Biology offers the door to many professional options, including medicine and other fields. Students can achieve full marks in the CBSE Class 12 Biology Board Exam with adequate study because it is primarily a theoretical subject. However, a well-planned study schedule, effective CBSE Class Biology preparatory advice, and information on all important topics. “Chromosomes” is an important CBSE Class 12 Biology topic. The definition, structure, and functions of chromosomes are highlighted in the article. These notes would benefit students studying for the CBSE Class 12 Biology Board Exams in 2022. Students must use NCERT Solutions for Class 12 Biology for a better understanding of the topic!
Did You Know?
Strasburger was the first person to discover chromosomes in 1815, while Waldeyer coined the name “chromosome” in 1888. |
CBSE Class 12 Biology: Chromosomes
Chromosomes can be defined as thread-like structures present in the nucleus of the eukaryotic cells that contain tightly packed DNA. These are found in the nucleus of both plant and animal cells. It can alternatively be defined as a DNA molecule containing some or all of an organism’s genetic material. DNA is coiled around proteins known as histones, which provide structural support. Chromosomes play a vital role in ensuring that DNA is duplicated and distributed correctly during biological processes.
“A chromosome is an entire strand of DNA accompanied by a group of stabilising proteins. They are in charge of hereditary features passed down from parents to children from generation to generation. DNA codes for specific proteins and is responsible for species and organism differences”. |
The primary role of chromosomes is to transport the basic genetic information known as DNA, which is required for the cells’ growth, development, survival, and reproduction activities.
The thread-like structure present in the chromosome is known as chromatin fibre. It is structured so that it resembles a spool of yarn. The chromatin fibre comprises histone protein molecules coiled in a string-like structure. These strings contain a chain of DNA made up of millions of long nucleotides consisting of hundreds of genes along its length. Use NCERT Solutions for Class 12 Biology to understand the topic of chromosomes effectively.
Some Interesting Facts
Humans’ bodies have 46 chromosomes organised into 23 pairs, comprising 22 pairs of autosomes. Autosomes pairs are labelled from 1 to 22. Each chromosome pair comprises one chromosome coming (inherited) from the mother and one from the father. Humans have one pair of sex chromosomes called an allosome in addition to the 22 autosomes pairs. Instead of numbers, the autosomes are labelled with letters such as XX and XY. Females have two X chromosomal copies (one from the mother and one from the father). Males have one X chromosome copy (inherited from their mother) and one Y chromosome copy (inherited from the father). |
CBSE Class 12 Biology: Structure of Chromosomes
A chromosome structure is made up of eight parts:
Curiously Enough
The unpacked DNA of a single cell will stretch 6 feet; this explains the need for packaging into each cell’s nucleus. |
- Centromere (also known as Main Constriction or Kinetochore)
The centromere, also known as the kinetochore, is the main constriction at the centre of the cell to which the chromatids or spindle fibres are linked. Its function is to allow chromosomal mobility during the anaphase phase of cell division.
- Chromatids
In cell division, the chromosome is split into two identical half strands connected by a centromere. A chromatid is formed by joining the two halves of a chromosome. Each chromatid contains DNA and separates to form a separate chromosome during Anaphase. The centromere links the two chromatids together.
- Chromatin
It is a complex of DNA comprising DNA, RNA and protein that creates chromosomes inside the nucleus of eukaryotic cells (plants and animal cells). Nuclear DNA is extremely compacted and coiled around nuclear proteins to fit inside the nucleus. In other words, it does not exist in the form of free linear strands. Chromatin is majorly composed of DNA, RNA, and protein.
- Secondary Constriction
It is commonly found in nucleolar structures.
- Telomere
A telomere is a terminal region on each half of the chromosome.
- Chromomere
It is a thread-like coiled filament structure that contains chromosomes. Chromonema regulates chromosomal growth and serves as a gene-bearing region.
- Chromonema
It refers to the bead-like structures found on threads or chromonema. These are lined up all along the length of the chromonema. The number of chromosomes is fixed, and it is responsible for passing on genes to the next generation during cellular division.
- Matrix
A pellicle is a membrane that surrounds each chromosome. The matrix is the jelly-like substance found inside the pellicle. It is made from non-genetic materials.
Also See: CBSE Class 12 Syllabus | CBSE Class 12 Biology Syllabus
CBSE Class 12 Biology: Functions of Chromosomes
Did You Know?
In 1902, Sutton and Bover proposed the function of chromosomes in inheritance for the first time. |
The primary function of chromosomes is to transmit the basic genetic material – DNA. DNA contains genetic information, which is necessary for various biological processes. These functions are necessary for organisms’ growth, development, survival, and reproduction. Read the NCERT Solutions for Class 12 Biology for concept clarity.
Histones and other proteins form a protective layer around the chromosomes. These proteins shield it from chemical and physical stimuli (e.g., enzymes). Thus, chromosomes also safeguard the genetic material (DNA) during the division of cells.
Spindle fibres connected to the centromeres play a key role during cell division. The shrinking of chromosomal centromeres ensures the accurate distribution of DNA (genetic material) to daughter nuclei.
Histone and non-histone proteins are present inside chromosomes. These proteins control gene activity. These proteins are activated or deactivated by cellular substances that regulate genes. This activation and deactivation cause the chromosome to grow or contract.
Conclusion
Cells need to repair constantly to expand and rejuvenate. Cell division is essential for an organism’s growth and development. During cell division, chromosomes guarantee that DNA is distributed evenly among daughter cells. But sometimes, even a minor flaw in the process can result in various diseases and abnormalities. Cancer is caused by uncontrolled cell division, which results in tumour cells. A chromosomal abnormality, such as a change in structure or quantity, can result in genetic abnormalities such as Down syndrome, Turner syndrome, etc. Defective chromosomes can potentially cause severe disease in humans.
A chromosome is an important topic from the CBSE Class 12 Biology Exam point of view. Studying these notes along with NCERT Solutions for Class 12 Biology will help students gain comprehensive information on the topic, ensuring higher grades in their Biology Exams!
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