Transcript Lesson 15a Components of DNA #1 PPT

DNA
DNA is often called the blueprint
of life.
In simple terms, DNA contains
the instructions for making
proteins within the cell.
DNA stands for deoxyribose
nucleic acid
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This chemical substance is present in the nucleus of
all cells in all living organisms.
DNA controls all the chemical changes which take
place in cells.
The kind of cell which is formed, (muscle, blood,
nerve etc) is controlled by DNA.
The kind of organism which is produced (buttercup,
giraffe, herring, human etc) is controlled by DNA
Composition of DNA
DNA is a very large molecule
made up of a long chain of sub-units
The sub-units are called nucleotides.
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Each nucleotide is made up of
a sugar called deoxyribose
a phosphate group -PO4 and
a nitrogen base.
Components and structure of
DNA
•A very long molecule. 4 nitrogenous bases:
The most common organic bases are
Adenine
(A)
Thymine
(T)
Cytosine
(C)
Guanine
(G)
Chargaff’s rules
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The relative amounts of adenine and
thymine are the same in DNA
The relative amounts of cytosine
and guanine are the same.
Named after Erwin Chargaff
Discovery of DNA Structure
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Erwin Chargaff showed the
amounts of the four bases on DNA
( A,T,C,G)
In a body or somatic cell:
A = 30.3%
T = 30.3%
G = 19.5%
C = 19.9%
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The bases always pair up in the same way
Adenine forms a bond with Thymine
Adenine
Thymine
and Cytosine bonds with Guanine
Cytosine
Guanine
PO4
PO4
adenine
thymine
PO4
PO4
cytosine
guanine
PO4
PO4
PO4
PO4
Two Kinds of Bases in DNA
Pyrimidines aresingle
ring bases.
 Purines are double
C
ring bases.
N
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N
C
C
C
N
N C
N C
Thymine and Cytosine are
pyrimidines
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Thymine and cytosine each have
one ring of carbon and nitrogen
atoms.
N
N
O
C
C
O
C C
N
C
thymine
N
O
C
C
C
N
C
cytosine
Adenine and Guanine are
purines
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Adenine and guanine each have two
rings of carbon and nitrogen
atoms.
N
C
Adenine
N
C
C
N
O
N
C
N
N
C
N
C
C
C
N
Guanine
C
N
N
C
The deoxyribose, the phosphate and one of the bases
Combine to form a nucleotide
PO4
adenine
deoxyribose
Joined
nucleotides
A molecule of DNA is
formed by millions of
nucleotides joined together
in a long chain.
PO4
PO4
In fact, the DNA usually
consists of a double strand
of nucleotides.
The sugar-phosphate chains
are on the outside
and the strands are held
together by chemical
bonds between the bases.
PO4
PO4
sugar-phosphate
backbone
+ bases
Rosalind Franklin
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Used X-Ray
diffraction to get
information about
the structure of
DNA:
Structure of DNA
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Discovered in 1953 by
two scientists:
James Watson (USA)
Francis Crick (GB)
Known as the
double-helix model.
THE DOUBLE
HELIX
bases
sugar-phosphate
chain
2-stranded DNA
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
The double-helix
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A twisted ladder with two long chains of
alternating phosphates and sugars. The
nitrogenous bases act as the “rungs”
joining the two strands connected by H
bonds.
How long is the DNA
molecule?
Chromosomes & DNA
replication
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The nucleus of one human cell contains
approximately 1 meter of DNA.
Histones = DNA tightly wrapped around
a protein
Nucleosome:
DNA replication
Must occur before a
cell divides.
Each new cell needs
a copy of the
information in order
to grow.
Before a cell divides, the DNA strands
unwind and separate.
Each strand makes a new partner by
adding the appropriate nucleotides.
The result is that there are now two
double-stranded DNA molecules in the
nucleus.
So that when the cell divides, each
nucleus contains identical DNA.
This process is called replication.
PO4
PO4
PO4
The strands
separate
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
Each strand builds up its partner by adding
the appropriate nucleotides
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO
PO4
TRANSCRIPTION
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DNA is copied into mRNA with
the aid of RNA polymerase.
Copies genetic code of
DNA by matching bases.
Occurs in the nucleus.
DNA changing to mRNA
RNA
Very similar to DNA.
 Exceptions:
1. Uracil replaces thymine
2. Single-stranded
3. Ribose is the 5-carbon sugar
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tRNA (transfer)
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approx. 80
nucleotides in
length.
Cross-like shape
At one end an amino
acid is attached
At the other end
there is an
anticodon
Acts like a truck
Polypeptide assembly
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Translation =
reading or
“translating” the
RNA code to form
a chain of amino
acids.
Known as protein
synthesis
Occurs in the
cytoplasm.
tRNA
mRNA
Amino acid
3 Types of RNA
Types of RNA
Where are they produced
Function
mRNA
nucleus
Carries the information in
codons that determine the order
of amino acids
tRNA
Other genes in the nuclear
DNA code for tRNA
molecules.
Carries the anticodon and picks
up the amino acid to bring to
the rRNA
rRNA
Other genes in the
nucleus produces the
rRNA
.rRNA molecules combine with
protein to form the ribosomes,
which serve as the base for
interactions between mRNA codons
and tRNA anticodons in translation
in the cytoplasm
Structure
The sequence of bases in DNA forms the
Genetic Code
A group of three bases (a triplet) controls
the production of a particular amino acid in
the cytoplasm of the cell
The different amino acids and the order in
which they are joined up determines the
sort of protein being produced
Codons
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A three letter “word” that
specifies an amino acid.
This is a small, imaginary protein molecule
showing how a sequence of 5 different amino acids
could determine the shape and identity of the
molecule.
Ser-Cyst-Val-Gly-Ser-Cyst Ala
Val
Val-Cyst-Ser-Ala-Ser-Cyst-Gly
Val- Cyst-Ala-Ala-Ser-Gly
Each amino acid (Serine, Cysteine, Valine, Glycine and
Alanine) is coded for by a particular triplet of bases.
For example
Cytosine
Adenine
Codes for
Valine
Codes for
Alanine
Thymine
Cytosine (C)
Guanine (G)
Adenine (A)
This is known as the triplet code
Each triplet codes for a specific amino
acid
CGA - CAA - CCA - CCA - GCT - GGG - GAG - CCA Ala
Val
Gly
Gly
Arg
Pro
Leu
Gly
The amino acids are joined together in the correct
sequence to make part of a protein
Ala
Val
Gly
Gly
Arg
Pro
Leu
Gly
The proteins build the cell structures.
They also make enzymes.
The DNA controls which enzymes are
made and the enzymes determine what
reactions take place.
The structures and reactions in the cell
determine what sort of a cell it is and what
its function is.
So DNA exerts its control through the
enzymes.
A sequence of triplets in the DNA molecule
may code for a complete protein
Such a sequence forms a gene
There may be a thousand or more
bases in one gene.
Reading the genetic code
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The genetic code is responsible for
building all the proteins in the body
using 20 different amino acids.
How many 3 letter words can you
make from the letters A,T,G and C?
Answer: 64
Question 1
Which of the following are components of
nucleotides?
(a) deoxyribose
(b) amino acids
(c) phosphate
(d) enzymes
(e) organic bases
Question 2
Which of the following represent a correct
pairing of bases?
(a) adenine with thymine
(b) adenine with guanine
(c) thymine with adenine
(d) guanine with cytosine
(e) thymine with thymine
Question 3
DNA molecules are formed from
(a) organic bases
(b) amino acids
(c) deoxyribose
(d) nucleotides
Question 4
Which of the following are organic bases?
(a) Valine
(b) Guanine
(c) Thymine
(d) Serine
Question 5
Replication of DNA occurs
(a) During cell division
(b) before cell division
(c) at any time
Question 6
A nucleotide triplet codes for
(a) a protein
(b) an amino acid
(c) an enzyme
(d) an organic base
Question 7
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Transcribe this DNA sequence into RNA, then translate the RNA into an amino
acid chain using the genetic code circle on the next slide.
TAGCCGACAGGCCTCTTTACT
An example of a DNA sequence might be
CCC TGT GGA GCC ACA CCC TAG translate into mRNA
GGG ACA CCU CGG UGU GGG AUC
You can decode triplet by triplet. Start from the inside of the wheel: find the first
letter of your codon in the centre of the wheel and work outwards, through the
second ring (with the next letter) and so on, to find the corresponding amino acid.
This would make the amino acid chain:
P - C - G - A - T - STOP
Proline-Cysteine-Glycine-Alanine-Threonine-STOP
Genetic code:
Answers
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1. A,C,E
2. A,C,D
3. D
4. B,C
5. B
6. B
7. AUC, GGC, UGU, CCG, GAG, AAA,UGA
MET, GLY, CYS, PRO, GLU, LYS, STOP