Saturday, September 1, 2012

43. Cell-Biology Basics

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: