Question

1.How can a biologist determine whether a thing is living or nonliving? Discuss the major characteristics of life.

2.Discuss in details the two types of cell division; Mitosis and Meiosis. Highlight the differences between them

Answer 1

1.)

t is a widely debated question which throws up lots of controversy over what we actually class as being a living organism. One definition of life is that in order to be living, an organism must possess all or the majority of the following capabilities:

1) Organism must be able to regulate its own internal environment (homeostasis)

2) Organism must be able to convert chemicals into energy (metabolism)

3) Organism must be capable of growth

4) Organism must be able to change over time in order to better suit its environmet (adaption)

5) Organism must be composed of one or more cells. The cell being the basic component of life.

6) Organism must be able to respond to stimuli (such as when a sunflower grows in order to face light)

7) Organism must be capable of producing new organisms (reproduction)

These definitions of life mean that some organisms, such as viruses, are therefore not classed as being alive. A virus requires a host in order to carry out most of these functions and so, itself, under these criteria would not be classed as being alive. Those which fulfil the majority of these criteria are therefore determined by biologists to be alive.

2.)

There are two types of cell division: mitosis and meiosis. Most of the time when people refer to “cell division,” they mean mitosis, the process of making new body cells. Meiosis is the type of cell division that creates egg and sperm cells.

Mitosis is a fundamental process for life. During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. Because this process is so critical, the steps of mitosis are carefully controlled by a number of genes. When mitosis is not regulated correctly, health problems such as cancer can result.

The other type of cell division, meiosis, ensures that humans have the same number of chromosomes in each generation. It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells. When the sperm and egg cells unite at conception, each contributes 23 chromosomes so the resulting embryo will have the usual 46. Meiosis also allows genetic variation through a process of DNA shuffling while the cells are dividing.

Following are the differences between Mitosis and Meiosis:

S.N.	Differences	Mitosis	Meiosis
1	Type of Reproduction	Asexual	Sexual
2	Genetically	Similar	Different
3	Crossing Over	No, crossing over cannot occur.	Yes, mixing of chromosomes can occur.
4	Number of Divisions	One	Two
5	Pairing of Homologs	No	Yes
6	Mother Cells	Can be either haploid or diploid	Always diploid
7	Number of Daughter Cells produced	2 diploid cells	4 haploid cells
8	Chromosome Number	Remains the same.	Reduced by half.
9	Chromosomes Pairing	Does Not Occur	Takes place during zygotene of prophase I and continue upto metaphase I.
10	Creates	Makes everything other than sex cells.	Sex cells only: female egg cells or male sperm cells.
11	Takes Place in	Somatic Cells	Germ Cells
12	Chiasmata	Absent	Observed during prophase I and metaphase I.
13	Spindle Fibres	Disappear completely in telophase.	Do not disappear completely in telophase I.
14	Nucleoli	Reappear at telophase	Do not reappear at telophase I.
15	Steps	Prophase, Metaphase, Anaphase, Telophase.	(Meiosis 1) Prophase I, Metaphase I, Anaphase I, Telophase I; (Meiosis 2) Prophase II, Metaphase II, Anaphase II and Telophase II.
16	Karyokinesis	Occurs in Interphase.	Occurs in Interphase I.
17	Cytokinesis	Occurs in Telophase.	Occurs in Telophase I and in Telophase II.
18	Centromeres Split	The centromeres split during anaphase.	The centromeres do not separate during anaphase I, but during anaphase II.
19	Prophase	Simple	Complicated
20	Prophase	Duration of prophase is short, usually of few hours.	Prophase is comparatively longer and may take days.
21	Synapsis	No Synapsis	Synapsis of Homologous chromosomes takes place during prophase.
22	Exchange of Segments	Two chromatids of a chromosome do not exchange segments during prophase.	Chromatids of two homologous chromosome exchange segments during crossing over.
23	Discovered by	Walther Flemming	Oscar Hertwig
24	Function	Cellular reproduction and general growth and repair of the body.	Genetic diversity through sexual reproduction.
25	Function	Takes part in healing and repair.	Takes part in the formation of gametes and maintenance of chromosome number.

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