NUCLEIC ACIDSAS BiologyBY1

NUCLEIC ACIDS - MADE IN THE NUCLEUS AND SLIGHTLY ACIDIC

Long molecules (polymers) made up of nucleotides

Linked by condensation reactions2 main types:

DNA = deoxyribonucleic acidRNA = ribonucleic acid

NUCLEOTIDES

Nucleotides consist of a:• Pentose sugar• Phosphate group• Nitrogen-containing base

• (purine or pyrimidine)

INTRODUCTION TO NUCLEOTIDESAll nucleotides contain the following three groups:

In DNA the sugar is deoxyribose, whereas in RNA the sugar is ribose.

a phosphate group

a pentose sugar

a nitrogen-containing

base

FORMATION OF NUCLEOTIDES

DNA

Has deoxyribose as the sugarExists as a double helix of 2 chains

linked by H bonds between complementary bases

Contains the nucleotides:AdenineThymineCytosineGuanine

Figure 2.16

COMPLEMENTARY BASE PAIRING :

A bonds with T

C bonds with G

BASESThere are five bases, split into two types:

adenine (A) and guanine (G) are purine bases. A G

T U

thymine (T), cytosine (C) and uracil (U) are pyrimidine bases.

DNA contains A, G, T and C, whereas RNA contains A, G, U and C.

C

IDENTIFYING BASES

BASE PAIRING RULES

FORMATION OF POLYNUCLEOTIDES

DNA STRUCTURE

Held by covalent bonds

DNA2 major functions:

1. replication, in dividing cells

2. carrying the information for protein synthesis in all cells

Replication allows accurate copying of DNA for cell division

DETERMINING THE STRUCTURE OF DNA

The double-stranded structure of DNA was determined in 1953 by the American biologist James Watson and the British physicist Francis Crick.

X-ray diffraction studies by British biophysicist Rosalind Franklin strongly suggested that DNA was a helical structure.

The Austrian chemist Erwin Chargraff had earlier showed that DNA contained a 1:1 ratio of pyrimidine:purine bases.

STRUCTURE OF DNA

HOW IS DNA PACKAGED?In eukaryotic cells:• DNA is packaged as chromosomes in the

nucleus

In prokaryotic cells:• DNA is loose in the cytoplasm –there are no histones or chromosomes.

•It is tightly coiled and folded so that all 2m can fit in the nucleus•It is wrapped around proteins called histones•DNA + Histones = chromatin

STRUCTURE OF EUKARYOTIC CHROMOSOMES

RNA Has ribose as the

sugar Is single-strandedUracil replaces

Adenine

A can hydrogen bond with U

C can hydrogen bond with G

RNA

3 types:Ribosomal (rRNA)Messenger (mRNA)Transfer (tRNA)

RNA is made by making a complementary copy of a section of the DNA

DNA

RNA

RIBOSOMAL RNA

•Makes up part of the ribosome – made in the nucleolus

mRNA strand

ribosome

TRANSFER RNA

nucleotides

amino acidattachment site

anticodon

•Found in the cytoplasm•Used to carry amino acids to the ribosome during protein synthesis

3’ end

5’ end

hydrogen bond

MESSENGER RNA

•Is a mobile copy of a gene that codes for a protein•It moves out of the nucleus to the ribosome to act as a template for protein synthesis

mRNA strand

ribosome

mRNA

DNA

ATP – ADENOSINE TRIPHOSPHATE

Has ribose sugar, adenine, and 3 phosphate groups

(Has 2 extra phosphates on the adenine containing nucleotide

ATP Is made by condensation reactions

(dehydration synthesis) Is broken by hydrolysis reaction to liberate useful

energy for the cell

DNA REPLICATION

DNA REPLICATION•Watson and Crick provided evidence that complementary base pairing was key to DNA’s ability to replicate

•The DNA ‘unzips’ as hydrogen bonds between base pairs are broken

DNA REPLICATION

•New polynucleotide strands can then be synthesized using the originals as a template

•Replication allows accurate copying of the DNA for cell division (mitosis and meiosis)

DNA REPLICATION

WHAT’S THE KEYWORD?

MULTIPLE-CHOICE QUIZ