Download Genes and Chromosomes

Nucleic acids
Genetic Information
Genetic Information
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Genetic information is stored in DNA in a linear sequence
of nucleotides, packaged into structures called
chromosomes
Most bacteria and viruses have a single chromosome while
eukaryotes usually have many
A single chromosome contains thousands of segments of
DNA which are referred to as genes
The sum of all the genes on all chromosomes of a cell is
referred to as the organism’s genome
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Size of DNA molecules
Prokaryotes
{ Sizes of DNA molecules differ in both
prokaryotes (viruses and bacteria) and
eukaryotes
{ Genetic information in viruses is small and it
can either be DNA or RNA
{ DNA viruses can have either single or double
stranded DNA
{ DNA in viruses can be circular in shape
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Bacterial DNA
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Bacteria have much more DNA than viruses (e.g E. coli
DNA is 200 times more than bacteriophage λ virus
E. coli has a single double stranded circular molecule of
DNA
Many species of bacteria contain one or more small
circular DNA molecules that are free in the cytosol
called plasmids
Plasmids also carry important genetic information
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Eukaryotic cell DNA
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Yeast, one of the simplest eukaryotes has
DNA 4 times as E.coli (human cell has as
many as 600 times)
Nuclear DNA molecules of eukaryotic cells
have linear, double stranded DNA
The length of DNA in a human cell is about
2m compared with 1.7 mm for E.coli.
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Structure of chromosomes
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Bacteria have one chromosomes per cell and
in most cases containing only one copy of
any given gene
Eukaryotic cells contain many chromosomes
in a cell and many genes exist as multicopy
genes
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Chromatin Structure
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DNA exists within cells as chromatin
The structure of chromatin is determined and
stabilized through the interaction of the DNA
with nucleoproteins
Examples of nucleoproteins include DNAbinding proteins in the nucleus
There are 2 classes of DNA-binding proteins
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DNA binding proteins
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The histones are the major class of DNAbinding proteins involved in maintaining the
compacted structure of chromatin.
There are 5 different histone proteins
identified as H1, H2A, H2B, H3 and H4
Histones package and order DNA into
structural units called nucleosomes
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Non-histone proteins
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The other class of DNA-binding proteins is a
diverse group of proteins called simply, nonhistone proteins.
This class of proteins includes the various
transcription factors, polymerases, hormone
receptors and other nuclear enzymes.
In any given cell there are greater than 1000
different types of non-histone proteins bound
to the DNA.
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Nucleosome
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The binding of DNA by the histones
generates a structure called the nucleosome.
The nucleosome core is a protein structure
consisting of 2 subunits each of H2A, H2B,
H3 and H4.
These nucleosomal structures would appear
as beads on a string if the DNA were pulled
into a linear structure
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Nucleosomes beads
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Summary of
genetic flow of
information
Nucleic acids
Manipulation of purified DNA
DNA manipulations
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DNA manipulative techniques including:1.
2.
3.
4.
5.
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Cutting
Joining
Shortening
Lengthening
Copying DNA into RNA or into new DNA
molecules
Modifying DNA by addition or removal of specific
chemical groups
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DNA manipulations
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Almost all the DNA manipulative
techniques make use of purified enzymes
DNA manipulative enzymes are grouped
into
1.
2.
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Nucleases
Ligases
Polymerases
Modifying enzymes
Topoisomerases
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Nucleases
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These degrade DNA molecules by
breaking the phosphodiester bonds
There are two types
1.
2.
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Exonucleases which remove nucleotides one at a
time from either end of a DNA molecule
Endonucleases which are able to break internal
phosphodiester bonds within a DNA molecule
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Restriction enzymes/endonucleases
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Restriction enzymes are DNA-cutting enzymes found in
bacteria
Because they cut within the DNA molecule, they are often
called restriction endonucleases
A restriction enzyme recognizes and cuts DNA only at a
particular sequence of nucleotides.
For example, the bacterium Hemophilus aegypticus
produces an enzyme named HaeIII that cuts DNA
wherever it encounters the sequence
5'GGCC3‘
3'CCGG5'
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DNA Ligase
DNA ligase is used to repair single
stranded breaks in the cell
{ DNA ligases will also join together two
individual fragments of double-stranded
DNA
{ They form covalent bonds along the
backbone of each strand
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Polymerases
These are enzymes that synthesize a new
strand of DNA complimentary to an
existing DNA or RNA template
{ DNA polymerase I is used to synthesize a
completely new strand of DNA
{ Reverse transcriptase is an enzyme that can
synthesize DNA from an RNA template is
essential for cDNA cloning
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DNA modifying enzymes
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Numerous enzymes modify DNA
molecules by addition or removal of
specific chemical groups
1.
2.
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Alkaline phosphatase will remove phosphate gps
present at the 5`-terminus of a DNA molecule
Polynucleotide kinase will add a phosphate gp on a
free 5`-terminus of a DNA molecule
Terminal deoxynucleotidyl transferase adds one or
more deoxyribonucleotides on the 3`-terminus of a
DNA molecule
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Topoisomerases
These are able to change the conformation
of covalently closed-circular DNA e.g.
plasmids by introducing or removing
supercoils
{ This process is necessary during DNA
replication
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DNA manipulations
1.
2.
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Genetic engineering
Recombinant DNA technology
Genetic modification/manipulation (GM)
and gene splicing are terms that are
applied to the manipulation of genes
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Genetic manipulations
It involves the isolation, manipulation and
reintroduction of DNA into cells or model
organisms, usually to express a protein
{ The aim is to introduce new characteristics
or attributes physiologically or physically
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such as making a crop resistant to a herbicide,
introducing a novel trait, enhancing existing ones, or
producing a new protein or enzyme
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Recombinant DNA
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Recombinant DNA is a form of artificial
DNA which is engineered through the
combination or insertion of one or more
DNA strands, thereby combining DNA
sequences which would not normally occur
together.
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Summary of recombinant DNA
technology
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Example
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GloFish are a type of
zebrafish with
recombinant DNA.
Genes for fluorescent
proteins have been
inserted into their
genome to produce
their fluorescent
colors
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Polymerase chain reaction (PCR)
The (PCR) is a technique for isolating and
exponentially amplifying a DNA fragment
or sequence of interest via enzymatic
replication, without using a living organism
{ PCR is an in vitro technique, it is
performed in the lab
{ It can be extensively modified to perform a
wide array of genetic manipulations
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Applications of rDNA technology
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The first genetically engineered drug was human
insulin,
Another early application was to create human
growth hormone
In 1986 the first genetically engineered vaccine
for humans, for hepatitis B was approved in the
US.
One of the best known applications of genetic
engineering is the creation of genetically modified
organisms (GMOs).
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GMOs
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Golden rice is a genetically modified rice crop
that produces beta-carotene, which can be
metabolized into Vitamin A within the body.
Scientists hope that golden rice will eventually be
a cheap source of beta-carotene in malnourished
countries, reducing the number of children
worldwide that go blind from Vitamin A
deficiency.
Bt maize is a genetically modified corn crop that
produces a toxin that kills stalk borers feeding on
the maize
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DNA fingerprinting
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Genetic fingerprinting, DNA testing, DNA
typing, and DNA profiling are DNA techniques
used to distinguish between individuals
Genetic fingerprinting exploits highly variable
repeating DNA sequences called minisatellites.
Minisatellites patterns are unique for each person
Genetic fingerprinting is used in forensic science,
to match suspects to samples of blood, hair, saliva
or semen
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Somatic cell nuclear transfer
Somatic cell nuclear transfer (SCNT) is a
laboratory technique for creating an ovum
with a donor nucleus.
{ It can also be used as the first step in the
process of reproductive cloning
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