Transcription: synthesis of RNA
Transcription starts with the process by which the genetic information is
transcribed onto a form of RNA, called mRNA. Ribonucleic acid, RNA, is
structurally similar to DNA with the exceptions that its nucleotides contain
ribose, instead of a 2
0 -deoxyribose, and the base thymine is replaced by
uracil. There are three major types of RNA depending on their specific
functions. However, all three types of RNA are much smaller than DNA and
they are single stranded, rather than double stranded.
(a) Messenger RNA (mRNA) carries genetic information from DNA to
ribosomes where protein synthesis occurs.
(b) Ribosomal RNA (rRNA), complexed with proteins (nucleoproteins),
provides the physical make up of ribosomes.
(c) Transfer RNA (tRNA) transports amino acids to the ribosomes for
protein synthesis.
Protein synthesis takes place in the cell nucleus with the synthesis of
mRNA. Part of the DNA double helix unwinds adequately to expose on a
single chain a portion corresponding to at least one gene. Ribonucleotides,
present in the cell nucleus, assemble along the exposed DNA chain by
pairing with the bases of DNA in a similar fashion that is observed in DNA
base pairing. However, in RNA uracil replaces thymine. The ribonucleotide
units of mRNA are joined into a chain by the enzyme RNA polymerase.
Once the mRNA is synthesized, it moves into the cytoplasm, where it acts as
a template for protein synthesis. Unlike what is seen in DNA replication,
where both strands are copied, only one of the two DNA strands is
transcribed into mRNA. The strand that contains the gene is called the
coding strand or sense strand. The strand that gets transcribed is known as
the template strand or antisense strand. As the template strand and the
coding strand are complementary, and as the template strand and the
RNA molecule are also complementary, the RNA molecule produced during
transcription is a copy of the coding strand, with the only exception that
the RNA molecule contains a U everywhere the DNA coding strand has
a T.
Ribosomes are small granular bodies scattered throughout the cytoplasm,
and this is the place where protein synthesis starts. rRNA itself does not
directly govern protein synthesis. A number of ribosomes get attached to a
chain of mRNA and form a polysome, along which, with mRNA acting as
the template, protein synthesis occurs. One of the major functions of rRNA
is to bind the ribosomes to the mRNA chain.
4.8 NUCLEIC ACIDS
177
Transcription starts with the process by which the genetic information is
transcribed onto a form of RNA, called mRNA. Ribonucleic acid, RNA, is
structurally similar to DNA with the exceptions that its nucleotides contain
ribose, instead of a 2
0 -deoxyribose, and the base thymine is replaced by
uracil. There are three major types of RNA depending on their specific
functions. However, all three types of RNA are much smaller than DNA and
they are single stranded, rather than double stranded.
(a) Messenger RNA (mRNA) carries genetic information from DNA to
ribosomes where protein synthesis occurs.
(b) Ribosomal RNA (rRNA), complexed with proteins (nucleoproteins),
provides the physical make up of ribosomes.
(c) Transfer RNA (tRNA) transports amino acids to the ribosomes for
protein synthesis.
Protein synthesis takes place in the cell nucleus with the synthesis of
mRNA. Part of the DNA double helix unwinds adequately to expose on a
single chain a portion corresponding to at least one gene. Ribonucleotides,
present in the cell nucleus, assemble along the exposed DNA chain by
pairing with the bases of DNA in a similar fashion that is observed in DNA
base pairing. However, in RNA uracil replaces thymine. The ribonucleotide
units of mRNA are joined into a chain by the enzyme RNA polymerase.
Once the mRNA is synthesized, it moves into the cytoplasm, where it acts as
a template for protein synthesis. Unlike what is seen in DNA replication,
where both strands are copied, only one of the two DNA strands is
transcribed into mRNA. The strand that contains the gene is called the
coding strand or sense strand. The strand that gets transcribed is known as
the template strand or antisense strand. As the template strand and the
coding strand are complementary, and as the template strand and the
RNA molecule are also complementary, the RNA molecule produced during
transcription is a copy of the coding strand, with the only exception that
the RNA molecule contains a U everywhere the DNA coding strand has
a T.
Ribosomes are small granular bodies scattered throughout the cytoplasm,
and this is the place where protein synthesis starts. rRNA itself does not
directly govern protein synthesis. A number of ribosomes get attached to a
chain of mRNA and form a polysome, along which, with mRNA acting as
the template, protein synthesis occurs. One of the major functions of rRNA
is to bind the ribosomes to the mRNA chain.
4.8 NUCLEIC ACIDS
177