Rosalind Franklin's research (King's College) produced x-ray diffraction images of DNA
Helped Watson and Crick build their DNA model, for which they received the Nobel Prize
Nucleotides of DNA
Each nucleotide is a complex of three subunits
Phosphoric acid (phosphate)
A pentose sugar (deoxyribose)
A nitrogen-containing base
Four Possible Bases
Adenine (A) - a purine
Guanine (G) - a purine
Thymine (T) - a pyrimidine
Cytosine (C) - a pyrimidine
Semi-conservative replication (DNA)
Each daughter DNA molecule consists of one new chain of nucleotides and one from the parent DNA molecule
The two daughter DNA molecules will be identical to the parent molecule
Steps of DNA Replication
Before replication begins, the two strands of the parent molecule are hydrogen-bonded together
Enzyme DNA helicase unwinds and "unzips" the double-stranded DNA
New complementary DNA nucleotides fit into place along separated strands by complementary base pairing. These are positioned and joined by DNA polymerase.
DNA ligase seals any breaks in the sugar-phosphate backbone
The two double helix molecules identical to each other and to the original DNA molecule
Ladder Configuration and DNA Replication
Parental DNA molecule contains
so-called old strands
hydrogen-bonded by complementary
Region of replication. Parental DNA
is unwound and unzipped. New
nucleotides are pairing with those
in old strands.
Replication is complete. Each
double helix is composed of an old
(parental) strand and a new
Enzymes use the base sequence of a gene as a template to make a strand of RNA
This is the first step in protein synthesis (mRNA)
Forms a messager for RNA (mRNA)
The process of copying one DNA double helix into two identical double helices is called ______.
To complete replication, the enzyme _______ seals any breaks in the sugar-phosphate backbone.
During transcription, a segment of the DNA called a _____ serves as a template for the production of an RNA molecule.
A promoter is ________________.
a region of DNA that contains a special sequence of nucleotides that begins transcription.
A cluster of genes usually coding for proteins related to a particular metabolic pathway is called a (an) ______.
Active genes in eukaryotic cells are associated with more loosely packed chromatin, called ___________
Which of the following is (are) primary characteristics of cancer cells?
Cancer cells are genetically unstable
Cancer cells do not correctly regulate the cell cycle
Cancer cells escape the signals for cell death
In the Watson-Crick model of DNA, the "sides" of the ladder are composed of
Because one original strand of the double-stranded helix is found in each daughter cell, the replication process is called
The _____ enzyme is responsible for unwinding the double-helix structure of DNA during replication.
Information from X-ray crystallographic data collected by _______ was used by Watson and Crick in their development of the model of DNA.
Which of the following have nitrogenous bases correctly paired in DNA?
The fact that a DNA strand contains equal amounts of the bases A and T allows one to realize that
these molecules bond together.
In the spiraling helix of a DNA molecule, each cross-member (or step) consists of two
DNA is considered to be the master molecule in cells because it
ultimately controls all cell activity.
The two molecules resulting from the replication of DNA
are the same in every regard unless a mutation has occurred.
Which of the following is mass-produced directly by cells altered by recombinant DNA techniques?
Which of the following is a result of recombinant DNA
Developing hybrid species of food crops.
Mass-producing molecules for medical treatments.
Producing laboratory clones of organisms for scientific investigations.
Transcription of part of a DNA molecule with a nucleotide sequence of AAACAACTT results in a mRNA molecule with the complementary sequence of
The type of gene mutation in which a nucleotide replaces another nucleotide is termed a(n) ________ mutation
If the normal nucleotide sequence was TACGGCATG, what type of gene mutation is present if the resulting sequence becomes TAGGCATG?
Permanent change in the sequence of bases in DNA
Effect can range from none to complete inactivity of the protein
Germ-line mutations can be passed to subsequent generations
Some mutations can lead to cancer
Radiation and certain organic chemicals
DNA repair enzymes constantly monitor and repair any irregularities
Transposons - jumping genes
Specific DNA sequences that move within and between chromosomes
Sometimes alters neighboring gene expression
Likely all organisms (including humans) have them
Point Mutations - Base pair substitution
Change in a single DNA nucleotide
Possible change in a specific amino acid
May have no effect (silent mutation)
May produce an abnormal protein
May produce an incomplete protein
New codon now codes for a stop
One or more nucleotides are either inserted or deleted from DNA
Result can be a completely new sequence of codons and a nonfunctional protein
A single nonfunctioning protein can have a dramatic effect on the phenotype, because enzymes are often a part of metabolic pathways
Phenylalanine can't be broken down and builds up causing mental impairment
Faulty receptor makes cells unable to respond
Genetic male looks like a normal female
Mutations and Cancer
Development of cancer involves a series of accumulating mutations that can be different for each type of cancer
Carcinogenesis begins with the loss of tumor suppressor gene activity and/or the gain of oncogene activity
In the United States, the three deadliest forms of cancer are
lung cancer, colon and rectal cancer, and breast cancer
Altered cells grow and divide abnormally
Malignant cells may metastasize (break loose and colonize distant tissues)
Cells have gained
the ability to invade
Characteristics of Cancer Cells
Do not correctly regulate the cell cycle
Escape the signals for cell death
Can survive and proliferate elsewhere in the body
Changes in phenotype (appearance) or gene expression caused by mechanisms other than changes in the underlying DNA sequence, hence the name epi- (Greek: over; above) -genetics.
______________ is the application of computer technologies, specially developed software, and statistical techniques to the study of biological information.
In which procedure is the gene injected directly into a particular region of the body?
C) in vivo therapy
_____________ is the insertion of genetic material into human cells for the treatment of genetic disorders.
D) Gene therapy
Which of the following procedures is the most common type of DNA microarray used to generate a person's genetic profile?
What are the two types of DNA testing for mutated genes?
testing for a genetic marker and using a DNA probe
Which of the following human proteins are being engineered by transgenic plants?
Which of the following is a result of a transgenic plant that produces potatoes below ground and tomatoes above ground?
Which of the following is an example of how bacteria can be selected to degrade a specific substance?
Oil-eating bacteria can be genetically engineered to clean up beaches after an oil spill
Which of the following is (are) a biotechnology product produced by bacteria?
human growth hormone
tissue plasminogen activator
During a process called ___________ an electrical current is used to force DNA through a porous gel material and thus separate them according to their size
Why is short tandem repeat (STR) profiling advantageous as a method of identifying DNA segments?
Short tandem repeat profiling is advantageous because it doesn't require the use of restriction enzymes.
Which of the following describes the correct order of steps involved in polymerase chain reaction?
denaturation, annealing, extension
Which procedure can create billions of copies of a segment of DNA in a test tube in a matter of hours?
polymerase chain reaction
The use of a DNA chip to discover all the particular mutations of an individual is called ______.
To clone an adult, scientists first transform one of its differentiated cells into an undifferentiated cell by turning its unused DNA back on
somatic cell nuclear transfer (SCNT)
Method of reproductive cloning in which a researcher removes the nucleus from an unfertilized egg, then inserts into the egg a nucleus from an adult body cell
Researchers are already using SCNT to produce human embryos for research, a practice called therapeutic cloning
The researchers harvest undifferentiated (stem) cells from the cloned human embryos
Use of SCNT to produce human embryos for research purposes
Growing organs for patients
How fatal diseases progress
Still long way to go
Cloned animals tend to have problems:
The first cloned mammal, Dolly the sheep, had old-age problems when she was only two
Clones may be overweight or have enlarged organs
Cloned mice develop lung and liver problems, and almost all die prematurely
Cloned pigs tend to limp and have heart problems: One never did develop a tail or, even worse, an anus
Uses of gene cloning
Produce large quantities of the gene's protein product
Learn how a cloned gene codes for a particular protein
Use the genes to alter the phenotypes of other organisms in a beneficial way
Produces transgenic organism
Gene therapy - cloned genes are used to modify a human
Recombinant DNA technology
Recombinant DNA (rDNA)
Contains DNA from two or more different sources
Vector - piece of DNA that foreign DNA can be added to
Plasmids are accessory rings of DNA in bacteria
2 enzymes used to introduce foreign DNA
Restriction enzyme - molecular scissors
Transgenic organisms are called GMOs
Genetically modified organisms
Products they produce are biotechnology products
Grown in bioreactors
Bacteria express cloned gene
Gene product collected from the media
Products include insulin, human growth hormone, tPA, and hepatitis B vaccine
Can be selected for their ability to degrade a particular substance
Ability can be enhanced by bioengineering
Eat oil, remove sulfur from coal
Foreign genes introduced into:
Immature plant embryos
Protoplasts - plant cells with cell wall removed
Go on to develop into mature plants
Produces potatoes below ground and tomatoes above ground
Pest resistance in cotton, corn, and potato strains
Soybeans resistant to herbicide
Can also be engineered to produce human proteins
Why do it?
Produce more food at lower cost
Use smaller amounts of toxic pesticides
Resistant to environmental conditions (droughts)
Most widely planted GMO crops:
Corn, sorghum, cotton, soy, canola, alfalfa
Insert genes into eggs
By hand or by vortex mixing with silicon-carbide eggs
Insert gene for bovine growth hormone (bGH) to produce larger fishes, cows, pigs, rabbits, and sheep
Use of transgenic farm animals to produce pharmaceuticals
Proteins harvested from animals milk
Transfer of one or more normal or modified genes into an individual's body cells to fix a genetic defect of boost disease resistance
Deliver cells carrying recombinant DNA into a patient's tissues or use viruses as vectors that inject genes into cells
SCID-X1 - severe immune deficiency, bone marrow therapy with the normal gene
Selecting the most desired human traits
People with enhance memory, bigger muscles, improved learning abilities..
The Structure of DNA
DNA is composed of four nucleotides, each containing: adenine, cytosine, thymine, or guanine.
The amounts of A = T, G = C, and purines = pyrimidines [Chargaff's Rule].
DNA is a double-stranded helix with antiparallel strands [Watson and Crick].
Nucleotides in each strand are linked by 5'-3' phosphodiester bonds
Bases on opposite strands are linked by hydrogen bonding: A with T, and G with C.
The Basic Principle: Base Pairing to a Template Strand
The relationship between structure and function is manifest in the double helix
Since the two strands of DNA are complementary each strand acts as a template for building a new strand in replication
The parent molecule unwinds, and two new daughter strands are built based on base-pairing rules
Steps in DNA replication
The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C
The first step in replication is separation of the two DNA strands.
Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand
The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each "daughter" DNA molecule consists of one parental strand and one new strand
Each 2-stranded daughter molecule is only half new
One original strand was used as a template to make the new strand
The copying of DNA is remarkable in its speed and accuracy
Involves unwinding the double helix and synthesizing two new strands.
More than a dozen enzymes and other proteins participate in DNA replication
The replication of a DNA molecule begins at special sites called origins of replication, where the two strands are separated
Mechanism of DNA Replication
DNA replication is catalyzed by DNA polymerase which needs an RNA primer
RNA primase synthesizes primer on DNA strand
DNA polymerase adds nucleotides to the 3' end of the growing strand
Nucleotides are added by complementary base pairing with the template strand
The substrates, deoxyribonucleoside triphosphates, are hydrolyzed as added, releasing energy for DNA synthesis.
DNA synthesis on the leading strand is continuous
The lagging strand grows the same general direction as the leading strand (in the same direction as the Replication Fork). However, DNA is made in the 5'-to-3' direction
Therefore, DNA synthesis on the lagging strand is discontinuous
DNA is added as short fragments (Okasaki fragments) that are subsequently ligated together
Many proteins assist in DNA replication
DNA helicases unwind the double helix, the template strands are stabilized by other proteins
Single-stranded DNA binding proteins make the template available
RNA primase catalyzes the synthesis of short RNA primers, to which nucleotides are added.
DNA polymerase III extends the strand in the 5'-to-3' direction
DNA polymerase I degrades the RNA primer and replaces it with DNA
DNA ligase joins the DNA fragments into a continuous daughter strand
Enzymes in DNA replication
parental double helix
to template strand
DNA polymerase III
to form new strands
DNA polymerase I (Exonuclease) removes RNA primer and inserts the correct bases
Ligase joins Okazaki
fragments and seals
other nicks in sugar-phosphate backbone
DNA must be faithfully replicated...but mistakes occur
DNA polymerase (DNA pol) inserts the wrong nucleotide base in 1/10,000 bases
DNA pol has a proofreading capability and can correct errors
Mismatch repair: 'wrong' inserted base can be removed
Excision repair: DNA may be damaged by chemicals, radiation, etc. Mechanism to cut out and replace with correct bases
A mismatching of base pairs, can occur at a rate of 1 per 10,000 bases.
DNA polymerase proofreads and repairs accidental mismatched pairs.
Chances of a mutation occurring at any one gene is over 1 in 100,000
Because the human genome is so large, even at this rate, mutations add up. Each of us probably inherited 3-4 mutations!
Proofreading and Repairing DNA
DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides
In mismatch repair of DNA, repair enzymes correct errors in base pairing
In nucleotide excision DNA repair nucleases cut out and replace damaged stretches of DNA
The information content of DNA is in the form of specific sequences of nucleotides along the DNA strands
The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins
The process by which DNA directs protein synthesis, gene expression includes two stages, called transcription and translation
Transcription and Translation
In a eukaryotic cell the nuclear envelope separates transcription from translation
Extensive RNA processing occurs in the nucleus
Transcription is the DNA-directed synthesis of RNA
Is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides
Follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine
What is the difference between RNA and DNA?
RNA is single stranded, not double stranded like DNA
RNA is short, only 1 gene long, where DNA is very long and contains many genes
RNA uses the sugar ribose instead of deoxyribose in DNA
RNA uses the base uracil (U) instead of thymine (T) in DNA.
After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand
transcription, the DNA strands re-form a double helix
The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5 3 . In the wake of transcription, the DNA strands re-form a double helix
RNA polymerase synthesizes a single strand of RNA against the DNA template strand (anti-sense strand), adding nucleotides to the 3' end of the RNA chain
As RNA polymerase moves along the DNA it continues to untwist the double helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides
Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA
Specific sequences in the DNA signal termination of transcription
When one of these is encountered by the polymerase, the RNA transcript is released from the DNA and the double helix can zip up again.
signal the initiation of RNA synthesis
Transcription factors help eukaryotic RNA polymerase recognize promoter sequences
Post Termination RNA Processing
Most eukaryotic mRNAs aren't ready to be translated into protein directly after being transcribed from DNA. mRNA requires processing.
Transcription of RNA processing occur in the nucleus. After this, the messenger RNA moves to the cytoplasm for translation.
The cell adds a protective cap to one end, and a tail of A's to the other end. These both function to protect the RNA from enzymes that would degrade
Most of the genome consists of non-coding regions called introns
Non-coding regions may have specific chromosomal functions or have regulatory purposes
Introns also allow for alternative RNA splicing
Thus, an RNA copy of a gene is converted into messenger RNA by doing 2 things:
Add protective bases to the ends
Cut out the introns
Alteration of mRNA Ends
Each end of a pre-mRNA molecule is modified in a particular way
The 5 end receives a modified nucleotide cap
The 3 end gets a poly-A tail
RNA Processing - Splicing
The original transcript from the DNA is called pre-mRNA.
It contains transcripts of both introns and exons.
The introns are removed by a process called splicing to produce messenger RNA (mRNA)
Proteins often have a modular architecture consisting of discrete structural and functional regions called domains
In many cases different exons code for the different domains in a protein
The Genetic Code
Codons: 3 base code for the production of a specific amino acid, sequence of three of the four different nucleotides
Since there are 4 bases and 3 positions in each codon, there are 4 x 4 x 4 = 64 possible codons
64 codons but only 20 amino acids, therefore most have more than 1 codon
3 of the 64 codons are used as STOP signals; they are found at the end of every gene and mark the end of the protein
One codon is used as a START signal: it is at the start of every protein
Universal: in all living organisms
is the 3 RNA bases that matches the 3 bases of the codon on the mRNA molecule
The four base-paired regions and three loops are characteristic of all tRNAs, as is the base sequence of the amino acid attachment site at the 3 end. The anticodon triplet is unique to each tRNA type.
Consists of a single RNA strand that is only about 80 nucleotides long
Each carries a specific amino acid on one end and has an anticodon on the other end
A special group of enzymes pairs up the proper tRNA molecules with their corresponding amino acids.
tRNA brings the amino acids to the ribosomes,
Ribosomes facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis
The 2 ribosomal subunits are constructed of proteins and RNA molecules named ribosomal RNA or rRNA
Building a Polypeptide
We can divide translation into three stages
The AUG start codon is recognized by methionyl-tRNA or Met
Once the start codon has been identified, the ribosome incorporates amino acids into a polypeptide chain
RNA is decoded by tRNA (transfer RNA) molecules, which each transport specific amino acids to the growing chain
Translation ends when a stop codon (UAA, UAG, UGA) is reached
A summary of transcription and translation in a eukaryotic cell
1. TRANSCRIPTION--RNA is transcribed from a DNA template.
2.RNA PROCESSING--In eukaryotes, the RNA transcript (pre-mRNA) is spliced and modified to produce
mRNA, which moves from the nucleus to the
3.FORMATION OF INITIATION COMPLEX--After leaving the nucleus, mRNA attaches to the ribosome.
4.AMINO ACID ACTIVATION--Each amino acid attaches to its proper tRNA with the help of a specific
enzyme and ATP.
5.TRANSLATION--A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved
through the ribosome one codon at a time.(When completed, the polypeptide is released
from the ribosome.)
The new polypeptide is now floating loose in the cytoplasm if translated by a free ribosme.
It might also be inserted into a membrane, if translated by a ribosome bound to the endoplasmic reticulum.
Polypeptides fold spontaneously into their active configuration, and they spontaneously join with other polypeptides to form the final proteins.
Sometimes other molecules are also attached to the polypeptides: sugars, lipids, phosphates, etc. All of these have special purposes for protein function