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Model Answer
B. Sc. Semester- II_Botany
Department of Botany, Guru Ghasidas University, Bilaspur
LBC 202: CELL & MOLECULAR BIOLOGY
Answer 1:
1.
2.
3.
4.
5.
6.
D. The plasma membrane.
D. Have fewer internal membranous compartments.
C. Endoplasmic reticulum
B. Microsomes
D. Binary fission.
C. S Phase
7. D. All A, B and C
8. D. Metacentric
9. D. Is initially not continuous
10. D. Strong processivity
Answer 2:
These are some of the parts common to plant cells:
•Cell Wall- tough, rigid layer that provides shape and protection from osmotic swelling.
•Cell (Plasma) Membrane- it is composed of a phospholipid lipid bilayer (including polar
hydrophilic heads facing outside and hydrophobic tails facing each other inside) that makes it
semipermeable and thus capable of selectively allowing certain ions and molecules in/out of the
cell.
•Cytoplasm- it consists of the jelly-like cytosol where the organelles are located.
•Cytoskeleton- is made up of microtubules. It provides shape the shape of the cell and helps in
transporting materials in and out of the cell.
•Golgi Apparatus (body/complex)- it is the site where membrane-bound vesicles are packed
with proteins and carbohydrates. These vesicles will leave the cell.
•Vacuole- stores metabolites and degrades and recycles macromolecules.
•Mitochondria- is the powerhouse of the cell, responsible for cellular respiration by converting
the energy stored in glucose into ATP.
•Ribosome- contain RNA for protein synthesis. One type is embedded in Rough ER and another
type puts proteins directly into the cytoplasm.
•Rough Endoplasmic Reticulum - covered with ribosomes, it stores, separates, and transports
materials through the cell. It also produces proteins in cisternae, which then go to the Golgi
apparatus or insert into the cell membrane.
•Smooth Endoplasmic Reticulum - it has no ribosomes embedded in its surface. Lipids and
proteins are produced and digested here. Smooth ER buds off from rough ER to move newlysynthesized proteins and lipids. The proteins and lipids are transported to the Golgi apparatus
(where they are made ready for export) and membranes.
•Peroxisome- is involved in metabolizing certain fatty acids and producing and degrading
hydrogen peroxide.
•Nuclear Membrane- the membrane that surrounds the nucleus. Its many opening allow traffic
in/out of the nucleus.
•Nucleus - it contains DNA in the form of chromosomes and controls protein synthesis.
•Nucleolus - it is the site of ribosomal RNA synthesis.
•Centrosome- consisting of a dense center and radiating tubules, it organizes the microtubules
into a mitotic spindle during cell division.
•Chloroplast- conducts photosynthesis and produces ATP and carbohydrates from captured light
energy.
•Stretch Granule- temporarily stores produced carbohydrates from photosynthesis.
Answer 3:
The endoplasmic reticulum (ER) is an eukaryotic organelle that forms an interconnected network
of tubules, vesicles, and cisternae within cells. Rough endoplasmic reticulua synthesize proteins,
while smooth endoplasmic reticulua synthesize lipids and steroids, metabolize carbohydrates and
steroids, and regulate calcium concentration, drug detoxification, and attachment of receptors on
cell membrane proteins. Sarcoplasmic reticulua solely regulate calcium levels. The endoplasmic
reticulum was first observed by Keith R. Porter.
Rough endoplasmic reticulum
The surface of the rough endoplasmic reticulum (often abbreviated RER) is studded with
protein-manufacturing ribosome giving it a "rough" appearance (hence its name). The binding
site of the ribosome on the rough endoplasmic reticulum is the translocon. However, the
ribosomes bound to it at any one time are not a stable part of this organelle's structure as they are
constantly being bound and released from the membrane. A ribosome binds to the endoplasmic
reticulum only when it begins to synthesize a protein destined for the secretory pathway. The
membrane of the rough endoplasmic reticulum forms large double membrane sheets that are
located near, and continuous with the outer layer of the nuclear envelope. Although there is no
continuous membrane between the endoplasmic reticulum and the Golgi apparatus, membranebound vesicles shuttle proteins between these two compartments.
Smooth endoplasmic reticulum
The smooth endoplasmic reticulum (abbreviated SER) has functions in several metabolic
processes. It synthesizes lipids, phospholipids and steroids—cells which secrete these products,
such as those in the testes, ovaries, and skin oil glands have a great deal of smooth endoplasmic
reticulum. It also carries out the metabolism of carbohydrates, drug detoxification, attachment of
receptors on cell membrane proteins, and steroid metabolism. In muscle cells, it regulates
calcium ion concentration. It is connected to the nuclear envelope. Smooth endoplasmic
reticulum is found in a variety of cell types (both animal and plant), and it serves different
functions in each. The smooth endoplasmic reticulum also contains the enzyme glucose-6phosphatase, which converts glucose-6-phosphate to glucose, a step in gluconeogenesis. It
consists of tubules that are located near the cell periphery.The network of smooth endoplasmic
reticulum allows increased surface area for the action or storage of key enzymes and the products
of these enzymes.
Functions
The endoplasmic reticulum serves many general functions, including the facilitation of protein
folding and the transport of synthesized proteins in sacs called cisternae. Correct folding of
newly made proteins is made possible by several endoplasmic reticulum chaperone proteins,
including protein disulfide isomerase.
Secretory proteins, mostly glycoproteins, are moved across the endoplasmic reticulum
membrane. Proteins that are transported by the endoplasmic reticulum throughout the cell are
marked with an address tag called a signal sequence. Proteins that are destined for places outside
the endoplasmic reticulum are packed into transport vesicles and moved along the cytoskeleton
toward their destination.
Answer 4:
Structure of Golgi Apparatus:
Golgi apparatus varies in sized and form in different cell types, but usually has similar
organization for any one kind of cells. The Golgi apparatus appears as a coarse network under a
light microscope. Electron microscope shows it is central stack of parallel, flattened, inter
communicating sacs or cisternae and many peripheral tubules and vesicles.
The cisternae vary in number from three to seven in most animal cells and from ten to twenty in
plant cells. Secretary materials reach the Golgi apparatus from the SER through their inter
connections, and also by way of transport vesicles which bud off from the SER and fuse with the
Golgi cisternae on the forming face.
Tubules form a complicated network towards the periphery and maturing face of the apparatus.
Golgian vacuoles are expanded part of the cisternae which have become modified to form
vacuoles. The vacuoles develop from the concave or maturing face. Golgian vacuoles contain
amorphous or granular substance. Some of the golgian vacuoles function as a lysosomes.
Functions of Golgi apparatus:
Golgi apparatus is metabolically very active and many functions have been assigned to it.
The Golgi complex modifies sorts and packages proteins and lipids coming from the ER.
Chemical labels are added to send the products to other specific parts of the cell or out of the
cell.
The Golgi apparatus synthesizes some simple carbohydrates such as galactus, sialic acid and
certain poly saccharides, pectin compounds from simple sugars.
The Golgi apparatus links carbohydrates with proteins coming from ER to form glycol proteins.
This process is called glycol sylation.
Lipids and proteins coming from the ER or complexed into lipoproteins in the Golgi apparatus.
The production of hormones by endocrine glands is mediated through Golgi apparatus.
In many mammalian tumor and cancer cells the Golgi complex has been described as the site of
origin of pigment granules (melanin).
Answer 5:
A). Difference between Prokaryotic and eukaryotic cell:
Prokaryotic Cells
Eukaryotic Cells
B). DIFFERENCES BETWEEN PLANT AND ANIMAL CELLS
Plant Cell
1. A plant cell has a rigid wall on the outside.
2. It is usually larger in size.
3. It cannot change its shape.
Animal Cell
1. A cell wall is absent, though other structures
occur in some acellular orgganisms formerly
included amongst animals.
2. An animal cell is comparatively smaller in
size
3. An animal cell can often change its shape.
4. Plastids are usually absent.
4. Plastids are found in plant cells.
5. Cholorophyll is absent.
5. Plant cells exposed to sunlight possess
chlorophyll containing plastids called
chloroplasts.
6. A mature plant cell contains a large central
vacuole.
7. Nucleus lies on one side in the peripheral
cytoplasm.
6. An animal cell often possesses many small
vacuoles.
7. Nucleus usually lies in the centre.
8. Mitochondria are comparatively fewer.
9. Plant cells do not burst if placed in
hypotonic solution due to the presence of cell
wall.
10. Centrioles are usually absent except in
lower plants.
11. Golgi apparatus consists of a number of
distinct or unconnected units called
dictyosomes.
12. Cytoskeleton does not contain intermediate
fibres.
13. Lysosomes are rare. Their activity is
performed by specialised vacuoles.
14. Glyoxysomes may be present
15. Crystals of inorganic substances may
occurs inside the cells.
8. Mitochondria are generally more numerous.
9. Animal cells usually burst if placed in
hypotonic solution unless and until they
possess contractile vacuoles.
10. Centioles are found in animal cells.
Spindle formed during nuclear division is
amphiastral.
11. Golgi apparatus is either localised or
consists of a well connected single complex.
12. Cytoskeleton contains intermediate fibres.
13. Typical lysosomes occur in animal cells.
14. They are absent.
15. Crystals usually do not occur in animal
cells.
16. A tissue fluid does not bathe the individual 16. A tissue fluid containing NaCl bathes the
cells.
cells.
17. Animal cells cannot synthesise all the
amino acids, vitamins and coenzymes required
17. Plant cells can synthesise all the amino
by them.
acids, vitamins and coenzymes required by
18. Cytokinesis occurs by constriction.
them.
18.Cytokinesis occurs by cell plate method.
Answer 6:
Differences between Meiosis & Mitosis:
Meiosis
Mitosis
Definition:
A type of cellular reproduction in
which the number of
chromosomes are reduced by half
through the separation of
homologous chromosomes,
producing two haploid cells.
A process of asexual reproduction
in which the cell divides in two
producing a replica, with an equal
number of chromosomes in each
resulting diploid cell.
Function:
sexual reproduction
Cellular Reproduction & general
growth and repair of the body
Type of
Reproduction:
Sexual
Asexual
Occurs in:
Humans, animals, plants, fungi
all organisms
Genetically:
different
identical
Crossing Over:
Yes, mixing of chromosomes can
occur.
No, crossing over cannot occur.
Pairing of
Homologues:
Yes
No
Number of
2
1
Meiosis
Mitosis
Number of
Haploid Daughter
Cells produced:
4
2
Chromosome
Number:
Reduced by half
Remains the same
Steps:
The steps of meiosis are
Interphase, Prophase I, Metaphase
I, Anaphase I, Telophase I,
Prophase II, Metaphase II,
Anaphase II and Telophase II.
The steps of mitosis are
Interphase, Prophase, Metaphase,
Anaphase, Telophase and
Cytokinesis
Karyokenesis:
Occurs in Interphase I
Occurs in Interphase
Cytokenesis:
Occurs in Telophase I &
Telohpase II
Occurs in Telophase
Centromeres
Split:
The centromeres do not separate
during anaphase I, but during
anaphase II
The centromeres split during
Anaphase
Creates:
Sex cells only: Female egg cells or
Male sperm cells
Makes everything other than sex
cells
Discovered by:
Oscar Hertwig
Walther Flemming
Divisions:
Answer 7: Different stages of Meiosis I
Meiosis I
Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes,
23 in humans), so meiosis I is referred to as a reductional division.
Prophase I
Prophase 1 is similar in some ways to prophase in mitosis.
– Chromosomes condense
– Spindle fibers appear
– Nucleus and nucleolus disappear
Opposite to mitosos-It is the longest phase of meiosis. During prophase I, DNA is exchanged between homologous
chromosomes in a process called homologous recombination forming tetrad. Tetrad is two
chromosomes or four chromatids (sister and nonsister chromatids). This often results in
chromosomal crossover. The process of pairing the homologous chromosomes is called synaps
Leptotene:
The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words
meaning "thin threads". Leptotene is of very short duration and progressive condensation and
coiling of chromosome fibers takes place.
Zygotene
The zygotene stage, also known as zygonema, from Greek words meaning "paired threads occurs
as the chromosomes approximately line up with each other into homologous chromosome pairs.
At this stage, the synapsis (pairing) of homologous chromosomes takes place.
Pachytene
The pachytene stage, also known as pachynema, from Greek words meaning "thick threads” is
the stage when chromosomal crossover (crossing over) occurs. Nonsister chromatids of
homologous chromosomes may exchange segments over regions of homology. The sites where
exchange happens, chiasmata form. The exchange of information between the non-sister
chromatids results in a recombination of information.
Diplotene
During the diplotene stage, also known as diplonema, from Greek words meaning "two threads”
the synaptonemal complex degrades and homologous chromosomes separate from one another.
Diakinesis
Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving
through. This is the first point in meiosis where the four parts of the tetrads are actually visible.
Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the
nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle
begins to form.
Metaphase I In metaphase I (Gr meta= middle) the homolog chromosomes with the already
exchanged DNA align as bivalents in the equatorial plane. The chromatides are strongly
condensed. Bivalents are positioned in such a way that homolog centromeres lay at either side of
the equatorial plane.
Anaphase I Anaphase I (Gr ana = apart) begins when the chromosomes, including piecesf
exchanged DNA, are pulled to the opposite poles in the cell. The units of a homolog pair move
apart (separation of bivalents) in opposite direction; however, the chromatides of each
chromosomes stay joined.
Telophase I. In telophase I (Gr telos = end) the bivalents are separated in two opposite domains
in the cell (two poles) and the chromosomes decondense. In some species a new nuclear envelop
is formed.
Answer 8:
DNA replication:
DNA replication is a biological process that occurs in all living organisms and copies their DNA.
DNA replication during mitosis is the basis for biological inheritance. The process of DNA
replication starts when one double-stranded DNA molecule produces two identical copies of the
molecule. Each strand of the original double-stranded DNA molecule serves as template for the
production of the complementary strand, a process referred to as semiconservative replication.
Steps of DNA replication
Segments of single-stranded DNA are called template strands.
Gyrase (a type of topoisomerase) relaxes the supercoiled DNA.
Initiator proteins and DNA helicase binds to the DNA at the replication fork and untwist
the DNA using energy derived from ATP (adenosine triphosphate).
(Hydrolysis of ATP causes a shape change in DNA helicase)
DNA primase next binds to helicase producing a complex called a primosome (primase is
required for synthesis),
Primase synthesizes a short RNA primer of 10-12 nucleotides, to which DNA polymerase
III adds nucleotides.
Polymerase III adds nucleotides 5’ to 3’ on both strands beginning at the RNA primer.
The RNA primer is removed and replaced with DNA by polymerase I, and the gap is
sealed with DNA ligase.
Single-stranded DNA-binding (SSB) proteins (>200) stabilize the single-stranded
template DNA during the process.
The DNA polymerase starts at the 3’ end of the RNA primer of the leading stand
CONTINUOUSLY.
DNA is copied in 5’ to 3’ direction.
DNA polymerase copies the lagging strand DIS- continuously.
The dis-continuous pieces of DNA copied on the lagging strand are known as Okazaki
fragments.
Another DNA Polymerase removes the RNA primers and replaces them with DNA.
Finally the gaps in the sugar phosphate backbone are sealed by DNA ligase. There are
now 2 identical double helices of DNA.