2. DNA
• DNA stands for:Deoxyribonucleic acid
• DNA is a molecule that encodes the genetic
instructions used in the development and
functioning 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.
3. • Most DNA molecules consist of two biopolymer
strands coiled around each other to form a double
helix
4. • The two DNA strands are known as polynucleotides since they are
composed of simpler units called nucleotides. Each nucleotide is
composed of a nitrogen-containing nucleobase—either guanine (G),
adenine (A), thymine (T), or cytosine (C)—as well as a monosaccharide
sugar called deoxyribose and a phosphate group.
• The nucleotides are joined to one another in a chain by covalent bonds
between the sugar of one nucleotide and the phosphate of the next,
resulting in an alternating sugar-phosphate backbone. According to base
pairing rules (A with T and C with G), hydrogen bonds bind the
nitrogenous bases of the two separate polynucleotide strands to make
double-stranded DNA.
5. RNA
• RNA stands for:Ribonucleic acid
• RNAis a ubiquitous family of large biological
molecules that perform multiple vital roles in
the coding, decoding, regulation, and
expression of genes.
• Together with DNA, RNA comprises the
nucleic acids, which, along with proteins,
constitute the three major macromolecules
essential for all known forms of life.
6. • DNA, RNA is assembled as a chain of nucleotides, but is usually single-
stranded. Cellular organisms use messenger RNA (mRNA) to convey
genetic information (often notated using the letters G, A, U, and C for the
nucleotides guanine, adenine, uracil and cytosine) that directs synthesis of
specific proteins, while many viruses encode their genetic information
using an RNA genome.
• Some RNA molecules play an active role within 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
8. DNA&RNA replication
• Before a cell divides it has to replicate its DNA so that the daughter cell receives a copy of the genome. The DNA helix
consists of two complementary DNA strands. Therefore, each of the two strands serves as a template for the construction
of the other strand. Under normal conditions the DNA is packed into a compact structure called chromatin. To be able to
replicate, the cell has to unfold and unwind the DNA, and also has to separate the two strands from each other. The cell
has a complex machinery to perform these tasks. When it is time to replicate, special initiator proteins attach to the DNA
at regions called replication origins. These regions are characterised by a weak bond between the two DNA strands. There
are around 10,000 replication origins on the DNA in a cell; this arrangement increases the rate of replication
tremendously. The initiator proteins pry the two strands apart and a small gap is created at the replication origin. Once
the strands are separated another group of proteins, that carry out the DNA replication, attaches and go to work.
9. • This group of proteins includes helicase, which serves as an unzipper by breaking the bonds
between the two DNA strands. This unzipping takes place in both directions from the
replication origins, creating a replication bubble. The replication is therefore said to be bi-
directional. Once the two strands are separated a small piece of RNA, called an RNA primer,
is attached to the DNA by an enzyme called DNA primase. These primers are the beginnings
of all new DNA chains since the enzyme responsible for the copying of the DNA, DNA
polymerase, can not start from scratch. It is a self-correcting enzyme and copies the DNA
template with remarkable fidelity.
• The DNA polymerase can only read in the 3' to 5' direction. This gives rise to some trouble
since the two strands of the DNA are antiparallel. On the upper strand which runs from 3'
to 5', nucleotide polymerisation can take place continuously without any problems. This
strand is called the leading strand. But how does the polymerase copy the other strand
then when it runs in the opposite direction, from 5' to 3'? On this so called lagging strand
the polymerase produces short DNA fragments, called okazaki fragments, by using a
backstitching technique. These lagging strand fragments are primed by short RNA primers
and are subsequently erased and replaced by DNA.
