Download Summary Ribonucleic acid (RNA) is one of the three major

Summary
Ribonucleic acid (RNA) is one of the three
major macromolecules (along with DNA and proteins)
that are essential for all known forms of life. Like
DNA, RNA is made up of a long chain of components
called nucleotides. Each nucleotide consists of a
nucleobase (sometimes called a nitrogenous base), a
ribose sugar, and a phosphate group. The sequence of
nucleotides allows RNA to encode genetic information.
For example, some viruses use RNA instead of DNA as
their genetic material, and all organisms use
messenger RNA (mRNA) to carry the genetic
information that directs the synthesis of proteins. Like
proteins, some RNA molecules play an active role in
cells by catalyzing biological reactions, controlling
gene expression, or sensing and communicating
responses to cellular signals. One of these active
processes is protein synthesis, a universal function
whereby mRNA molecules direct the assembly of
proteins on ribosomes. This process uses transfer RNA
(tRNA) molecules to deliver amino acids to the
ribosome, where ribosomal RNA (rRNA) links amino
acids together to form proteins. The chemical
structure of RNA is very similar to that of DNA, with
two differences--(a) RNA contains the sugar ribose
while DNA contains the slightly different sugar
deoxyribose (a type of ribose that lacks one oxygen
atom), and (b) RNA has the nucleobase uracil while
DNA contains thymine (uracil and thymine have
similar base-pairing properties). Unlike DNA, most
RNA molecules are single-stranded. Single-stranded
RNA molecules adopt very complex three-dimensional
structures, since they are not restricted to the
repetitive double-helical form of double-stranded DNA.
RNA is synthesised within living cells by RNA
polymerases, enzymes which act to copy a DNA or
RNA template into a new RNA strand, through
processes known as transcription or RNA replication,
respectively.
Ribosomes are cytoplasmic granules composed of
RNA and protein, at which protein synthesis takes
place. They were first observed by Palade in the
electron microscope as dense particles or granules.
Upon isolation, they were shown to contain
approximately equal amounts of RNA and protein.
There are two types of ribosomes in the living
organisms, the eukaryotic (80s) and prokaryotic (70s)
ribosomes. The subunits of 80s ribosomes are 60s and
40s, whereas 70s have 50s and 30s subunits.
Prokaryotic and eukaryotic ribosomes do not differ in
any fundamental way; both perform the same
functions by the same set of chemical reactions. The
genetic code is the same in all living organism, and it
has been demonstrated that eukaryotic ribosomes are
able to translate bacterial mRNAs correctly. Eukaryotic
ribosomes are, however, much larger than that of
prokaryotic ones and most of their proteins are
different. Antibiotics, such as chloramphenicol inhibit
bacterial, but not eukaryotic ribosomes. Protein
synthesis by eukaryotic ribosomes in inhibited by
cycloheximide.
Mitochondrial
and
chloroplast
ribosomes resemble those of bacteria.