1 DNA Structure

8/3/2019 1 DNA Structure

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Dr:AbouSree Ellethy

Dentistry- 1432-1433

DNA & RNA structure &

function Objectives

Types of purine & pyrimidine bases and sugars present in nucleic acids.

Define nucleoside, nucleotide, DNA & RNA and name their structural components

Define nuclear DNA, name its different forms and show structure of its B form.

Define (Codon & types) - gene- (coding & non-coding) .



The building units for Carbohydrates are««««««.

Amino AcidAmino Acid

Fatty acidFatty acid

MonosaccharidesMonosaccharides

The building of fats units are «««««

SugarsSugars

Fatty acidsFatty acids

Amino AcidsAmino Acids

The building units for Proteins are««««««.

SugarsSugars



The building units for Genetic materials are ««««???????????

««««««««««««

««««««««««««

««««««.««««««.



The building units for Carbohydrates are««««««.Amino AcidsAmino Acids


SugarsSugars

The building of fats units are «««««

SugarsSugarsFatty acidsFatty acids


The building units for Proteins are««««««.

SugarsSugars

Fatty acidsFatty acidsAmino AcidsAmino Acids

The building units for Genetic materials are ««««???????????

««««««««««««

««««««««««««

««««««.««««««.

Revision



- Pyramidines: Thymine and cytosine have single-ring structures.

- Purines: Adenine and guanine have double-ring structures.

Nitrogenous Bases



AdenineAdenine (A) would(A) wouldformform 2 hydrogen2 hydrogen bondsbonds

only withonly with thyminethymine (T(T))

GuanineGuanine (G) would form(G) would form

3 hydrogen bonds3 hydrogen bonds onlyonly

withwith cytosinecytosine (C(C).).

Base pairing hasBase pairing hasbalanced ratio of (A = T)balanced ratio of (A = T)

& (G = C) bases& (G = C) bases basedbased

Chargaff·s RuleChargaff·s Rule

Sugars & Bases Pairing

Deoxy ribose Ribose



If there is 30% AdenineAdenine, how much CytosineCytosine ispresent?

Answer:Answer:

There would be 20% CytosineCytosine

Adenine (30%) = Thymine (30%)Adenine (30%) = Thymine (30%)

Guanine (20%) = Cytosine (20%)Guanine (20%) = Cytosine (20%)

Therefore, 60% ATherefore, 60% A--T + 40% CT + 40% C--GG

Question:Question:



Nucleobase+ deoxy sugar Nucleobase + Sugar

Deoxy-adenosine Adenosine

Deoxy-thymidineThymidine

Deoxy-cytidineCytidine

Deoxy-guanosineGuanosine

Deoxy-uridineUridine

A base is linked via a glycosidic bond to sugar unit like Ribose or

Deoxyribose sugars.

Nomenclature by adding -idine to the root name of pyrimidines or -osine

to the root name of purines.

Nucleosides



Nucleotides represent mono phosphonucleotides, di-phosphonucleotides & tri-phosphonucleotides.

The phosphoryl group is added on C-5 of the pentose.

P

S B

Covalent Bonds

Nucleotide consists of 3Nucleotide consists of 3 components:components:

1. A phosphate group

2. A sugar :Deoxyribose.

3. A Nitrogenous Base

Held Together by Covalent Bonds.

Nucleotides



Purines nucleotides: Adenine or Guanine + sugar+ phosphate group

Cytosine

Pyramidines nucleotides: Cytosine and Thymine + sugar+ phosphate group

Thymine

GuanineAdenine

Nucleotides



O

H

H

H

HH

CH2

DeoxyriboseDeoxyribose sugar sugar ((O on C2 is missedO on C2 is missed))

O

OH

H

H

HH

CH2

Ribose sugar Ribose sugar

((no missed Ono missed O))

Ribo-Nucleic-AcidDeoxiribo-Nucleic-Acid

Single stranded nucleic acidSingle stranded nucleic acidDouble stranded nucleic acidDouble stranded nucleic acid

Bases: A, G, C, T Bases: A, G, C, U

DNA & RNA Structure



B-DNA is the most common right-handed (20

angstrom diameterA

The length for one complete turn of the double

helix (34 Å).

The width of the double helix (20 Å).

A single turn of B-DNA about the axis of the

molecule contains ten base pairs (3.4 Å /bps.)

The distance spanned by one turn of B-DNA is 3.4

nm. The width (helical diameter) of the double helix

in B-DNA is 2 nm.

complementary base-pairing (watson-crick)

A-T

G-C

B form of DNA



Definition: It is three successive nucleotides of an mRNA that encode a

specific amino acid in polypeptide chain.

There are 64 codons code for the 20 amino acids.

There are more than one codone for the same amino acid

Start codon is (methionine, AUG)

Stop codons are (UAA, UAG & UGA)

Codon





Defination: Altering the nucleotide sequence

Example Original DNA Template C A A T G C T A C

1. Substitution: one base pair for another.

C A A C G C T A C

2. Deletion: missing a base.

C A A T --- C T A C

3. Addition: adding an extra baseC A A T- A- G C T A C

4. Inversion: bases are rearranged.

C A G A T C T A C

Types of Mutations:



Types of Mutations:

A- Point mutation

1. Silent= codons contain the changed base will code for the same amino acid.

Example: UCA UCU codes for Serine

2. Mis sense= codons contain the changed base may code for a different

amino acid UCA= serine CCA=proline

3. Non sense= codons contain the changed base may become a Termination

codon UCA= serine UAA=stop codon



B-Frame- shift mutation

Deletion or addition of one or two base to DNA sequence, leading to

change in reading frame (reading sequence)

Types of Mutations:



Gene:A segment of DNA on the chromosome that codes for a specific protein.

(1) (2) (3) (4)

(5)Gene composed of

Enhancers: control elements or sequences that activate promotors.

Promotors: DNA sequences activate synthesis of new DNA strand.

Exons: coding DNA sequences (share protein synthesis)

Introns: they are non coding DNA sequences (does not share protein synthesis)

Structure of gene

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1 DNA Structure