NUCLEIC proteins. DNA is found in all

NUCLEIC ACIDS

            Nucleic acids, such as DNA and RNA,
are one of the most essential macromolecules for the continuity of life. DNA
bears the genetic information that’s passed on from parents to offspring,
providing instructions for how (and when) to make the many proteins needed to
build and maintain functioning cells, tissues, and organisms.

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Roles of DNA and RNA in Cells

The
nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are
macromolecules composed of monomers called nucleotides. Therefore, DNA and RNA
are polynucleotides. DNA carries the genetic blueprint for the cell while RNA
is the molecule that converts the blueprint or codes into defined amino acid
sequences in proteins. DNA is found in all living organisms, all the way from
single-celled bacteria to multicellular mammals. Viruses contain RNA and not
DNA as their genetic material, but they are not considered living organisms
because they lack the ability to reproduce without the help from the host.

From
DNA to RNA to proteins

            Many genes encode protein products,
meaning that they specify the sequence of amino acids used to build a
particular protein. Before this information can be used for protein synthesis,
however, an RNA copy (transcript) of the gene must first be made. This type of
RNA is called a messenger RNA (mRNA), as it serves as a
messenger between DNA and the ribosomes, molecular machines that read mRNA
sequences and use them to build proteins. This progression from DNA to RNA to
protein is called the “central
dogma” of molecular
biology.

            Importantly, not all genes encode
protein products. For instance, some genes specify ribosomal RNAs (rRNAs),
which serve as structural components of ribosomes, or transfer RNAs (tRNAs),
cloverleaf-shaped RNA molecules that bring amino acids to the ribosome for
protein synthesis. Still other RNA molecules, such as tiny microRNAs (miRNAs),
act as regulators of other genes, and new types of non-protein-coding RNAs are
being discovered all the time.

Nucleotides

            DNA and RNA are polymers (in the
case of DNA, often very long polymers), and are made up of monomers known as nucleotides. When these monomers
combine, the resulting chain is called a polynucleotide
(poly- = “many”).

            Each nucleotide is made up of three
parts: a nitrogen-containing ring structure called a nitrogenous base, a
five-carbon sugar, and at least one phosphate group. The sugar molecule has a
central position in the nucleotide, with the base attached to one of its
carbons and the phosphate group (or groups) attached to another. Let’s look at
each part of a nucleotide in turn.

Nitrogenous bases

            The nitrogenous bases of nucleotides
are organic (carbon-based) molecules made up of nitrogen-containing ring
structures.

            Each nucleotide in DNA contains one
of four possible nitrogenous bases: adenine (A), guanine (G) cytosine (C), and
thymine (T). Adenine and guanine are purines,
meaning that their structures contain two fused carbon-nitrogen rings. Cytosine
and thymine, in contrast, are pyrimidines
and have a single carbon-nitrogen ring. RNA nucleotides may also bear adenine,
guanine and cytosine bases, but instead of thymine they have another pyrimidine
base called uracil (U). As shown in the figure above, each base has a unique
structure, with its own set of functional groups attached to the ring structure.

            In molecular biology shorthand, the
nitrogenous bases are often just referred to by their one-letter symbols, A, T,
G, C, and U. DNA contains A, T, G, and C, while RNA contains A, U, G, and C (that
is, U is swapped in for T)