Cell Limitations and Cell Reproduction

Key Concepts

• Eukarotic
• Prokaryotic
• Nucleus
• Chromosome
• Chromatid
• Telomere
• Centromere

Introduction

Reproduction is one of the features of living things.  Reproduction takes place at the cellular level. Cellular reproduction is a method through which cell make exact copy of it and divide to make more cells. There are some parts of the body, such as the gastrointestinal tract where cells reproduce more frequently. In the nervous system, the cells reproduce less frequently. All cells of the human body reproduce except red blood cells which do not contain nucleus. Lack of nucleus increases the cells’ oxygen carrying capacity which is essential for the survival of the organism.  

Types of cells 

There are two types of cells- Eukaryotic and prokaryotic. Both these cells vary in size and the presence of a membrane-bound nucleus. 

Prokaryotes cells:

These are unicellular organisms that do not have a membrane-bound nucleus and other cell structures (organelles). Prokaryotic cells are small. These are simple cells, measuring about 0.1 to 5 μm in diameter. The two main branches of prokaryotes are archea and bacteria. The main structures present in prokaryotic cells that contain genetic material are as follows. 

Nucleoid: It is a central area of the cell that has its DNA. 

Plasmid: It is a small piece of DNA that separates from chromosomal DNA. 

Eukaryotic cells:

These are the cells that have a membrane-enclosed nucleus and other organelles. These membranes are similar to the plasma membrane, which is a flexible lipid bilayer. The other parts of the cell (organelles) are internal structures responsible for a variety of functions, such as energy production and protein synthesis. These cells are found in plants, animals, protists, and fungi. 

Nucleus 

In eukaryotic cells, the structure, function and contents of the nucleus are of primary importance to understand cellular reproduction. The nucleus comprises of DNA, the hereditary material of the cell. The DNA condenses to form chromosomes. In addition, the nucleus generally contains one or more prominent nucleoli (dense bodies that are the place of ribosome synthesis). 

The nucleus is the brain of the cell as it helps in the processing of information. It is the site for the storage of hereditary material (DNA) which is passed from one generation to the next. It also helps in maintaining cellular activities. 

Structure of nucleus 

In an animal cell, the nucleus is usually positioned in the central region of the cell. In plant cell nucleus is located on one side of the cells. Since the large vacuole in a plant cell occupy large volume, the nucleus is pressed to the border. 

Animal cell Plant cell 

The nucleus structure consists of nuclear membrane, chromosomes, nucleoplasm, and nucleolus. The nucleus is enclosed by a nuclear envelope comprising of a double membrane that is continuous with the endoplasmic reticulum. Movement of molecules between the nucleus and cytoplasm is completed through a series of nuclear pores, lined with proteins, that help the movement of molecules out of the nucleus. The proteins give some measure of selectivity in the passage of molecules across the nuclear membrane. The Nucleus is separated from the cell or the cytoplasm by a nuclear membrane. 

Nuclear Membrane: 

The nuclear membrane is a double layered outer structure of the nucleus that separates the nuclear material from the rest of the cell. It has perforations on it called the nuclear pores. These nuclear pores act as passage for movement of proteins and nucleic acid between nucleus and cytoplasm. 

Chromosomes: 

Chromosomes are long thread like structures present in the nucleus of plant and animal cells. DNA and protein are the constituents of chromosome. The DNA and proteins condense to form chromatin, which further condenses to form chromosomes within the nucleus. Chromatin is present in two forms; euchromatin and heterochromatin. Euchromatin is less condensed whereas heterochromatin is highly condensed. Heterochromatin usually attaches to the nuclear membrane whereas euchromatin is present mostly in transcribing cell. 

Centromere is the constricted part of each chromosome present approximately at the center of each chromosome. Centromere divides the chromosome into two parts, namely, the ‘p’ and ‘q’ arm. The centromere helps to describe the site of particular genes. 

Telomeres are repetitive stretches of DNA present at the ends of linear chromosomes. They protect the ends of chromosomes from unraveling. 

In many categories of cells, telomeres lose a bit of their DNA every time a cell divides. Ultimately, when all of the telomere DNA is gone, the cell cannot duplicate and dies. 

WBCs and other cell types with the capacity to divide very frequently have a special enzyme that inhibits their chromosomes from losing their telomeres. Since they retain their telomeres, such cells generally live longer than other cells. 

Humans have 23 pairs of chromosomes, so a total of 46 chromosomes. Each species of plants and animals has definite number of chromosomes.  

Functions of chromosome 

The main function of chromosome is to carry genetic material in the form of DNA with protein known as histones. 

It provides genetic material to various cells. Chromosomes are important as they ensure the copying of DNA and its distribution to cell during division. Genes are located on chromosomes. 

Nucleolus: 

Nucleolus is present in the nucleus. It plays an indirect role in the protein synthesis by producing ribosomes. The main constituents of ribosomes are RNA and proteins. The ribosomes move to the cytoplasm and get attached to the endoplasmic reticulum. Such endoplasmic reticulum is called rough endoplasmic reticulum.  The main function of ribosomes is protein synthesis.  

Chromatin Threads: 

It is a network like structure of chromatin. The word Chromatin thread was introduced by W. Flemming.  During cell division, chromatin thread become chromosome by getting disconnected from one another and become thick and small. 

Nucleic Acids: 

Nucleic acids are of two types – DNA and RNA. DNA (Deoxyribonucleic acid) mainly found in the cytoplasm in soluble form and is known as soluble RNA. It is also present in certain amounts in the ribosomes of nucleus, chromatin, and nucleolus.  

Functions of cell nucleus: 

The primary function of the nucleus is to store genetic information (DNA), and pass this information, in the exact form, to daughter cell by cell division and manage the whole activities by controlling gene expression and protein synthesis. 

Cell cannot survive without a nucleus. A cell needs protein synthesis to continue its functions and activities. Nucleus is essential for protein synthesis. 

Summary:

• Reproduction occurs at the cellular level.
• There two types of cells— Eukaryotic and prokaryotic cells.
• Prokaryotic cells do not have a membrane-bound nucleus.
• Eukaryotic cells have a membrane-bound nucleus.
• The nucleus comprises of DNA, the hereditary material of the cell, in the form of chromosomes.
• Chromosomes are present in the form of threads of DNA and protein molecules called chromatin.
• Each chromosome has a compression point known as the centromere.
• Telomeres are repetitive stretches of DNA present at the end of linear chromosomes.
• The primary function of the nucleus is to store genetic information (DNA), and pass this exact information to its daughter cell by cell division.