Download DNA Profiling

Chpt. 16
DNA & RNA
DNA
(Deoxyribonucleic Acid)
• DNA was discovered by Watson and Crick (1953)
•It is found in the nucleus .
• DNA is coiled and folded in order to fit into nucleus.
• Proteins responsible for holding DNA in its folded state.
Structure of DNA
• DNA has two strands.
• Strands are linked by pairs of chemicals called bases.
• Each pair of bases form a rung on the DNA molecule.
Structure of DNA
• Only four different bases used in DNA:
Adenine - A
Thymine - T
Guanine - G
Cytosine - C
• A joins with T only
and
G joins with C only
Structure of DNA
• Pairs of bases A/T and G/C are said to be
complementary.
A
T
G
C
Structure of DNA
• DNA structure can be thought of as a ladder twisted to
form a spiral – called Double Helix
Side Strand
Pairs of Bases
DNA Double
Helix
This structure is important as
1. The DNA is copied exactly when ever the cell
divides
2. The DNA provides a code for protein
synthesis
Chromosomes and Genes
Chromosome: consists of many base pairs arranged into
a double helix.
Gene: is a sequence of many bases.
Genetic Code:
• is the precise sequence of bases.
The Genetic Code
Gene:
• are pieces of inherited information that code for the
production of proteins.
Protein:
• made up of combination of amino acids.
• up to 20 different types of amino acids used.
Genetic Code:
• To allow correct amino acids to be assembled to make
protein DNA carries a genetic code.
The Genetic Code
1. The genetic code is made up of 3 DNA bases in
sequence to form a triplet or a codon.
2. Each triplet is the code for an amino acid.
3. A sequence of codons that produce a protein is
called a GENE
Genetic Code
Examples:
DNA triplet CAA
code for amino acid Valine
DNA triplet CGA
code for amino acid
Alanine
Genetic Code
The genetic code is like another language and can be
compared with the English language as follows:
Genetic Code
Base
Triplet or codon
Gene
Genome
Note: Genome = all the genes in the cell
English Equivalent
Letter
Word
Paragraph
Book
Genetic Code
Non-coding DNA:
• Approximately 97% DNA in human nucleus does not carry
the code for the production of proteins Junk DNA.
• Junk DNA has no known function.
• Located in two places:
i) between genes
ii) within genes
• Sequence of bases in non-coding DNA varies greatly
from person to person – used in preparing DNA profiles.
Non-Coding DNA
DNA Replication
• occurs in nucleus of cell during INTERPHASE, which is just
before cell division (mitosis) occurs.
• results in the single-stranded chromosome forming two
identical strands (identical genes) held together at the
centromeres.
Single-stranded
chromosome
Double-stranded
chromosome
DNA in each strand is
identical
DNA Replication in Detail:
1. The double helix unwinds.
2. Enzyme breaks bonds between base pairs and the DNA
strands separate.
3. New DNA bases (nucleotides), present in the cytoplasm,
enter nucleus and attach to their complementary bases
on the exposed strands.
4. Each new strand is:
a) half new DNA and half old DNA
b) identical to original DNA strand and to other
new partner strand.
6. Each new piece of DNA rewinds to form double helix.
DNA Replication in Detail:
Significance of DNA Replication:
• DNA is able to produce exact copies of itself.
•The same genetic information is passed on from one
generation of cells to the next.
DNA Replication Theory
Step 1:
Step 2:
Step 3:
Step 4:
Step 5:
DNA Replication Diagram
DNA Profiling
(DNA/Genetic Fingerprinting)
DNA Profiling: is a method of making a unique pattern of
bands from the DNA of a person, which can then be used
to distinguish that DNA from other DNA.
DNA Profiling
Preparing a DNA Profile:
Involves four steps:
1) Release DNA from cells
2) Cut the DNA into fragments
3) Separate the fragments
4) Compare patterns
1. Release DNA from cells:
- Cells are broken down to release their DNA (see
activity 15) e.g. DNA from blood, hair, semen.
DNA Profiling
2. Cut the isolated DNA into fragments:
- special enzymes (restriction enzymes) are used
to cut the isolated DNA into pieces.
- different enzymes cut DNA at specific
base sequences e.g.:
i) One restriction enzyme will always cut DNA at
the base sequence: GAATTC.
ii) Another restriction enzyme only cuts at the
sequence: GATC.
- sections of DNA cut out = Restriction Fragments
DNA Profiling
3. Separate the Fragments:
- DNA fragments are separated according to their
length.
- They are separated by a process called gel
electrophoresis:
i) fragments are placed in a gel and an
electric current passed through the gel.
ii) small fragments move faster through the
gel than large ones.
iii) Photographic copy of final pattern of DNA
bands is obtained.
Note: Each DNA profile looks like a bar code and is different
for each person except in the case of identical twins.
DNA Profiling
This band represents
longer segments of
DNA
Short segments
DNA Profile
DNA Profiling
4. Compare Patterns:
- Highly unlikely that any two people will have the
same DNA profile (exception: identical twins).
- if pattern of bands from two different DNA
samples is the same then two samples must have
come from same person.
DNA Profiling
Step 1:
Step 2:
Step 3:
Step 4:
Applications of DNA Profiles
(DNA Fingerprints)
A. Crime:
• DNA profiles may be used to link somebody to a
crime or to the scene of a crime.
• If some biological tissue (saliva on a cigarette butt,
semen stain or a hair) is found at a crime scene, its
DNA profile is compared with one taken from a
suspect.
• If patterns match, suspect is associated with the crime
scene.
Note: See Figure 16.18 pg. 155 – comparing DNA profiles
Applications of DNA Profiles
B. Medical:
• DNA profiles can be used to determine whether a
particular person is, or is not, the parent of a child i.e.
establishing the paternity or maternity of a child.
• Paternity cases are important in immigration, inheritance
and rape cases.
Note: See figure 16.19 and 16.21(a) (b) pg 155 – how to
use DNA profiles to determine the father of a child.
Genetic Screening
Remember: DNA Replication is when DNA makes exact
copies of itself during interphase.
Genetic Screening:
• means testing a person’s DNA for the presence of abnormal
or altered DNA.
• the presence of abnormal or altered DNA is an indication
that a particular gene is mutated.
• this may have a severe effect on a person who inherits such
genes – examples of genetic disorders include:
- albinism, cystic fibrosis, haemochromatosis, sickle cell
anaemia.
Genetic Screening
Two main forms
Adult
Foetal
Screening
Screening
A. Adult Screening
- even though they may not have a genetic disorder
adults may be screened to see if they carry the
defective gene for the disorder.
- such people are known as carriers for the condition.
- such tests give people information regarding the
chances of them having a child with the disorder.
Genetic Screening
B. Foetal Screening
- cells are removed from the placenta or the fluid
around the foetus.
- these cells can then be tested to detect if the child
has a genetic disorder.
Genetic Screening
Ethics of genetic screening:
Genetic screening may cause ethical problems such as:
1. Abortion.
2. Unfair treatment
3. Should a person be told they have a disorder that will
develop later in life and lead to death?
4. Discrimination from insurance.
RNA ( Ribonucleic Acid)
DNA and RNA are both nucleic acids however they differ
inn several ways:
DNA
RNA
Bases: contains the base
Bases: contains URACIL (U)
thymine
instead of thymine
Double Stranded
Single Stranded
DNA always stays in the
RNA can move out of the
nucleus
nucleus into the cytoplasm.
Only one type
Three types mRNA, tRNA,
rRNA
Note: in RNA the bases A and U are complementary.
RNA (Ribonucleic acid)
Note:
• RNA bases are complementary to DNA bases e.g.
- If DNA has the base sequence GGAATC, the RNA
complementary sequence will be CCUUAG.
Protein Synthesis
(Ordinary Level)
• Genes control cells by producing proteins most of which
are enzymes.
• Proteins are composed of amino acids, to produce
correct protein amino acids must assemble in the
correct sequence.
• Genes control the order of amino acids, each group of
three bases (codon) code for a particular amino acid.
• Must know/understand how genes work (are expressed)
i.e. how DNA (genes) makes proteins.
Protein Synthesis
(Ordinary Level)
Involves the genetic code in DNA being transcribed to
mRNA and this code being translated into the correct
sequence of amino acids.
Occurs in stages:
DNA
Transcription
Nucleus
RNA
Translation
Ribosomes
(cytoplasm)
PROTEIN
How proteins are made --- Protein Synthesis
1. In the nucleus the DNA strands separate
2. TRANSCRIPTION– rewriting of the DNA code to RNA
RNA bases attach to the exposed bases of the DNA
forming messenger RNA( mRNA)
3. The mRNA detaches and enter a ribosome which is
present in the cytoplasm.
4. TRANSLATION –
As the mRNA passes through the ribosome, each
group of 3 bases cause amino acids to be made this
is called TRANSLATION
5. The amino acids are linked together forming a
protein
6. The protein folds into the correct shape which
allows the protein to carry out it’s function
DNA Structure – Higher Level
o
DNA is made up of many nucleotides which are
arranged into chains called polynucleotides.
o
Nucleotides are made up of:
- a 5 carbon sugar deoxyribose
- a phosphate group (PO4)
- a nitrogen containing base ( A, T ,G , C)
Phosphate
Sugar
(deoxyribose)
Nitrogen
Base
DNA Structure – Higher Level
There are four nitrogenous bases:
Adenine (A)
Guanine (G)
Purines
Thymine (T)
Cytosine (C)
Pyrimidines
DNA Structure – Higher Level
o
In the double helix Adenine & Thymine are held
together by double hydrogen bonds
A
o
T
Cytosine & Guanine are held together by triple
hydrogen bonds
G
C
Note: - each base pair has a purine and a pyrimidine
- nucleotides join together with a bond between
phosphate group of one and sugar group of the
next – polynucleotide.
DNA Structure – Higher Level
DNA Structure – Higher Level
• DNA consists of two spiral
chains of polynucleotides.
• Outside strands –
deoxyribose and phosphate
• Rungs of molecule – are base
pairs on the inside
Protein Synthesis – Higher Level
Initiation:
• Occurs in nucleus
• Double helix unwinds at the site of the gene that
is to form a protein.
Transcription: (information is copied from DNA to RNA)
• Occurs in nucleus
• Complementary RNA bases attach to the exposed DNA
bases
• The RNA bases join together by the enzyme RNA
polymerase to form messenger RNA (mRNA)
• Each group of three bases on mRNA represents a start
codon, an amino acid sequence or a stop codon.
• mRNA moves from the nucleus into the cytoplasm
Translation: (is the making of an amino acid sequence
from RNA)
• Occurs in the ribosome
• Ribosomes - made up of rRNA and protein
- has two sub units: large sub-unit & small
sub-unit
• mRNA attaches to a ribosome
• Transfer RNA (tRNA) which are found free in the
cytoplasm, enter the large sub unit of the ribosome
two at a time
• Each tRNA carries:
- a anticodon
- a particular amino acid
Note: Every tRNA codes for a specific amino acid
• Each anticodon on a tRNA is complementary to a codon
on the mRNA
• Just after the start codon the first tRNA molecule
attaches to the mRNA (tRNA molecules attach two at a
time)
• Adjacent amino acids are detached from the tRNA and
bonded together by the ribosome forming part of new
protein.
• tRNA continues to enter the ribosome until a stop
codon is reached
• At this point:
- mRNA code sequence complete
- new protein produced.
Protein Synthesis
Protein Synthesis
Protein Synthesis
Initiation:
Transcription:
Translation:
Protein Synthesis