Chromosomes, Genes and DNA
Chromosomes, thread-like structures in a cell’s nucleus, are composed of a nucleic
acid molecule called deoxyribonucleic acid (DNA). Segments of DNA form genes. The
DNA molecule is the basic genetic/hereditary molecule found in all living things – from
humans to bacteria. It determines an organism’s characteristics by directing protein
synthesis; passes genetic information from one generation to the next by making exact
duplicates of itself; and stores genetic information in form of a sequence of bases.
OVERVIEW: THE CENTRAL DOGMA OF MOLECULAR BIOLOGY
An organism must be able to store and preserve its genetic information, pass that
information along to future generations, and express that information as it carries out
all the processes of life. The major steps involved in handling genetic information are
illustrated by the central dogma of molecular biology (Figure I -1-1). Genetic information
is stored in the base sequence of DNA molecules. Ultimately, during the process of gene
expression, this information is used to synthesize all the proteins made by an organism.
Classically, a gene is a unit of the DNA that encodes a particular protein or RNA
molecule. Although this definition is now complicated by our increased appreciation of
the ways in which genes may be expressed, it is still useful as a general, working
definition.
The central dogma
NUCLEOTIDE STRUCTURE AND NOMENCLATURE
The building blocks of nucleic acids are nucleotides. Nucleic acids (DNA and
RNA) are assembled from nucleotides, which consist of three components: a
nitrogenous base, a five-carbon sugar (pentose), and phosphate.
,Five-Carbon Sugars
Nucleic acids (as well as nucleosides and nucleotides) are classified according to the
pentose they contain. If the pentose is ribose, the nucleic acid is RNA (ribonucleic acid);
if the pentose is deoxyribose, the nucleic acid is DNA (deoxyribonucleic acid).
Five-carbon sugars found in nucleic acids
Bases
There are two types of nitrogen-containing bases commonly found in nucleotides:
purines and pyrimidines.
Commonly found bases in nucleic acids
Purines contain two rings in their structure. The two purines commonly found in nucleic
acids are adenine (A) and guanine (G); both are found in DNA and RNA. Other purine
metabolites, not usually found in nucleic acids, include xanthine, hypoxanthine, and uric
acid.
Pyrimidines have only one ring. Cytosine (C) is present in both DNA and RNA. Thymine
(T) is usually found only in DNA,whereas uracil (U) is found only in RNA.
Nucleosides and Nucleotides
Nucleosides are formed by covalently linking a base to the number 1 carbon of a sugar.
The numbers identifying the carbons of the sugar are labeled with "primes" in
nucleosides and nucleotides.
, Examples of nucleosides
Nucleotides are formed when one or more phosphate groups is attached to the 5'
carbon of a
nucleoside. Nucleoside di- and triphosphates are high-energy compounds because of
the hydrolytic energy associated with the acid anhydride bonds.
Examples of nucleotides
Chromosomes, thread-like structures in a cell’s nucleus, are composed of a nucleic
acid molecule called deoxyribonucleic acid (DNA). Segments of DNA form genes. The
DNA molecule is the basic genetic/hereditary molecule found in all living things – from
humans to bacteria. It determines an organism’s characteristics by directing protein
synthesis; passes genetic information from one generation to the next by making exact
duplicates of itself; and stores genetic information in form of a sequence of bases.
OVERVIEW: THE CENTRAL DOGMA OF MOLECULAR BIOLOGY
An organism must be able to store and preserve its genetic information, pass that
information along to future generations, and express that information as it carries out
all the processes of life. The major steps involved in handling genetic information are
illustrated by the central dogma of molecular biology (Figure I -1-1). Genetic information
is stored in the base sequence of DNA molecules. Ultimately, during the process of gene
expression, this information is used to synthesize all the proteins made by an organism.
Classically, a gene is a unit of the DNA that encodes a particular protein or RNA
molecule. Although this definition is now complicated by our increased appreciation of
the ways in which genes may be expressed, it is still useful as a general, working
definition.
The central dogma
NUCLEOTIDE STRUCTURE AND NOMENCLATURE
The building blocks of nucleic acids are nucleotides. Nucleic acids (DNA and
RNA) are assembled from nucleotides, which consist of three components: a
nitrogenous base, a five-carbon sugar (pentose), and phosphate.
,Five-Carbon Sugars
Nucleic acids (as well as nucleosides and nucleotides) are classified according to the
pentose they contain. If the pentose is ribose, the nucleic acid is RNA (ribonucleic acid);
if the pentose is deoxyribose, the nucleic acid is DNA (deoxyribonucleic acid).
Five-carbon sugars found in nucleic acids
Bases
There are two types of nitrogen-containing bases commonly found in nucleotides:
purines and pyrimidines.
Commonly found bases in nucleic acids
Purines contain two rings in their structure. The two purines commonly found in nucleic
acids are adenine (A) and guanine (G); both are found in DNA and RNA. Other purine
metabolites, not usually found in nucleic acids, include xanthine, hypoxanthine, and uric
acid.
Pyrimidines have only one ring. Cytosine (C) is present in both DNA and RNA. Thymine
(T) is usually found only in DNA,whereas uracil (U) is found only in RNA.
Nucleosides and Nucleotides
Nucleosides are formed by covalently linking a base to the number 1 carbon of a sugar.
The numbers identifying the carbons of the sugar are labeled with "primes" in
nucleosides and nucleotides.
, Examples of nucleosides
Nucleotides are formed when one or more phosphate groups is attached to the 5'
carbon of a
nucleoside. Nucleoside di- and triphosphates are high-energy compounds because of
the hydrolytic energy associated with the acid anhydride bonds.
Examples of nucleotides
