 3rd codon, where tRNA
molecule brings it the AA specified by the 3rd codon
d. pp chain continues to grow until the ribosome hits a stop codon
on mRNA molecule
i. ribosome releases newly formed polypeptide and the
mRNA molecule, completing translation
12.4 Mutations
I. Cells sometimes make mistakes in copying DNA
A. Insert incorrect/skip base
1. Mutations
a. changes in the DNA sequence that effect genetic information
2. Gene mutations
a. result from changes of a single gene
3. Chromosomal mutations
a. involve changes in whole chromosomes
II. Gene Mutations (Fig 12-19; pg 307)
A. Mutations that effect just one nucleotide are called point mutations
1. They occur at a single point in the DNA sequence
a. many substitute one nucleotide for another
b. change one of the AA in a protein
B. Insertions or deletions cause much bigger changes
1. Codon groupings will all be changed
a. these are called frameshift mutations
i. shift the reading frame of the genetic message
b. affect every AA that follows the point of the insertion/deletion
c. proteins may not be able to perform their normal functions
III. Chromosomal Mutations
A. Involve changes in the number or structure of chromosomes
1. May change the location of genes on chromosomes and eve the
number of copies of some genes
B. There are 4 types of Chromosomal Mutations (Fig 12-20; pg 308)
1. Deletion
a. loss of all or part of a chromosome
2. Duplication
a. segment of a chromosome is repeated
3. Inversion
a. when part becomes oriented in the reverse of its usual direction
4. Translocation
a. part of one chromosome breaks off and attaches to another,
non-homologous chromosome
b. in most cases, non-homologous chromosomes exchange
segments so that 2 translocations occur at the same time
12.5 Gene Regulation
I. Gene Regulation: An Example
A. How does an organisms know whether to turn a gene on or off?
1. E. coli example (PROKARYOTE) (Fig 12-22; pg 310)
a. 4288 protein-encoding genes include cluster of 3 that turn on/off
together
b. called operon (genes that turn on/off together)
c. genes must be expressed for it to be able to process lactose
i. called lac operon
d. turned on so it can break it apart (glucose/galactose)
e. if its not needed it doesn’t turn them on
i. turned on by presence of lactose; off by repressors
2. On one side of the operon’s 3 genes are 2 regulatory regions
a. in promoter (P) RNA polymerase binds then transcription begins
3. Other region is the operator (O)
a. E. coli cells contain several copies of a DNA-binding protein that
can bind to the O region
i. known as lac repressor
4. When the lac repressor binds to the O region, RNA polymerase is
prevented from binding to the promoter
a. binding of the repressor protein turns the operon off by
preventing the transcription of its genes
5. Lac repressor protein has a binding site for lactose itself
a. when lactose is present a few bind to the repressor proteins
b. binding of lactose causes repressor protein to change shape
i. completely alters its DNA-binding site
ii. causes repressor to fall off the operator
c. if no longer on O site, RNA polymerase can bind to the promoter
and transcribe genes of the operon
II. Eukaryotic Gene Regulation
A. Operons are generally not found in eukaryotes
1. Most eukaryotic genes controlled individually and have regulatory
sequences that are much more complex than the lac operon
2. Many have a “TATA box”
a. about 30 base pairs long before the start of transcription
b. seems to help position RNA polymerase by marking a point right
before the start
3. Genes are regulated in a variety of ways by enhancer sequences
located before the beginning of transcription
a. huge number of proteins can bind to diff. enhancer sequences
i. some help open up chromatin; attract RNA polymerase;
block access to genes
4. Reason for all of this?
a. genes that code for liver enzymes are not expressed in nerves
b. keratin important in skin cells, but is not produced by RBCs
c. cell specialization requires genetic specialization, but all cells
have the complete genetic code in their nucleus
d. only a small fraction of genes needs to be expressed in cells of
different tissues of the body
e. complexity of gene regulation makes specificity possible
III. Regulation and Development
A. Study of developmental genes reveals more in this area
1. Embryos have genes known as hox genes
a. control organs/tissues that develop in parts of the embryo
b. determine an animals basic body plan
c. mutation here could cause fruit fly to replace antennae with pair
of legs growing out of the head
d. by removing and placing Pax 6 (eye development gene) on fly’s
knee, an eye grew on the leg