BIOLOGICAL SCIENCE · © 2014 Pearson Education, Inc. In this chapter you will learn that by asking comparing/contrasting and by asking specialized for 4.1 4.2 4.3 4.4 What is a nucleic

Freeman Quillin Allison

© 2014 Pearson Education, Inc.

BIOLOGICAL SCIENCEFIFTH EDITION

4

Lecture Presentation by

Cindy S. Malone, PhD, California State University Northridge


In this chapter you will learn that

by asking

comparing/contrastingandby asking

specialized for

4.1

4.2 4.3 4.4

What is a nucleic acid?

DNA

structure and

function

RNA

structure and

function

Could life have

evolved from

an RNA?

Stability and

storage

Versatility and

catalysis

Nucleic acids store the information that encodes life


▪ A nucleic acid is a polymer of nucleotide monomers

▪ Three components of a nucleotide:

1. A phosphate group

2. A five-carbon sugar

3. A nitrogenous (nitrogen-containing) base

▪ The phosphate is bonded to the sugar molecule

– In turn, the sugar molecule is bonded to the

nitrogenous base


▪ Ribonucleotides

– The sugar is ribose

▪ Deoxyribonucleotides

– The sugar is deoxyribose (deoxy means lacking

oxygen)

▪ These two sugars differ by a single oxygen atom

– Ribose has an –OH group bonded to the 2′ carbon

– Deoxyribose has an H instead at the same location

▪ In both of these sugars

– An –OH group is bonded to the 3′ carbon


▪ There are two groups of nitrogenous bases:

1. Purines

– Adenine

– Guanine

2. Pyrimidines

– Cytosine

– Uracil

– Thymine

▪ The base uracil (U) is found only in ribonucleotides

▪ The base thymine (T) is found only in deoxyribonucleotides


Figure 4.1

(a) Nucleotide (c) Nitrogenous bases

(b) Sugars

Cytosine (C) Uracil (U) in RNA Thymine (T) in DNA

Guanine (G) Adenine (A)

Pyrimidines

PurinesRibose in RNA Deoxyribose in DNA

Nitrogenous

base

Phosphate

group5-carbon

sugar

Phosphate group is bondedto 5 carbon of sugar

Nitrogenousbase is bonded to1 carbon of sugar

Purines are larger thanpyrimidines


▪ The sugar-phosphate backbone of a nucleic acid is

directional (has polarity)

– One end has an unlinked 5′ carbon

– The other end has an unlinked 3′ carbon

▪ The nucleotide sequence is written in the 5′ 3′

direction

– Reflects the order that nucleotides are added to a

growing molecule

– The nucleic acid’s primary structure is the

nucleotide sequence


Figure 4.3

5

3

3

3

3

3

5

5

5

5

3

The sugar-phosphate

backbone of RNA

5 end ofnucleic acid

3 and 5 carbonsjoined byphosphodiesterlinkage

3 end of nucleic acid:new nucleotides are addedto the unlinked 3 hydroxyl


▪ James Watson and Francis Crick determined

1. DNA strands run in an antiparallel configuration

2. DNA strands form a double helix

– The hydrophilic sugar-phosphate backbone faces the

exterior

– Nitrogenous base pairs face the interior


3. Purines always pair with pyrimidines

– Strands form complementary base pairs A-T and

G -C

– A-T have two hydrogen bonds

– C-G have three hydrogen bonds

4. DNA has two different-sized grooves:

– The major groove

– The minor groove


Figure 4.6

(a) Only purine-pyrimidine pairs fit inside the

double helix.

(b) Hydrogen bonds form between G-C pairs and

A-T pairs.

Guanine Cytosine

Adenine Thymine

5 3

3 5

Purine-purine pair

NOT ENOUGH SPACE

Pyrimidine-pyrimidine pair

TOO MUCH SPACE

Purine-pyrimidine pair

JUST RIGHT

Space inside sugar-

phosphate backbones

Antiparallel

strands

Hydrogen

bonds

DNA contains thymine,

whereas RNA contains uracilS

ugar-

ph

osp

hate

ba

ckb

on

e


▪ DNA’s secondary structure consists of

– Two antiparallel strands twisted into a double helix

▪ The molecule is stabilized by

– Hydrophobic interactions in its interior

▪ By hydrogen bonding between

– The complementary base pairs

– A-T and G-C


(a) Cartoons of DNA structure (b) Space-filling model of DNA

double helix

5

3 3

3

5 5

5 5

5

3

3

Base pairing Double helix

Major groove

Minor groove

Distance betweenbases0.34 nm

Width of helix2.0 nm

Len

gth

of

on

e c

om

ple

te t

urn

of

heli

x (

10 r

un

gs

per

turn

) 3

.4 n

m


▪ DNA can store and transmit biological

information

▪ DNA carries the information required for the

organism’s growth and reproduction

▪ The language of nucleic acids is contained in the

sequence of the bases

▪ DNA carries the information required for the

growth and reproduction of all cells


▪ Step 1

– Heating or enzyme-catalyzed reactions

– Cause the double helix to separate

▪ Step 2

– Free deoxyribonucleotides form hydrogen bonds with complementary bases on the original strand of DNA

– Called a template strand

– Sugar-phosphate groups form phosphodiester linkages to:

– Create a new strand

– Called a complementary strand

▪ Step 3

– Complementary base pairing allows

– Each strand of a DNA double helix to be copied exactly

– Producing two identical daughter molecules


▪ DNA replication requires two steps:

1. Separation of the double helix

2. Hydrogen bonding of deoxyribonucleotides with complementary bases

– On the original template strand

– Followed by phosphodiester bond formation between the deoxynucleotides to form the complementary strand

3. Polymerization

2. Base pairingwith template

1. Strand separation

35

3

3 3

3 3

3 3

3 3

5

5 5

5 5

5 5

5 5

The originalmolecule hasbeen copied.

New NewOld Old


▪ RNA (like DNA) has a primary structure consisting

of a sugar-phosphate backbone

– Formed by phosphodiester linkages

– Extending from that backbone, a sequence of four types of nitrogenous bases

▪ The primary structure of RNA differs from DNA

1. RNA contains uracil instead of thymine

2. RNA contains ribose instead of deoxyribose

– The presence of the –OH group on ribose makes RNA

– Much more reactive

– Less stable than DNA


Figure 4.9

Nitrogenous bases

Loop

Ste

m

Hair

pin

3

5

Single-strandedregion forms a loop

Double-stranded regionforms a double helix


▪ RNA molecules can also have tertiary structure

– Forms when secondary structures fold into more

complex shapes

▪ RNA (like DNA) can function as

– An information-containing molecule

– Capable of self-replication

▪ Structurally/chemically, RNA is intermediate

between

– The complexity of proteins

– The simplicity of DNA


Summary Table 4.1


1. Complementarybases pair.

2. Copied strandpolymerizes.

3. Copy and templateseparate.

4. Copy serves as newtemplate.

5. New copy polymerizes.

6. New copy is identical tooriginal template.

3

5

5

5

5

5

5

5

55 5

5

5

5

5

3

3

3

3

333

33

3

3

3

3

Template strand

Copied strand

New copy strand

New template strand

53


Figure 4.11

Folding bringswidely spaced

nucleotidestogether at theactive site ofthis catalytic

RNA


▪ The theory of chemical evolution

– Life began as a naked self-replicator

– A molecule that existed by itself in solution

– Without being enclosed in a membrane

▪ To make a copy of itself, the first living molecule had to

– Provide a template that could be copied

– Catalyze polymerization reactions that would link monomers into a copy of that template

▪ RNA is capable of both processes

– Most origin-of-life researchers propose that the first life-form was made of RNA


▪ RNA is not very stable

– But it might have survived long enough in the

prebiotic soup to replicate itself

– And so it may have been the first life-form

▪ RNA replicase is a ribozyme that

– Can catalyze the addition of ribonucleotides to a

complementary RNA strand

– Can replicate RNA

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BIOLOGICAL SCIENCE · © 2014 Pearson Education, Inc. In this chapter you will learn that by asking comparing/contrasting and by asking specialized for 4.1 4.2 4.3 4.4 What is a nucleic