Nucleic Acids

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Nucleic acids are made up of polynucleotides – polymers of nucleotides.

There are two types of nucleic acids◦Deoxyribonucleic acid (DNA)◦Ribonucleic acid (RNA)

Genes◦Are the units of inheritance made up of DNA◦they consist of sequences of DNA nucleotides that

program the amino acid sequence of polypeptides

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Stores information for the synthesis of specific proteins◦Directs RNA synthesis◦Directs protein synthesis through mRNA

Exists as double stranded structure in the form of the double helix in cells and has a long life span

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1

2

3

Synthesis of mRNA in the nucleus

Movement of mRNA into cytoplasm

via nuclear pore

Synthesisof protein

NUCLEUSCYTOPLASM

DNA

mRNA

Ribosome

AminoacidsPolypeptide

mRNA

Relationship between DNA, RNA and proteins

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Nucleic acids are polymers called polynucleotides

Each polynucleotide is made of monomers called nucleotides

Each nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups

The portion of a nucleotide without the phosphate group is called a nucleoside

Sugar-phosphate backbone5 end

5C

3C

5C

3C

3 end

(a) Polynucleotide, or nucleic acid

(b) Nucleotide

Phosphategroup Sugar

(pentose)

Nucleoside

Nitrogenousbase

5C

3C

1C

Nitrogenous bases

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

Adenine (A) Guanine (G)

Sugars

Deoxyribose (in DNA) Ribose (in RNA)

(c) Nucleoside components

Pyrimidines

Purines

Nitrogenous bases

Cytosine (C)

Thymine (T, in DNA)

Uracil (U, in RNA)

Adenine (A) Guanine (G)

Sugars

Deoxyribose (in DNA)

Ribose (in RNA)

(c) Nucleoside components

Pyrimidines

Purines

Are made up of nucleosides and phosphate groups The phosphate group carries negative charges and as a result both

DNA and RNA are negatively charged The nucleosides are made up of nitrogenous bases and pentose

sugars There are 2 types of pentose sugars – deoxyribose (in DNA) and

ribose (in RNA) Nitrogenous bases contain nitrogen and act as bases by picking up

hydrogen ions The bases can be categorized into 2 groups – purines and

pyrimidines Purines consist of adenine (A) and Guanine (G) while pyrimidines

consist of cytosine (C), thymine (T) and uracil (U) There are 4 types of bases in DNA and RNA In DNA, the type of bases are – A, G, T and C In RNA, the type of bases are – A, G, C and U

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Are made up of nucleotides linked by the –OH group on the 3´ carbon of one nucleotide and the phosphate on the 5´ carbon on the next nucleotide through condensation reactions resulting in the formation of 5´ 3´ phosphodiester linkages

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Sugar-phosphate backbone5 end5C

3C

5C

3C

3 end(a) Polynucleotide, or nucleic acid

(b) Nucleotide

Phosphategroup Sugar

(pentose)

Nucleoside

Nitrogenousbase

5C

3C

1C

The sequence of bases/nucleotides along a nucleotide polymer◦ Is unique for each gene◦ Determines the sequence of amino acids for a

polypeptide chain

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Cellular DNA molecules◦ Have two polynucleotides that spiral around an imaginary

axis◦ Form a double helix held together by hydrogen bonding

The nitrogenous bases in DNA◦ Form hydrogen bonds in a complementary fashion (A

with T only, and C with G only)◦ Between A and T, there are 2 hydrogen bonds while

between G and C, there are 3 hydrogen bonds◦ Each turn of the double helix contains 10 pairs of bases

held together by hydrogen bonds

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Sugar-phosphatebackbonesHydrogen bonds

Base pair joinedby hydrogen bonding

Base pair joinedby hydrogen

bonding

(b) Transfer RNA(a) DNA

5 3

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Base pairing between the bases

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O

–O O

OH

O

–OO

O

H2C

O

–OO

O

H2C

O

–OO

O

OH

O

O

OT A

C

GC

A T

O

O

O

CH2

OO–

OO

CH2

CH2

CH2

5 end

Hydrogen bond3 end

3 end

G

P

P

P

P

O

OH

O–

OO

O

P

P

O–

OO

O

P

O–

OO

O

P

(b) Partial chemical structure

H2C

5 end

O

The DNA double helix◦ Consists of two antiparallel nucleotide strands which

are complementary to each other.

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3’ end

Sugar-phosphatebackbone

Base pair (joined byhydrogen bonding)Old strands

Nucleotideabout to be added to a new strand

A

3’ end

3’ end

5’ end

Newstrands

3’ end

5’ end

5’ end

Generally single stranded Has short life span Consist of the bases A,G,C and U. There are 3 main types of RNA:1. mRNA (messenger RNA) – transcribes the genetic

information from DNA2. rRNA (ribosomal RNA) – combines with proteins to

form ribosomes3. tRNA (transfer RNA) – transfers amino acids to the

growing polypeptide chains attached to ribosomes to aid in translation process

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Molecular comparisons ◦ Help biologists sort out the evolutionary connections

among species ◦ DNA and amino acid sequences can be compared to

establish evolutionary relationships

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