Nucleic acids are biopolymers, or large biomolecules, essential for

all known forms of life. Nucleic acids, which

include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are
made from monomers known as nucleotides. Each nucleotide has
three components: a 5-carbon sugar, a phosphate group, and
a nitrogenous base. If the sugar is deoxyribose, the polymer is DNA.
If the sugar is ribose, the polymer is RNA. When all three
components are combined, they form a nucleotide. Nucleotides are
also known as phosphate nucleotides.
Nucleic acids are among the most important biological
macromolecules (others being amino acids/proteins,
sugars/carbohydrates, and lipids/fats). They are found in abundance
in all living things, where they function in encoding, transmitting and
expressing genetic informationin other words, information is
conveyed through the nucleic acid sequence, or the order of
nucleotides within a DNA or RNA molecule. Strings of nucleotides
strung together in a specific sequence are the mechanism for storing
and transmitting hereditary, or genetic information via protein
synthesis.
Proteins are large biomolecules, or macromolecules, consisting of
one or more long chains of amino acid residues. Proteins perform a
vast array of functions within living organisms, including catalyzing
metabolic reactions, DNA replication, responding to stimuli, and
transporting molecules from one location to another. Proteins differ
from one another primarily in their sequence of amino acids, which is
dictated by the nucleotide sequence of their genes, and which usually
results in protein folding into a specific three-dimensional
structure that determines its activity.
A linear chain of amino acid residues is called a polypeptide. A
protein contains at least one long polypeptide. Short polypeptides,
containing less than 20-30 residues, are rarely considered to be

proteins and are commonly called peptides, or

sometimes oligopeptides. The individual amino acid residues are
bonded together by peptide bonds and adjacent amino acid residues.
The sequence of amino acid residues in a protein is defined by
the sequence of a gene, which is encoded in the genetic code. In
general, the genetic code specifies 20 standard amino acids;
however, in certain organisms the genetic code can
include selenocysteine andin certain archaeapyrrolysine. Shortly
after or even during synthesis, the residues in a protein are often
chemically modified by posttranslational modification, which alters the
physical and chemical properties, folding, stability, activity, and
ultimately, the function of the proteins. Sometimes proteins have nonpeptide groups attached, which can be called prosthetic
groups or cofactors. Proteins can also work together to achieve a
particular function, and they often associate to form stable protein
complexes.
DNA vs RNA: DNA is a molecule that carries most of
the genetic instructions used in the development, functioning
and reproduction of all known living organisms and many viruses.
DNA is a nucleic acid; alongside proteins and carbohydrates, nucleic
acids compose the three major macromolecules essential for all
known forms of life. Most DNA molecules consist of
two biopolymer strands coiled around each other to form a double
helix.
Ribonucleic acid (RNA) is a polymeric molecule implicated in
various biological roles in coding, decoding, regulation,
and expression of genes. RNA and DNA are nucleic acids, and, along
with proteins and carbohydrates, constitute the three
major macromolecules essential for all known forms of life. Like DNA,
RNA is assembled as a chain of nucleotides, but unlike DNA it is
more often found in nature as a single-strand folded onto itself, rather

than a paired double-strand. Cellular organisms use messenger

RNA (mRNA) to convey genetic information (using the letters G, U, A,
and C to denote the nitrogenous bases guanine, uracil, adenine,
and cytosine) that directs synthesis of specific proteins.
Many viruses encode their genetic information using an
RNA genome.
Gene expression is the process by which information from a gene is
used in the synthesis of a functional gene product. These products
are often proteins, but in non-protein coding genes such as transfer
RNA (tRNA) or small nuclear RNA (snRNA) genes, the product is a
functional RNA. The process of gene expression is used by all known
life - eukaryotes (including multicellular
organisms), prokaryotes (bacteria and archaea), and utilized
by viruses - to generate the macromolecular machinery for life.
Several steps in the gene expression process may be modulated,
including the transcription, RNA splicing, translation, and posttranslational modification of a protein. Gene regulation gives
the cell control over structure and function, and is the basis
for cellular differentiation, morphogenesis and the versatility
and adaptability of any organism. Gene regulation may also serve as
a substrate for evolutionary change, since control of the timing,
location, and amount of gene expression can have a profound effect
on the functions (actions) of the gene in a cell or in a multicellular
organism.
Step 1: Transcription is the first step of gene expression, in which a
particular segment of DNA is copied into RNA (mRNA) by
the enzyme RNA polymerase.
Both RNA and DNA are nucleic acids, which use base
pairs of nucleotides as a complementary language. The two can be
converted back and forth from DNA to RNA by the action of the
correct enzymes. During transcription, a DNA sequence is read by

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an RNA polymerase, which produces a

complementary, antiparallel RNA strand called a primary transcript.
Transcription proceeds in the following general steps:
One or more sigma factor protein binds to the RNA
polymerase holoenzyme, allowing it to bind to promoter DNA.
RNA polymerase creates a transcription bubble, which separates the
two strands of the DNA helix. This is done by breaking the hydrogen
bonds between complementary DNA nucleotides.
RNA polymerase adds matching RNA nucleotides to the
complementary nucleotides of one DNA strand.
RNA sugar-phosphate backbone forms with assistance from RNA
polymerase to form an RNA strand.
Hydrogen bonds of the untwisted RNA-DNA helix break, freeing the
newly synthesized RNA strand.
If the cell has a nucleus, the RNA may be further processed. This
may include polyadenylation, capping, and splicing.
The RNA may remain in the nucleus or exit to the cytoplasm through
the nuclear pore complex.
Step 2: splicing is a modification of the nascent pre-messenger RNA
(pre-mRNA) transcript in which introns are removed and exons are
joined. For nuclear-encoded genes, splicing takes place within the
nucleus after or concurrently with transcription. Splicing is needed for
the typical eukaryotic messenger RNA (mRNA) before it can be used
to produce a correct protein through translation. For many eukaryotic
introns, splicing is done in a series of reactions which
are catalyzed by the spliceosome, a complex of small nuclear
ribonucleoproteins (snRNPs), but there are also self-splicing introns.
Step 3: translation is the process in
which cellular ribosomes create proteins.
In translation, messenger RNA (mRNA)produced

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by transcription from DNAis decoded by a ribosome to produce a

specific amino acid chain, or polypeptide. The polypeptide
later folds into an activeprotein and performs its functions in
the cell. The ribosome facilitates decoding by inducing the binding
of complementary tRNA anticodon sequences to mRNA codons. The
tRNAs carry specific amino acids that are chained together into a
polypeptide as the mRNA passes through and is "read" by the
ribosome. The entire process is a part of gene expression.
In brief, translation proceeds in four phases:
Initiation: The ribosome assembles around the target mRNA. The
first tRNA is attached at the start codon.
Elongation: The tRNA transfers an amino acid to the tRNA
corresponding to the next codon.
Translocation: The ribosome then moves (translocates) to the next
mRNA codon to continue the process, creating an amino acid chain.
Termination: When a stop codon is reached, the ribosome releases
the polypeptide.
Step 4: Post-translational modification (PTM) refers to
the covalent and generally enzymatic modification of proteins during
or after protein biosynthesis. Proteins are synthesized
by ribosomes translating mRNA into polypeptide chains, which may
then undergo PTM to form the mature protein product. PTMs are
important components in cell signaling.
Post-translational modifications can occur on the amino acid side
chains or at the protein's C- or N- termini.[1] They can extend the
chemical repertoire of the 20 standard amino acids by introducing
new functional groupssuch as phosphate, acetate, amide groups,
or methyl groups. Phosphorylation is a very common mechanism for
regulating the activity of enzymes and is the most common posttranslational modification.[2] Many eukaryotic proteins also
have carbohydrate molecules attached to them in a process

called glycosylation, which can promote protein folding and improve

stability as well as serving regulatory functions. Attachment
of lipidmolecules, known as lipidation, often targets a protein or part
of a protein to the cell membrane.