All tissues in
animals and plants are made up of cells, and all cells come from other cells. Generally,
a cell may be either a prokaryote or
an eukaryote. The former is an
organism that has neither a distinct 'nucleus', nor other specialized subunits
or organelles. Examples include
bacteria and blue-green algae.
Unicellular
organisms like yeast are eukaryotes. Such cells are generally separated from
the environment by a semi-permeable membrane. Inside the membrane there is a nucleus and the cytoplasm surrounding the nucleus. Multicellular organisms are all
made up of eukaryote-type cells. In them the cells are highly specialized
(we call it 'cell differentiation'), and perform the function of the organ to
which they belong.
The nucleus
contains nucleic acids, among other
things. With the exception of viruses, two types of nucleic acids are found in
all cells: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). Viruses have
either RNA or DNA, but not both (but then viruses are not cells). Apart from
having a nucleus, an eukaryotic cell has mitochondria,
ribosomes, and vacuoles. Plant cells also have chloroplasts.
Mitochondria
make energy out of food. Ribosomes make proteins. Vacuoles are used for storage
of water or food. Chloroplasts use sunlight to create food by photosynthesis.
DNA is a very long molecule that has the genetic information encoded in
it as a sequence of four different molecules called nucleotides:
adenine (A), thymine (T), guanine (G), and cytosine (C). There is a double
backbone of phosphate and sugar molecules, each carrying a sequence of the
‘bases’ A, T, G, C. This backbone is coiled into a double helix, like a twisted
ladder.
In this double-helix structure, the base nucleotide A bonds almost always
to base nucleotide T (via a weak hydrogen bond), and G bonds to C. The sequence
of base pairs along the backbone defines the primary structure of a DNA molecule.
The chemical composition
of RNA is quite similar to that of DNA, except that it has the base uracil (U) instead
of thymine (T).
DNA contains
the codes for manufacturing the various proteins. All the proteins in our body
are chain-like structures made up from just 20-odd amino acids.
Depending on the sequence of the amino acids (this sequence defines the primary
structure of the protein), the chain of molecules 'folds' in a specific way
which defines the secondary structure of the protein.
Production of
a protein in the cell nucleus involves transcription
of a stretch of DNA (this stretch is called a gene) into a portable form, namely the messenger RNA (or mRNA). The m-RNA is then translated into
the corresponding protein: The mRNA molecule travels to the cytoplasm of the
cell, where the information is conveyed to the ribosome. This is where the
encoded instructions are used for the synthesis of the protein. The code is
read, and the corresponding amino acid is brought into the ribosome. Each amino
acid comes connected to a specific transfer
RNA (tRNA) molecule; i.e. each tRNA carries a specific amino acid. There is
a three-letter recognition site on the tRNA that is complementary to, and pairs
with, the three-letter code sequence for that amino acid on the mRNA.
Such one-way flow of information from DNA to
RNA to protein is the basis of all life on Earth. This is THE CENTRAL DOGMA OF MOLECULAR BIOLOGY. It
insists that information cannot flow in the reverse direction, i.e. from
protein to RNA to DNA, or from protein to DNA.
Three letters
(out of the four, namely the bases A, T, C, G) are needed to code the synthesis
of any particular protein. The term codon
is used for the three consecutive letters on an mRNA. The possible number of
codons is 64, and only 20 amino acids are processed by these codons. So there
is redundancy: The linking of most of the amino-acid-triplets for synthesizing
a protein can be coded by more than one codon.
There are
~60-100 trillion human cells in the human body. In this multicellular organism
(as also in any other multicellular organism), almost every cell (red blood
‘cells’ are an exception) has the same DNA, with exactly the same primary
structure. The nucleus contains 95% of the DNA, and is the control centre of
the cell. The DNA inside the nucleus is complexed with proteins to form a
structure called chromatin.
The fertilized
mother cell (the zygote) divides (self-replicates; see below) into two cells. Each of these again divides into
two cells, and so on. Before this cell division (mitosis) begins, the chromatin condenses into elongated structures
called chromosomes. A gene is a functional unit on a
chromosome, which directs the synthesis of a particular protein. Humans have 23
pairs of chromosomes. Each pair has two non-identical chromosomes, derived one
from each parent.
During cell
division, the double-stranded DNA splits into the two component strands, each
of which acts as a replication template
for the construction of the complementary strand. ‘Complementary strand’ means
that for every A on the original template these is a T on the new strand;
similarly, there is a C for every G, A for T, and G for C. At every stage, the two daughter cells are of
identical genetic composition (they have identical genomes). In each of the 60 trillion cells in the human body, the
genome consists of around three billion nucleotides.
Watch this video for an entertaining lesson on biology:
Sir, could you elaborate a bit more on this point?
ReplyDelete//Such one-way flow of information from DNA to RNA to protein is the basis of all life on Earth. This is THE CENTRAL DOGMA OF MOLECULAR BIOLOGY. It insists that information cannot flow in ...the reverse direction, i.e. from protein to RNA to DNA, or from protein to DNA.//
Also, I recall reading somewhere, that whenever a new type of learning takes place, its conveyed to the DNA via RNA. Could you affirm this?
This question will be discussed in detail in future posts. I shall give a short answer here. It has to do with Darwinian evolution.
DeleteCharacteristics acquired during a lifetime, e.g. strong and large biceps, or a six-pack abdomen, have to do only with changes which occur in muscle and bone structure, etc. In particular, they are changes in the proteins, but not in the DNA which is inside the nuclei of the cells. Therefore, such acquired characteristics are not passed on to the progeny on procreation. That is why we say that information does not flow from proteins to DNA.
And something different happens during Darwinian evolution, spread over a very large number of generations of the population. Any feature or characteristic that is conducive to the survival and propagation of the species has a greater chance of getting 'selected' in succeeding generations, but this information gets into the genes (the DNA, or the 'genotype') and not directly into the proteins (the body, or the 'phenotype'). The improved genome or DNA manifests itself by directing the synthesis of more suitable proteins. Information flows from DNA to proteins, but not vice versa.