Page 35 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

194 Chapter 6 DNA Structure, Replication, and Recombination

Figure 6.16 Differences in the chemical structure of Figure 6.17 Complex folding patterns of RNA molecules.
DNA and RNA. Phosphodiester bonds join ribonucleotides into an Most RNA molecules are single-stranded but are sufficiently flexible
RNA chain that differs from DNA in three ways (bullets in each column). so that some regions can fold back and form base pairs with other
DNA RNA parts of the same molecule.
5'
• Deoxyribose • Ribose C
G
5' 5' A
HOCH 2 O OH HOCH 2 O OH C G
4' 1' 4' 1' G A
H H H H U
H H H H C
3' 2' 3' 2' G
HO H HO HO G C
A
C
C
• Thymine deoxyribonucleotide • Uracil ribonucleotide
U
O O
– O CH 3 C H – O H C H C G
– O P O C5 4 3 N – O P O C5 4 3 N U G A
C 6 1 2 C C 6 1 2 C U
O H N O O H N O
5' 5'
CH 2 O CH 2 O 3'
4' 1' 4' 1'
H H H H
H H H H
3' 2' 3' 2' 5'
HO H HO HO

• Usually double-stranded • Usually single-stranded

3'

Differences between RNA and DNA
Three major chemical differences distinguish RNA from
DNA (Fig. 6.16). First, RNA takes its name from the sugar essential concepts
ribose, which it incorporates instead of the deoxyribose
found in DNA. Second, RNA contains the base uracil (U) • DNA carries digital information in the sequence of its four
instead of the base thymine (T); U, like T, base pairs with bases.
A. Finally, most RNA molecules are single-stranded and • The base sequence of DNA can be read from a single,
contain far fewer nucleotides than the very long DNA mol- unwound strand during replication or transcription. In
ecules found in nuclear chromosomes. addition, specialized proteins can recognize and bind to
short base sequences accessible in the grooves of
Within a single-stranded RNA molecule, folding can double-stranded DNA.
bring together two oppositely oriented regions that carry
complementary nucleotide sequences to form a short, base- • RNA contains ribose rather than deoxyribose and uracil
(U) instead of thymine (T); it also is generally single-
paired stretch within the molecule. This means that, com- stranded instead of double-stranded.
pared to the relatively simple, double-helical shape of a
DNA molecule, many RNAs have a complicated structure
of short double-stranded segments interspersed with sin- 6.4 DNA Replication
gle-stranded loops (Fig. 6.17).
RNA has the same ability as DNA to carry information
in the sequence of its bases, but it is much less stable than learning objectives
DNA. In addition to serving as the genetic material for an
array of viruses, RNA fulfills several vital functions in all 1. Describe the key steps in the semiconservative
cells. For example, it participates in gene expression and replication of DNA.
protein synthesis, as presented in Chapter 8. RNA also 2. Explain how the Meselson-Stahl experiment with
plays a surprisingly significant role in DNA replication, heavy nitrogen showed that DNA replication is
which we now describe. semiconservative.