Section A: DNA as the Genetic MaterialSection A: DNA as the Genetic Material

CHAPTER 16 THE MOLECULE BASIS OF

INHERITANCE

Watson and Crick discovered the double helix by building models to conform to X-ray data

In April 1953, James Watson and Francis Crick shook the scientific world with an elegant double-helical model المزدوج ذالنمو الحلزونى ج for the structure of deoxyribonucleic acid or DNA.

Watson and Crick began to work on a model of DNA with two strands, the double helix المزدوج .الحلزونى

• The amino acid sequence of a polypeptide is

programmed by a gene.

• A gene is a small region in the DNA.

• Nucleic acids store and transmit hereditary

information الوراثية .المعلومات

• There are two types of nucleic acids: ribonucleic

acid (RNA) and deoxyribonucleic acid (DNA).

• Organisms inherit تتوارث DNA from their parents.– When a cell divides تنقسم, its DNA is copied and

passed to the next generation of cells.

DNA : Introduction

Structures of nucleic acids (DNA & RNA)

o

H

HH

HH

CH2

o

P oo

oBase

o

P oo

o

o

H

HH

HH

CH2

Base

DNA nucleotide

Bases

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

Purine

Pyrimidine

Deoxyribose

Phosphate group

1

23

4

5

3

5

35

3

Fig. 16.3, Page 290

• The PO4 group of one

nucleotide is attached

to the sugar of the

next nucleotide in line

مستقيم صف .فى

Nitrogenous bases النيتروجينية القواعد

Sugar-phosphate backbones

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

PurinePyrimidines

Hydrogen bonds

Nit

rog

eno

us

bas

es

5

35

3

Uracil (U)

• Adenine (A) would form 2 hydrogen bonds only with thymine (T)

• Guanine (G) would form 3 hydrogen bonds only with cytosine (C).

Fig. 16.6, Page 292

DNA

DNA & RNA

o

H

HH

HH

CH2

Deoxyribose sugar (O on C2 is missed)

o

OH

HH

HH

CH2

Ribose sugar (no missed O)

Ribo-Nucleic-AcidDeoxiribo-Nucleic-Acid

Single stranded nucleic acidDouble stranded nucleic acid

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

• Nucleic acids consist of من .nucleotides تتكون

• Each nucleotide consists of three parts اجزاء a nitrogen base :ثالثنتروجينية خماسي a pentose sugar ,قاعدة and a phosphate ,سكر

group.

• The nitrogen bases come in two types: Purines and Pyrimidines.

• The pentose sugar joined to بـ the nitrogen base is ribose المرتبطin nucleotides of RNA and deoxyribose in DNA.

• The only difference بين of نقص between the sugars is the lackالفرقan oxygen atom ذرة on carbon 2 in deoxyribose.

Nucleic acids االحماض النووية

• Sugars of one nucleotide join ترتبط the phosphate of the next with a phosphodiester bond.

• Genes are normally hundreds to thousands of nucleotides long.

• The linear order التتابعى of bases in الترتيبa gene specifies ُي2حدد the order of amino acids األمينية األحماض the monomers of a) ترتيب

protein).

• The flow of genetic information is from DNA mRNA protein.

– Protein synthesis occurs in Ribosomes.

Fig. 5.28

• An RNA molecule is single polynucleotide chain (single strand).• DNA molecules have two polynucleotide strands (double strand)

that spiral around حلزونيا مزدوج to form a double helix تدور .حلزون

Inheritance is based on replication of the DNA double helix

Fig. 16.5, Page 291

– Adenine (A) always pairs with thymine (T) and guanine (G) with cytosine (C).

• If we know the sequence of bases تتابع we know ,الخيط on one strand القواعدthe sequence on the opposite المقابل strand.

• The two strands are complementary لبعضهما .مكملين

Repeated Sugar - PhosphateRepeated Sugar - Phosphate

Sugar–Phosphate-BaseSugar–Phosphate-Base

PolynucleotidePolynucleotide

DNA backboneDNA backbone

One nucleotideOne nucleotide

DNA MoleculeDNA Molecule

DNA Double strandedDNA Double stranded

RNA single strandedRNA single stranded

T C G A T A G

A G TC T A C

UUmRNA

DNA