Metacentric, Telocentric and Acrocentric chromosomes are different types of chromosomes based on the position of the centromere. The position of the centromere varies in different chromosomes.
Metacentric chromosomes have a centromere in the middle of the chromosome, telocentric chromosomes have a chromomere at the end and acrocentric chromosomes have a chromomere near the end of the chromosomes.
Chromosomes are present in the nucleus and they carry genetic information. They play an important role in cell division, heredity, variation, growth and repair, etc. The term chromosome was coined by W. Waldeyer in 1883 to describe darkly stained material in the nucleus.
Prokaryotic cells mostly have a single circular chromosome. Some bacteria have linear chromosomes also. They are present in the cytoplasm as the nucleus is absent. DNA is not scattered all around in the cell but is present in the nucleoid region. The DNA makes large loops and is held by proteins. The organisation of chromosomes in prokaryotes is less complex compared to eukaryotes.
Eukaryotic chromosomes contain a linear DNA double helix associated with proteins. Chromosomes appear as long thin threads at the interphase called chromatin. DNA is wrapped around the histone octamer to form a nucleosome, which is the repeating unit of chromatin thread. Chromatin fibres coil and condense further to form chromosomes at the metaphase. The structure of chromosomes is mostly studied at mitotic metaphase.
Chromosome number, size, shape, etc. varies in different organisms. Each diploid cell contains two sets of homologous chromosomes.
Also Check: Chromosome Structure
Centromeres are present at the primary constriction site, the thinner segment of a chromosome. It is dense and contains highly repetitive DNA sequences and is referred to as heterochromatin.
Centromeres play an important role at the time of cell division. Special proteins bound to DNA at the centromere form a disc-shaped structure called the kinetochore. The mitotic spindle attaches to the kinetochore at the time of cell division and is essential for the alignment of chromosomes at the time of metaphase.
Sister chromatids are attached to each other by the centromere. Centromere splits and the sister chromatids separate at the anaphase stage and go to the opposite poles.
The centromere divides chromosomes into two arms. The shorter arm is called the ‘p’ arm and the longer arm is called the ‘q’ arm.
The position of the centromere categorises chromosomes into metacentric, acrocentric, telocentric and submetacentric.
Types of Chromosomes
Depending on the position of the centromere, chromosomes are categorised into the following types:
- Metacentric chromosomes – The centromere is present in the middle of the chromosome and divides the chromosome into two equal arms. The p and q arms are almost equal. The shape of the metacentric chromosome is X shape. E.g. 1st, 3rd, 16th, 19th and 20th are metacentric chromosomes in humans.
- Telocentric chromosomes – It is not observed in humans. They have centromeres present at the end of the chromosome. The shorter p arm is rarely visible. The shape of the chromosome at the anaphase is like the letter ‘i’.
- Acrocentric chromosomes – The centromere is present near the end of chromosomes. It forms a very short p arm and a very long q arm. E.g. 13th, 14th, 15th, 21st, 22nd and Y chromosomes are acrocentric in humans.
- Submetacentric chromosomes – The centromere is present near the middle and divides the chromosome into two unequal arms. It is an L-shaped chromosome. Most chromosomes in humans are submetacentric, e.g. 2nd, 4th to 12th, 17th, 18th and X chromosomes.
Sometimes fusion of two acrocentric chromosomes by translocation leads to the formation of one metacentric chromosome.
Apart from these, chromosomes are also categorised based on the number of centromeres present in them. They are of the following types:
- Acentric – Acentric chromosomes do not have a centromere. E.g. in the macronucleus of ciliates. It cannot attach to the mitotic spindle. Acentric chromosomes can also be formed by breaking a chromosome.
- Monocentric – This type of chromosome has only one centromere with narrow constriction. This is commonly found in highly repetitive DNA.
- Dicentric – It is an abnormal chromosome and is formed by the union of two chromosome fragments. Each chromosome fragment has a centromere in it. These chromosomes are unstable because due to the presence of two centromeres they tend to move to opposite poles and break.
- Holocentric – Here, the entire chromosome acts as a centromere. The centromere is diffused and present along the length of the chromosome.
Polytene and Lampbrush chromosomes are giant chromosomes.
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