Structure of Chromosomes, Cell Cycle and Cell Division | ICSE Biology
Must Read Structure of Chromosomes
Nucleus
- It controls all the cellular activities of cell.
- It consists of the following.
- a. Nuclear membrane – It has perforations called nuclear pores.
- b. Nucleoplasm
- c. Nucleolus – is the site of ribosomal RNA formation.
- Network of nucleoprotein fibres are called the chromatin.
- Chromatin contains DNA, histones (basic proteins), non-histone proteins, and RNA.
- Chromatin threads condense and organize to form chromosome.
- The primary constriction in the chromosome is called centromere.
Based on the position of centromere, chromosomes are of four types:
Metacentric: Centromere is located at the middle of chromosome.
Sub-metacentric: Centromere is slightly away from the middle of chromosome.
Acrocentric: Centromere is situated close to the end of chromosome.
Telocentric: Centromere is located at the terminal end.
Satellite
A small chromosomal segment separated from the main body of the chromosome by a secondary constriction is called satellite.
Structure of DNA
- It has a double-helix structure, similar to ladder.
- It is made up of nucleotides.
- Nuclotides are made of sugar, phosphate groups and nitrogen bases
- Also Read Cell
Components of DNA
- Sugar
- Phosphate groups
- Nitrogen bases
Genes
- It is a unit of DNA.
- Located on Chromosomes.
- Controls the development of one or more traits.
- It is the basis of Inheritrance.
- It can acquire mutation leading to variation.
- Also Read Genetics
Cell cycle
- It is defined as a series of events that takes place in a cell, leading to the formation of two daughter cells.
- The average duration of a cell cycle for a human cell is about 24 hours and for yeast cell, it is about 90 minutes
- Cell cycle is divided into two basic phases: Interphase and M phase
Significance of Cell Division
- It is the mean of asexual reproduction in unicellular organisms.
- It is essential for the growth of a single celled zygote into a whole new multicellular organism.
- It helps in the repair of injuries and worn out tissues.
- It replaces dead cells of the body and thus is essential for growth of organism.
- In sexual reproduction, meiosis occurs. This type of cell division not only results in production of gametes, but also brings new combinations of genes, thus resulting in variations among a population. This also leads to evolution of a species.
Mitosis
- It is a process of cell division where chromosomes replicate and get equally distributed into two daughter cells. Hence, it is also called equational division.
- The process of mitosis keeps the chromosome number equal in daughter as well as parental cell.
- Mitosis usually takes place in somatic cells.
Mitosis involves four stages:
Prophase : It involves initiation and condensation of chromosomes. Nucleolus and nuclear membrane disappear.
Metaphase : Chromosomal material condenses to form compact chromosomes that get aligned in the middle of nucleus at equatorial plate.
Anaphase: Centromere splits and chromosomes move apart towards two opposite poles due to shortening of spindle fibres.
Telophase : Chromosomes finally reach their respective poles. Nuclear envelope assembles around each chromosome cluster. Nucleolus and other organelles reform.
Karyokinesis and Cytokinesis
- Karyokinesis is the division of nucleus during mitosis or meiosis that is followed by cytokinesis.
- Cytokinesis involves the division of cytoplasm of a cell.
- Cytokinesis is achieved in animal cell by cleavage that deepens and divides the cell into two.
- It is achieved in plant cell by cell plate formation.
- When karyokinesis is not followed by cytokinesis, a multinucleate condition arises. This is called syncytium.
Significance of mitosis
- It results in the formation of diploid daughter cells with identical genetic material.
- Mitosis plays a significant role in cell repair, growth, and healing.
Meiosis
- It is a type of cell division that produces sex cells or gametes.
- It occurs in the reproductive organs.
Significance of meiosis:
- Chromosome number is halved.
- It helps in mixing up of genes.
Differences between Mitosis and Meiosis
| Mitosis | Meiosis |
|---|---|
| It occurs in the body cells throughout the life. | It occurs in the reproductive cells during reproductive age. |
| Parent chromosome and daughter cell genes are identical. | Parent chromosome and daughter cell genes are randomly assorted causing genetic variation. |
| It helps in growth and replacement. | It helps in gamete formation. |
| Full set of chromosome is passed. | Only half of the total chromosomes is passed. |
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Structure of Chromosomes