1. HIV belongs to the group of viruses called retroviruses.
2. It is spherical in shape and it measures about 100 to 140 nanometer.
3. It contains the genetic material double stranded RNA surrounded by a protein envelope. 4. HIV causes AIDS in man.
Structure of bacterium
1. There is no well defined nucleus, but the genetic material DNA is condensed in the central region of the cell. Hence it is a prokaryote.
2. It possesses cell wall which lies adjacent and external to the plasma membrane.
3. Ribosome exists in groups called polyribosome or polysomes. Each ribosomes consists of a large and a small subunit.
4. A long thin flagellum is seen. It is useful for locomotion.
Structure of a chloroplast
1. The chloroplast is the photosynthetic organelle of green plants and it is surrounded by outer and inner membrane.
2. Within the inner membrane there is a liquid matrix called stoma which contains circular DNA, ribosome and enzymes for photosynthesis.
3. Since the chloroplast has ribosome and DNA, it is known as semiautonomous organelle.
4. In the stroma, there is an extensive system of double membranous structures called thylokoids arranged in patches called grana. The grana are disc shaped.
Structure of a mitochondrion
1. The mitochondria are rod shaped and bounded by outer and inner membranes. 2. The inner membrane has numerous infoldings called cristea which are present in mitochondrial matrix. 3. The liquid mitochondrial matrix contains circular DNA, ribosome, lipids, proteins, and enzymes to catalyst the reactions of Krebs cycle. 4. Since the mitochondria have DNA and ribosomes, they are called as semiautonomous organelle.
Mitosis – metaphase
1. Nuclear membranes disappear and the spindle fibers appear at opposite poles in the cytoplasm 2. The chromosomes orient themselves in the equator of the spindle 3. The spindle fibers arising from the opposite poles are seen attached to the centromeres of the chromosomes.
Mitosis – Telophase
1. The chromosomes reach the opposite poles and gather together to form nucleus.
2. The nuclear membrane and nucleolus reappear.
3. The spindle fibres gradually disappear.
4. The newly formed nuclei contain the same number of chromosomes.
TAP ROOT SYSTEM
1. The primary root, secondary roots, tertiary roots and the fine rootlets are seen.
2. The secondary lateral roots are extensively branched.
3. Nodes, internodes, leaves and buds are not seen.
FIBROUS ROOT SYSTEM
1. A Tift of thread like and of equal size roots arise from the common origin i.e., from the lower most node of the stem.
2. The primary, the secondary and the tertiary roots are absent.
TAP ROOT MODIFICATION-CONICAL ROOT e.g. Carrot
1. The root tuber is conical in shape.
2. It is broad at the top and tapers gradually towards the lower end.
TAP ROOT MODIFICATION-FUSIFORM ROOT e.g. RADISH
1. The root tuber is swollen in the middle and tapers towards the two ends.
2. It is spindle shaped.
TAP ROOT MODIFICATION-NAPIFORM ROOT e.g. BEETROOT
1. The root tuber has a top like appearance.
2. It is very broad at the top and suddenly tapers like a tail at the apex.