Chromosome Definition: Structure, Significance, Function, Properties

Chromosomes are thread-like structures in the nucleus. They are significant because they contain DNA, which is the basic genetic material. These can be found in the nuclei of both plant and animal cells. Each chromosome is made up of protein and DNA, as previously stated (a single molecule). 

Keyterms: Chromosomes, Nucleus, DNA, Genetic material, Plant Cell, Animal cell, Protein, Nuclei, Molecule, Reproduction

Also Read: Difference Between Gene and DNA

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What is Chromosome? 

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A chromosome is an entire strand of DNA that is accompanied by a collection of stabilising proteins. Chromosomes are thread-like structures found within the nucleus of both animal and plant cells. DNA is responsible for conveying unique qualities from parents to kids, as well as containing precise instructions that give rise to each type of living thing. 

Here, we can emphasise the chromosome function, which is to carry the basic genetic material known as DNA, which is required for growth, survival, development, and reproduction.

Diagram: Chromosome and DNA 

The chromosome's thread-like structure is known as chromatin fibre, and it is placed in the chromosome in such a way that it resembles a spool of yarn. This chromatin fibre is made up of histones, which are protein molecules coiled in a string-like structure. When we zoom in on the string, we can see that it contains a DNA chain that is millions of nucleotides long and contains hundreds of genes along its length.

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Functions Of Chromosomes

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Here are some of the key functions of chromosomes:

• The most crucial function of chromosomes is to transport DNA, which is the most basic genetic material. DNA is a molecule that holds genetic information that is used to execute a wide range of biological functions. These functions are required for the organism's survival, reproduction, and growth.
• Chromosomes are protected by Histones and other proteins. Chemical (e.g., enzymes) and physical forces are protected by these proteins. As a result, chromosomes protect the genetic material (DNA) throughout cell division as well.
• Spindle fibres connected to the centromeres contract during cell division and play a vital role. The contraction of chromosome centromeres ensures that DNA (genetic material) is distributed evenly to daughter nuclei.
• Histone and non-histone proteins are found in chromosomes. These proteins control gene activity. Activating or deactivating these proteins is how cellular molecules that regulate genes work. The chromosome expands or contracts as a result of this activation and deactivation.

DNA Transportation in Bacteria

Also Read: Chromosomes and Genes

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Structure Of Chromosome 

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Centromere or primary constriction or kinetochore, chromatids, chromatin, secondary constriction, telomere, chromomere, chromonema, and matrix are the eight components of a chromosome.

• Centromere or Kinetochore: The chromatids or spindle fibres are linked to the major constriction at the centre. Its purpose is to allow chromosomal mobility during the anaphase stage of cell division.
• Chromatid: A chromosome is split into two identical half strands and reunited by a centromere during cell division. Each half of the chromosome is linked to form a chromatid. Each chromatid carries DNA and divides into its own chromosome during Anaphase. The centromere is the link between the two chromatids.
• Chromatin: Within the nucleus of eukaryotic cells, it is a complex of DNA and proteins that creates chromosomes. To fit inside the nucleus, nuclear DNA is extremely compressed and wrapped around nuclear proteins. To put it another way, it doesn't exist as free linear strands. DNA, RNA, and protein make up the chromatin.
• Secondary Constriction: It is present for the nucleolar organisation in most cases.
• Telomere: Each chromosome's terminal segment is known as the telomere.
• Chromonema: The chromomeres are organised along with a threadlike coiled filamentous structure. The chromosome's size is controlled by the chromonema, which also serves as a gene-bearing site.
• Chromomeres: The bead-like structures found on threads or chromonema are known as chromonema. Along the length of the chromonema, these are organised in a row. The number of chromosomes remains constant, and they are responsible for conveying genes to the next generation throughout cell division.
• Matrix: The pellicle is the membrane that surrounds each chromosome. Inside the pellicle, there is a jelly-like substance called matrix. It is made up of non-genetic components.

Structure and Organization of Chromosome 

Also Read: Sex Determination

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Examples Of Chromosomes 

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Examples of Chromosomes are listed below:

Prokaryotic Replication

Once a single bacteria cell has grown large enough, asexual reproduction is feasible. Despite the absence of membranes that separate certain organelles, bacteria will replicate their DNA as well as the molecules they require to survive. 

DNA is contained in a single chromosome, known as a genophore, which is reproduced by separating the individual strands and polymerase constructing new, corresponding strands. Individual cells are formed by separating the two chromosomes, and the cells carry out their duties by synthesizing proteins from DNA and interacting with the environment.

Prokaryotic Replication

Eukaryote Replication

Eukaryotes have a more sophisticated view of chromosomes. DNA replication occurs in eukaryotes near the conclusion of interphase, the period during which the cell develops and functions. The DNA strands are split and new strands are produced by enzymes, just like in prokaryotes. Eukaryotes, on the other hand, have many chromosomes. 

The centromere, a region that permits microtubules to join and holds chromosomes together, keeps the new chromosomes attached. Because they are identical duplicates, these are now known as sister chromatids. During meiotic cell division, when recombination is possible, these chromatids can change.

Eukaryotic Replication

Also Read: Chromosomal Theory of Inheritance

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Properties Of Chromosomes

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Read on to learn about properties of Chromosomes:

• Humans have 23 chromosome pairs. This chromosome's unusual structure keeps DNA securely packed with histone proteins so that it can fit within the cell and also serves to give it additional stability. 
• For example, a single cell's unravelled DNA will measure 6 feet, demonstrating the need for packaging inside the nucleus of each cell.
• To replace the old cells, cells must constantly mend, expand, and rejuvenate. Cell division is critical for an organism's growth and development.
• During cell division, we can clearly observe chromosomes. 
• During cell division, chromosomes guarantee that DNA is evenly distributed across daughter cells.
• Even a minor flaw in the process can result in a variety of diseases and malformations. Cancer is caused by uncontrolled cell division, which results in tumour cells. Down's syndrome, Turner's syndrome, and other genetic abnormalities are caused by chromosomal aberrations such as changes in structure or number. In humans, chromosomal defects can lead to a specific type of leukaemia.

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Things to Remember 

• DNA and its related proteins, of which chromosomes are a part, are referred to as chromatin.
• During anaphase of mitosis or anaphase II of meiosis, the still linked copies of a chromosome will be split into independent chromosomes.
• Homologous Chromosomes- In anaphase, I of meiosis, pairs of chromosomes from different parents that have the same genetic information are separated.
• Sister Chromosome- One of two new chromosomes generated during DNA replication that have now been separated and will shortly be in distinct cells.
• Chromosomes that separate during cell division are known as sister chromosomes. Different gametes, or sperm and egg cells, are made up of sister chromosomes. When sperm and egg combine, the zygote inherits one sister chromosome from each parent, but they are known as homologous chromosomes since they are not identical copies of each other. 
• Humans have 46 chromosomes in each cell because they have 23 homologous pairs. They have 92 sister chromatids when the DNA is reproduced, but they are still connected, thus there are only 46 chromosomes. There are just 23 chromosomes in human gametes, and no homologous pairs.

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Previous Year Questions

• At which stage of meiosis does the genetic constitution of gametes is finally decided?
• During anaphase I of meiosis
• During pachytene stage of meiosis, the chromosomes appear
• Identify the wrong statement about meiosis.
• The stage of meiosis in which pairing of homologous chromosomes starts, is called? [UPSEE 2016]
• The exchange of segments of non-sister chromatids between homologous pair is? [DUET 2010]
• Which of the following stages is the longest in animals? [ JIPMER 1996]
• During meiosis -I, the number of chromosomes is? [JKCET 2015]
• The largest phase of meiosis-I is? [WBJEE-2011]
• In meiosis, synapsis occurs during?
• In which stage of meiosis crossing over takes place?
• Synapsis occurs in ...... phase of meiosis? [BCECE 2004]
• Meiosis can be best studied in [CUCET 2011]
• During _________stage of meiosis I, crossing over take place. [COMEDK UGET 2007]
• Meiosis results in