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Structure & function of DNA & RNA
DNA, d
eoxyribon
ucleic a
cid, is the genetic material that organisms inherit from their parents. It is a long
molecule called a nucleic acid which is a polymer made up of many smaller units or
monomers. These monomers, the repeating units making up the nucleic acid, are called
nucleotides
. Each nucleotide, is made up of three parts: a phosphate group,
a nitrogen base
, and a five carbon sugar
. This sugar is called deoxyribose.
There are four different nucleotide monomers which make up DNA: adenine, guanine , (the "purines"), and thymine, & cytosine (the "pyrimidines").
The nucleotides differ from each other only by small changes in the structure of
their nitrogen base. Each base is made up of from 12 to 16 C, H, N, and 0 atoms.
DNA is made up of from one million to several billion nucleotides. One DNA strand is composed of alternating sugar and phosphate groups
which form strong covalent bonds (phosphodiester bonds); the nitrogen bases project perpendicular to the
DNA strand (actually from the 5C sugar) where they protrude and can readily form
weak hydrogen bonds
to other nitrogen bases on the other strand.
However, the structure of each nucleotide allows it to hydrogen bond to only one
other nucleotide. For example, adenine only bonds to thymine (2 hydrogen bonds);
guanine only bonds to cytosine (3 hydrogen bonds).
These are called complementary base pairs
.
It is this bonding of complementary bases which spontaneously produces the double helix
structure of DNA. Each molecule of DNA is thus actually TWO of the above strands
wound around each other with their pairs of bases facing inward, rather like a twisted
ladder. One strand is "upside down" to the other.
DNA has two functions: 1) it reproduces itself (replication) and, as each cell divides,
passes genetic information along to the next generation; 2) it acts as a template
upon which RNA is assembled using suitable enzymes, (transcription
).
During the process of DNA replication, the surface architecture formed by DNA nucleotides
acts to dock enzymes such as DNA helicase and DNA polymerase. These then proceed
to build a second double helix on templates formed by the original DNA strands. In
this way each daughter cell resulting from a cell division ends up with a complete set
of genes; in this manner the genetic heritage of the new cell is transferred from
its parent cell.
During the process of DNA transcription, a different set of enzymes, including RNA
polymerase, function to assemble a different nucleic acid from the DNA template.
RNA, which will be used to transfer information from where it is stored in DNA to
where it is used in the cytoplasm of the cell, is also formed on DNA templates.
It is the set of unique surface features formed by the sequence and kind of nucleotides
which determine which enzymes will attach to the DNA.
Because DNA functions as a library of blueprints or recipes for making RNA, its life-span
is usually the cell's life-span. RNA codes for the different proteins. You are different
from your parents or a slug because you made different proteins whilst in the uterus!
RNA
RNA, ribonucleic acid, is a smaller single stranded
polymer made up of similar nucleotide monomers to DNA. RNA nucleotides are made up
of a phosphate group, a five-carbon sugar (ribose
), and a nitrogen base. There are four different nucleotides, each differing by small
differences in the structure of the nitrogen base. RNA nucleotides include the bases
adenine, guanine, cytosine, and uracil. Uracil and thymine
both base-pair with adenine, but only uracil
is found in RNA. In other words, Uracil replaces the Thymine in DNA . A small difference
in their structure (uracil lacks a methyl group - CH3), defines their function in
the cell.
RNA is made up of from several hundred to 20 000 nucleotides. The average eukaryotic
RNA is about 6 000 nucleotides long. The RNA strand is made up of alternating 5C
sugar and phosphate groups which form strong covalent bonds; the nitrogen bases project
perpendicular to the RNA strand where they can readily form weak hydrogen bonds to other
nitrogen bases. RNA can form base pairs with DNA: guanine bonds to cytosine and adenine
bonds to uracil.
RNA can also base pair with ( a single strand of) DNA.
RNA is made on DNA templates in a process called DNA transcription.
The assembly of RNA requires the enzyme RNA polymerase.
RNA has three cellular functions: 1) it acts as a messenger molecule (messenger RNA),
carrying genetic information between DNA in the nucleus and ribosomes in the cytoplasm.
Here it functions as a blueprint for the synthesis of proteins. 2) It functions to deliver amino acids to ribosomes (transfer RNA), where they are then assembled into
polypeptides. 3) It acts as the primary structural material in the formation of ribosomes
(ribosomal RNA).
Messenger RNA is found in the nucleus and
in the cytoplasm. Its life-span, measured in minutes and hours, is relatively short;
tRNA and mRNA are found only in the cytoplasm
TASKS:
1. DRAW part of a DNA & RNA molecule to show their structure. Label
2. Compare and contrast DNA & RNA . (A "graphical organiser" will be used to help
you).
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