SCIENCE & TECHNOLOGY
BIOTECHNOLOGY- THE BASICS
BIOTECHNOLOGY: Biotechnology is technology that utilizes biological systems, living organisms or parts of this to develop or create different products that help to improve our lives and health of our planet.
Cell is the functional structural and fundamental unit of all living organisms. All organisms are composed of cells; some composed of single cell are called unicellular organisms say, amoeba, paramecium, etc. where others like humans, composed of many cells, are called multicellular organisms.
According to the cell theory, initially proposed by Schleiden and Schwann, later modified by Rudolf Virchow.
- All living organisms are composed of cells and products of cells.
- All cells arise from pre-existing cells, i.e. they are able to reproduce themselves.
The above said statements throw light on the two major features of cells-
- One, cells synthesizes different cell products (essentially proteins), that are required for the metabolic activities.
- Two, cells have definite life span during which they reproduce to form new cells of their own kind.
The cells based on their size, shape and the functions they perform. For instance, the nerve cell (longest in human body) contain branched filaments projecting outward to facilitate the transfer of signals between the brain and the target size (fig. 1). Mycoplasmas, the smallest cells having 0.3 m in length (fig.2).
Despite the diversity in the types of cells, the components of the cell remains the same, except a few differences between plant and animal cell.
COMPONENTS OF A CELL
A typical cell broadly consists of a matrix called cytoplasm, in which all other components are suspended and a nucleus. The cytoplasm is protected from the external environment by a semi- permeable membrane called cell membrane/ plasma membrane (plants have an additional membrane called cell wall). Based on whether the nucleus inside the cell is surrounded by a membrane or not, organisms are classified as-
- Prokaryotic– which lack cells with membrane bound nucleus, e.g. bacteria, blue- green algae, mycoplasma, etc.
- Eukaryotic– which possess cells with membrane bound nucleus, e.g. protists, fungi, all other multicellular organisms
Note: being a prokaryotic or a eukaryotic organism is not related to having single or multi cells; yeast is a single celled fungi which is a eukaryote.
The cytoplasm consists of several cell organelles suspended in it. The organelles are enlisted below:
- Endoplasmic reticulum (ER)
- Golgi apparatus
- Plastids (only in plants)
Inside a cell, each organelle performs a specific function to produce protein.
- The mitochondria provides the necessary energy required by the cell to carry out the function, in the form of ATP, and hence is called the power house of the cell.
- The main protein synthesizing site is the ribosome. The endoplasmic reticulum that possess ribosomes on their surface (called rough ER/ RER) hence, are actively involved in protein synthesis. On the other hand, smooth ER (SER) that do not have any ribosomes on their surface produce lipids and steroidal hormones.
- The synthesized products, are packed and modified (proteins to glycoproteins, lipids to glycolipids) in the Golgi bodies to be transported to other parts of the cells.
- The waste products, water, sap and other non-useful products are stored in the The lysosomes, called suicide bags contain powerful digestive enzymes to destroy any foreign matter that enters the cell.
- The plastids, present only in plants, contain pigments, either to trap light energy (chloroplasts), impart red, yellow or orange colour (chromoplasts) or to store starch (leucoplasts).
Fig 3. Plant cell
Fig 4. Animal cell
If the cell is the factory to manufacture products, then the nucleus is the controlling unit where all the information about the processes are stored. It is stored in the form of codes in specific gene segments. It also functions as the producer of ribosomal RNA.
The nucleus also is a cell organelle and has the following components:
- Nuclear membrane
- The nuclear matrix called nucleoplasm, bounded by a nuclear membrane (in eukaryotes) contain spherical structures called nucleoli. They are not membrane bound and are the sites of ribosomal RNA (RNA present in ribosomes) synthesis.
- The nucleoplasm also contains chromatin, which bears the most important unit of a cell- the genetic material- DNA, along with basic proteins called histones, non- histone proteins and
- Chromatin is an undifferentiated and loose network of nucleoprotein, which prior to cell division, transforms into chromosomes.
- The chromosome bears the DNA and is the carrier of genetic information. It occurs in pairs in organisms with sexual reproduction- one obtained maternally and the other paternally.
- In humans, there are 23 pairs of chromosomes, out of which 22 pairs are autosomes and the remaining 1 pair is called sex chromosomes.
Fig 5. Nucleus
CELL DIVISION & CELL CYCLE
Since cells are living entities, they have definite life spans and have the ability to reproduce. In a cell, the following processes take place:
There are two modes of cell divisions- mitosis and meiosis.
Mitosis is the simple duplication of a cell and all of its parts. It duplicates its DNA and the two new cells (daughter cells) have the same pieces and genetic code, i.e. two identical copies come from one original.
There are five basic phases in mitosis:
Meiosis results in cells that only have half the usual number of chromosomes, one from each pair. In the long run, meiosis increases genetic variation.
Meiosis takes place only in the gametes, or sex cells. The eggs and sperm are the gametes in human beings. Gametes are haploid; they have only half the number of chromosomes as a normal body cell (called a somatic cell). Fertilization restores the chromosomes in body cells to the diploid number.
THE GENETIC MATERIAL- DNA
Consider ourselves, no two humans are ever alike except the twins. However, some features such as the complexion or height are shared among the family members, Why is it that the organisms express variation in some characteristic traits but be alike in some other? What is the thing that is passed on through generations from parents to off springs, which is the basis of inheritance? The thing is called the genetic material.
There are two genetic materials present in the living system-
- Ribonucleic acid (RNA)– genetic material mainly in viruses
- Deoxyribonucleic acid (DNA)– genetic material in all other life forms
DNA contains all the information about how an individual must function, his features and traits. The information is coded in the genes of the DNA; each gene codes for a particular function which in turn enable the overall functioning of the individual.
WHERE IS DNA LOCATED?
Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).
Fig 6. Position of DNA in a cell
FUNCTIONS OF DNA
DNA contains the instructions to command the cells what proteins are to be made. Mostly, these proteins are enzymes. DNA is inherited by children from their parents. This is why children share traits with their parents, such as skin, hair and eye colour.
During meiosis of gametes, only half the number of chromosomes (23 in humans) are produced in the new cells, of both the egg and the sperm. During fertilisation, half the chromosome of the egg (23) and the other half chromosome of the sperm (23) merge together to form 23 pairs (46 in total). Hence the newly formed embryo (i.e. the offspring) contains the DNA of the mother as well as the father, and hence acquire the characteristics of both. Hence, variation is achieved.
- A portion of an organism’s DNA is “non-coding DNA” sequences, i.e. they do not code for protein sequences.
- The amount of non-coding DNA varies greatly among species. For example, over 98% of the human genome is non-coding DNA, while only about 2% of a typical bacterial genome is non-coding DNA.
STRUCTURE OF DNA
DNA has a double helix shape, as proposed by Watson & Crick which is like a ladder twisted into a spiral. Each step of the ladder is a pair of nucleotides.
A nucleotide is a molecule made up of:
- deoxyribose, a kind of sugar with 5 carbon atoms
- a phosphate group made of phosphorus and oxygen
- nitrogenous base
The sugar- phosphate complex forms the backbone whereas the bases project inside (Fig 7.). Several such nucleotides joined together forms a polynucleotide (Fig 8.). Two strands of such a polynucleotide forms the double stranded DNA.
DNA is made of four types of nucleotide:
- Adenine (A)
- Thymine (T)
- Cytosine (C)
- Guanine (G)
A fifth nucleotide called Uracil (U) is present in RNA at the place of thymine. Adenine and guanine comes under purines while thymine, uracil and cytosine are pyrimidines. Note that only a purine bonds with another pyrimidine.
- The ‘rungs’ of the DNA ladder are each made of two bases, one base coming from each leg.
- The bases connect in the middle: ‘A’ only pairs with ‘T’, and ‘C’ only pairs with ‘G’. The bases are held together by hydrogen bonds.
- Adenine (A) and thymine (T) can pair up because they make two hydrogen bonds, and cytosine (C) and guanine (G) pair up to make three hydrogen bonds.
- Although the bases are always in fixed pairs, the pairs can come in any order (A-T or T-A; similarly, C-G or G-C).
- This way, DNA can write ‘codes’ out of the ‘letters’ that are the bases. These codes contain the message that tells the cell what to do.
Fig 8. Double helical structure of DNA
When DNA is copied this is called DNA replication. Briefly, the hydrogen bonds holding together paired bases are broken and the molecule is split in half: the legs of the ladder are separated. This gives two single strands. New strands are formed by matching the bases (A with T and G with C) to make the missing strands.
- First, an enzyme called DNA helicase splits the DNA down the middle by breaking the hydrogen bonds.
- Then after the DNA molecule is in two separate pieces, another molecule called DNA polymerase makes a new strand that matches each of the strands of the split DNA molecule.
- Each copy of a DNA molecule is made of half of the original (starting) molecule and half of new bases.
When DNA is copied, mistakes are sometimes made – these are called mutations. There are four main types of mutations:
- Deletion: where one or more bases are left out.
- Substitution: where one or more bases are substituted for another base in the sequence.
- Insertion: where one or more extra base is put in.
- Duplication: where a sequence of bases pairs are repeated.
- Mutations may be bad for the organism, or neutral, or of benefit. Sometimes mutations are fatal for the organism – the protein made by the new DNA does not work at all, and this causes the embryo to die.
- Evolution is moved forward by mutations, when the new version of the protein works better for the organism.
A section of DNA that contains instructions to make a protein is called a gene. Each gene has the sequence for at least one polypeptide. Proteins form structures, and also form enzymes. The enzymes do most of the work in cells. Proteins are made out of smaller polypeptides, which are formed of amino acids. To make a protein to do a particular job, the correct amino acids have to be joined up in the correct order.
Following are the steps involved in protein synthesis:
- Proteins are made by tiny machines in the cell called ribosomes.
- Ribosomes are present in cytoplasm, while the DNA is in the nucleus.
- Because DNA cannot leave the nucleus, the cell makes a copy of the DNA sequence in RNA.
- This is smaller and can get through the holes – pores – in the membrane of the nucleus and out into the cell.
- Genes encoded in DNA are transcribed into messenger RNA (mRNA) by proteins such as RNA polymerase.
- Mature mRNA is then used as a template for protein synthesis by the ribosome.
- Ribosomes read codons, ‘words’ made of three base pairs that tell the ribosome which amino acid to add.
- The ribosome scans along an mRNA, reading the code while it makes protein. Another RNA called tRNA or transfer RNA helps match the right amino acid to each codon.
THE CENTRAL DOGMA OF MOLECULAR BIOLOGY
It was a concept proposed by Crick regarding the flow of genetic information. As mentioned above, the simple line of flow is represented as:
It can be stated in a very short and oversimplified manner as “DNA makes RNA makes proteins, which in turn facilitate the previous two steps as well as the replication of DNA“, or simply “DNA → RNA → protein”. This process is therefore broken down into three steps: transcription, translation, and replication.
- Transcription is the process by which the information contained in a section of DNA is transferred to a newly assembled piece of messenger RNA (mRNA).
- It is facilitated by RNA polymerase and transcription factors.
- The sum total of all the messenger RNA molecules expressed from the genes of an organism is called transcriptome.
- Mature mRNA finds its way to a ribosome, where it is translated to synthesize protein.
- In prokaryotic cells, which have no nuclear compartment, the process of transcription and translation occur together while in eukaryotic cells, the site of transcription (the nucleus) is usually separated from the site of translation (the cytoplasm), so the mRNA must be transported out of the nucleus into the cytoplasm, where it can be bound by ribosomes.
- The process of transmitting the genetic information between parents and progeny
- Replication is carried out by a complex group of proteins that unwind the double-stranded DNA helix, and, using DNA polymerase and its associated proteins, copy or replicate the master template itself so the cycle is repeated.
EXCEPTIONS TO THE CENTRAL DOGMA
The central dogma is not really a dogma in the traditional sense of the word, like all scientific theories it is modified as we learn more details of the processes.
- The biggest revolution in the central dogma was the discovery of retroviruses, which transcribe RNA into DNA through the use of a special enzyme called reverse transcriptase has resulted in an exception to the central dogma
- Also, some virus species are so primitive that they use only RNA → proteins, having not developed DNA.
- With the discovery of prions, a new exception was discovered; Protein → Protein, i.e. proteins directly replicating themselves by making conformational changes in other proteins.
PREVIOUS YEARS QUESTIONS
- Consider the statement about probiotic food (2008)
- Probiotic food contains live bacteria which are considered beneficial to humans
- Probiotic food helps in maintaining gut flora
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither1 nor 2
- Assertion (A): cellulose is used in making shatter proof glass
Reason(R): polysaccharides are not soluble in water (2006)
- Both A and R are true and R is the correct explanation of A
- Both A and R are true and R is not the correct explanation of A
- A is true but R is false
- A is false but R is true
- Which organelle in the cell other than nucleus has a DNA? (2001)
- Golgi apparatus
- Which of the following features of DNA makes uniquely suited to store and transmit genetic information from generations to generations? (2001)
(a) Complimentary of two strands
(b) Double helix
(c) Number of base pair per turn
(d) Sugar-phosphate backbone
- 5. Which of the following cell organelles play the most significant role in protein synthesis? (2001)
(a) Lysosome and Centrosome
(b) ER and Ribosome
(c) Golgi apparatus and Mitochondra
(d) Lysosome and Mitochondria