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12-1Discovering the role of DNA
Frederick Griffith
Discovered that a factor in heat-killed, disease causing bacteria can transform harmless
bacteria into disease causing bacteria.
Griffith has hypothesized that when the live, harmless bacteria and the heat-killed
bacteria were mixed together, some factor was transferred from the heat-killed cells to the live
cells. That factor might contain a gene with the information that could change harmless bacteria
into disease causing ones.
Oswald Avery
Avery and other scientist discovered that DNA is the nucleic acid that stores and
transmits genetic information from one generation of organisms to the next.
Alfred Hershey and Martha Chase
They studied viruses, non-living particles which can infect living organisms. Virus that
infects and kills bacteria is known as bacteriophage.
They reasoned that if they could determine which part of virus- “protein coat” or “DNA core” –
entered the infected cell they will learn whether genes were made of “protein” or “DNA”.
They finally concluded that genetic material of bacteriophage was DNA not protein.
Erwin Chargaff
Discovered that % of Guanine (G) and Cytosine (C) is almost equal in any sample of
DNA. Same thing is true for Adenine (A) and Thymine (T).
Chargraff’s rule: A = T and C = G
Rosalind Franklin
Studied structure of DNA molecule using a technique called X-ray diffraction and
showed that strains of DNA are twisted around each other in a shape known as helix
James Watson and Francis Crick
They developed the double helix model of the structure of DNA.
They discovered that H2 bonds could form between Adenine - Thymine and Guanine - Cytosine
and provide just enough force to hold two strands together.
This principal called base-pairing explained Chargaff’s rule.
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The structure of DNA
DNA= Deoxyribonucleic Acid
 nucleic acid is a polymer
 nucleotide is a monomer
Shape of DNA – double helix (two spirals – looks like a twisted ladder)
Gene is a segment of DNA that codes for proteins.
 They carry information from one generation to next
 determine heritable characteristics
 Replicated easily.
DNA is a long molecule made up of units called nucleotides.
Each nucleotide is made up of three basic parts
1. 5-carbon sugar called deoxyribose
2. phosphate group
3. Nitrogenous base
There are 4 kinds of nitrogenous bases in DNA.
 A (Adenine)
 T (Thymine)
 C (Cytosine)
 G (Guanine)
Two of the bases Adenine and Guanine – they belong to the group purines.
Two bases known as Cytosine and Thymine are known as pyrimidines
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12-2 DNA & Chromosomes
Prokaryotes – single circular DNA – referred to as chromosomes
Eukaryotic – DNA inside nucleus in the form of number of chromosomes which varies from one
species to next.
Eukaryotic chromosome contains DNA and protein tightly packed together to form
chromatin in which DNA is coiled around proteins called as histones.
DNA and histone together form a nucleosome.
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DNA replication
Each strand of DNA double helix has all the information needed to reconstruct the other half by
base pairing hence the strands are said to be complementary.
Before cells divides, it duplicates its DNA in a copying process called a replication.
During replication, DNA molecule separates into two strands.
Each strand of double helix serves as a template (model for the new strand).
Two complementary stands are produced following the rules of base pairing.
DNA replication is called out by a series of enzymes.
Enzyme Helicase  Enzyme that unzips a molecule of DNA
 Hydrogen bonds between base pairs are broken.
 Two strands of molecule unwind
 Each strand serves as a template for attaching complementary bases.
 The result is two DNA molecules identical to each other and to the original
Enzyme DNA polymerase –
 polymerizes individual nucleotides (using base pair) to produce DNA
 Proof reads new DNA strand. If something is incorrect, corrects it.
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12-3 RNA and Protein synthesis
Gene is a segment of DNA that codes for proteins
-
Proteins are made in the ribosomes.
DNA cannot leave nucleus
DNA needs a messenger to get its message to ribosomes.
It uses RNA
Structure of RNA
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Long chain of nucleotides (5 carbon sugar, phosphate group, nitrogenous base)
Sugar is Ribose
Single stranded
Bases – A, C, G, U (Uracil instead of Thymine)
Can leave nucleus
RNA is like a disposable copy of a segment of DNA
Most of RNA molecules are involved in protein synthesis. The assembly of amino acids into
proteins is controlled by RNA.
Types of RNA
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Messenger RNA (mRNA) – carries DNA’s message
Ribosomal RNA (rRNA) – mix with a few proteins to make a ribosome
Transfer RNA (tRNA) – carries amino acid to the ribosome
Recipe
Chef
DNA ------------------RNA --------------Protein
Transcription
Translation
Sous chef
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Transcription – RNA molecules are produced by copying part of nucleotide sequence of
DNA into a complementary sequence in RNA is called as transcription.
Process of transcription
1. Enzyme RNA polymerase binds to DNA and separates DNA strands
2. It uses one strand of DNA as template to assemble nucleotides into a strand of RNA
according to base-pairing
C->G
G->C
T->A
A->U (there is no RNA ‘T’)
3. Where to start and stop making a RNA copy of DNA?
RNA polymerase will bind only to region of DNA known as promoter
(which has specific base sequence/ signals that indicates where to bind to make RNA)
4. RNA strand detaches and leaves nucleus
5. DNA strand rejoin, unchanged
RNA editing
Many RNA molecules have sections called “introns” or intervening sequences that must be
removed before RNA becomes active.
The remaining RNA, the “exons” or expressed sequences are spliced together.
Then a cap and a tail are added to form final RNA molecule.
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Genetic code
Protein is a polymer. Amino acid is a monomer of protein.
There are 20 types of amino acids in living organisms.
The order in which these 20 amino acids are joined to form polypeptide chain determines type of
protein and its properties.
The language of mRNA instructions is called as genetic code.
The genetic code is read as 3 bases at a time.
3 nucleotides = 1 codon = 1 amino acid
A codon consists of 3 consecutive nucleotides that specify a single amino acid.
RNA has 4 bases A, U, G, C – that code for 20 amino acids
4 bases in RNA hence 64 possible codons (each one has 3 bases).
Some amino acid can be specified by more than 1 codon. E.g. leucine, Argenine
AUG – Methionine or start codon for protein synthesis.
3 stop codons – do not code for any amino acid.
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Translation
Decoding of mRNA message into polypeptide chain (protein) is known as translation.
1. Before translation mRNA must be transcribed from DNA in nucleus and released in to
cytoplasm.
2. Translation takes place in ribosome.
3. mRNA attaches to a ribosome
4. Sequence of nucleotide bases in mRNA serves as instruction for order to put amino acid.
5. The ribosome reads the first codon but does not know which amino acid to match that
codon.
6. That’s the job of tRNA. Each tRNA molecule has an amino acid attached to one end and
3 bases (anticodon) at the other end. Anticodon is complementary to MRNA codon.
7. The ribosome reads next codon. tRNA anticodon is matched with it.
8. The ribosome forms a peptide bond between 2 amino acids. Ribosome releases tRNA
which goes off to get another amino acid.
9. The polypeptide chain continues to grow until ribosome reaches stop codon.
Protein gets detached.
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12-4 Mutations
The change in the DNA sequence that affects genetic information.
Gene mutation - result from changes in one nucleotide (point mutation) or several
nucleotides in a single gene.
1. Point mutation that substitute one nucleotide for another
- Changes one amino acid in a protein.
2. Point mutation that inserts or deletes nucleotide – frameshift mutation
- shift the reading frame of genetic message
- affect every amino acid that follows the point of insertion or deletion.
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Chromosomal mutation - involve changes in the number or structure of whole
chromosome.
 Deletion – loss of all or part of chromosome
 Duplication – segment of chromosome is repeated.
 Inversion – part of chromosome becomes oriented in the reverse of it’s usual direction
 Traslocation – part of one chromosome breaks off and attaches to another, non
homologous chromosome.
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12-5 Gene regulation
an example - bacterium E. coli
The 4288 protein-encoding genes in this bacterium include three genes that are turned on or off
together.
A group of genes that operate together is known as an operon.
Three genes that control lactase gene (in order for the bacterium to be able to use the sugar
lactose as a food) are called as lac operon.
Why must E. coli turn on the lac genes in order to use lactose?
Lactose is a compound made up of galactose and glucose.
The bacterium takes lactose and breaks the bond between glucose and galactose, for which it
needs proteins coded by the genes of the lac operon.
If the bacterium is grown where lactose is the only food, it must transcribe these genes and
produce these proteins. If grown on another food source, such as glucose, it would have no need
for these proteins.
The lac operon is turned off by repressors and turned on by the presence of lactose.
Regulatory regions:
1. promoter (P) - RNA polymerase binds and then begins transcription.
2. Operator (O) - DNA-binding protein can bind to O region e.g. when lac repressor, binds
to the O region, it turns the operon “off” by preventing the transcription of its genes. If
lactose is present, lactose binds to the repressor and removes it. Now RNA polymerase
can bind to the promoter and transcribe the genes of the operon.
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Eukaryotic Gene Regulation
Most eukaryotic genes are controlled individually and have regulatory sequences that are much
more complex than prokaryotic genes.
TATA box
- 30 base pairs long,
- containing a sequence of TATATA or TATAAA
- before the start of transcription
- found before many eukaryotic genes
- help position RNA polymerase by marking a point just before the point at which
transcription begins.
Enhancer sequences
- some DNA-binding proteins enhance transcription by opening up tightly packed
chromatin.
- Some proteins attract RNA polymerase.
- Some proteins block access of RNA polymerase to genes
All of the cells in a multicellular organism carry the complete genetic code in their nucleus. But
only a tiny fraction of the available genes needs to be expressed in the appropriate cells of
different tissues throughout the body.
The genes that code for liver enzymes, for example, are not expressed in nerve cells. Keratin, an
important protein in skin cells, is not produced in blood cells.
Regulation and Development
The cells don't just grow and divide during embryonic development; they also undergo
differentiation, meaning they become specialized in structure and function.
A series of genes, known as the hox genes, tell the cells of the body how they should
differentiate as the body grows.
A mutation in one of these “master control genes” can completely change the organs that
develop in specific parts of the body.