The Structure and Function of Large Biological Molecules

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group

Sugar

(pentose)

(b) Nucleotide

(a) Polynucleotide, or nucleic acid

3 end

3C

3C

5C

5C

Nitrogenous bases

Pyrimidines

Cytosine (C)

Thymine (T, in DNA)

Uracil (U, in RNA)

Purines

Adenine (A)

Guanine (G)

Sugars

Deoxyribose (in DNA)

Ribose (in RNA)

(c) Nucleoside components: sugars

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Fig. 5-27ab

5' end

5'C

3'C

5'C

3'C

3' end

(a) Polynucleotide, or nucleic acid

(b) Nucleotide

Nucleoside

Nitrogenous

base

3'C

5'C

Phosphate

group

Sugar

(pentose)

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Fig. 5-27c-1

(c) Nucleoside components: nitrogenous bases

Purines

Guanine (G)

Adenine (A)

Cytosine (C)

Thymine (T, in DNA)

Uracil (U, in RNA)

Nitrogenous bases

Pyrimidines

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Fig. 5-27c-2

Ribose (in RNA)

Deoxyribose (in DNA)

Sugars

(c) Nucleoside components: sugars

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Nucleotide Monomers

Nucleoside = nitrogenous base + sugar

There are two families of nitrogenous bases:

Pyrimidines (cytosine, thymine, and uracil) have a single six-membered ring

Purines (adenine and guanine) have a six-membered ring fused to a five-membered ring

In DNA, the sugar is deoxyribose; in RNA, the sugar is ribose

Nucleotide = nucleoside + phosphate group

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Nucleotide Polymers

Nucleotide polymers are linked together to build a polynucleotide

Adjacent nucleotides are joined by covalent bonds that form between the –OH group on the 3 carbon of one nucleotide and the phosphate on the 5 carbon on the next

These links create a backbone of sugar-phosphate units with nitrogenous bases as appendages

The sequence of bases along a DNA or mRNA polymer is unique for each gene

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The DNA Double Helix

A DNA molecule has two polynucleotides spiraling around an imaginary axis, forming a double helix

In the DNA double helix, the two backbones run in opposite 5 → 3 directions from each other, an arrangement referred to as antiparallel

One DNA molecule includes many genes

The nitrogenous bases in DNA pair up and form hydrogen bonds: adenine (A) always with thymine (T), and guanine (G) always with cytosine (C)

Slide 115