The chromosomes are carriers of the genetic elements of the organism in the form of DNA. Chromosomes possess all the data that aids for cell development, survival, and reproduction. DNA portions with particular patterns are known as genes. The nucleus of the cell contains the chromosomes. In prokaryotic life forms, the DNA is absent in the nucleus; the DNA is present in the cytoplasm in a region called as the nucleoid.
The chromosomes vary between various life forms. Eukaryotic cells have an extensive number of direct chromosomes and cells of prokaryotes have little and round shaped DNA. More than one type of chromosomes can be found in the cells, as in most eukaryotic cells. In plant cells, the mitochondria and the chloroplasts have their own arrangement of chromosomes.
The chromosomes are stuffed by proteins to form a minimized structure called chromatin in the nucleus of eukaryotic organisms. This buildup enables long compounds of DNA to fit into the nucleus of the cell. The chromosomes are imitated, isolated and passed on to daughter cells, to guarantee genetic diversity and survival of the offspring.
Chromatids are duplicates chromosomes which are its identical copies and are joined by a centromere. Compaction of the chromosomes amid the cell division process gives the four-arm structure. Recombination of chromosome assumes an essential part in genetic diversity. Incorrect augmentation of the chromosomes may prompt mitotic failure or cell demise.
Functions of Chromosomes
Genetic Code Storage − The chromosome contains the hereditary material which is required by the living beings to create and develop. DNA particles are made of a chain of units called genes. They are the segments of the DNA which code for particular proteins required by the cell for its proper working.
Sex Determination − Humans have 23 sets of chromosomes out of which one set is the sex chromosome. Females have two X chromosomes and males have one X and one Y chromosome. The sex of the baby is controlled by the chromosome passed down by the male. In the event that X chromosome is passed out of XY chromosome, the baby will be a female and if a Y chromosome is passed, the child is a male.
Control of Cell Division − Chromosomes check the division of cells amid the process of mitosis. The chromosomes of the parent cells see the right information is passed on to the daughter cells required by the cell to develop and grow perfectly.
Development of Proteins and Storage − Proteins are fundamental for the activity of a cell. The chromosomes coordinate the arrangements of proteins developed in our body and furthermore keep up the order of DNA. The proteins are additionally stored in the coiled structures of the chromosomes. These proteins help in the appropriate packaging of the DNA.
What are chromosomal disorder?
In the human body, chromosomes are the structures that hold genes. They give instructions and tell the bodies how to develop and function. In fact, they govern physical characteristics such as eye color, hair color, skin color and also the medical characteristics of the body such as blood type and susceptibility to disease.
A chromosome abnormality is caused when there is any disorder in DNA which may have been caused due to mutation or a missing or extra portion of chromosomal DNA.
Chromosomal abnormalities may be caused due to Mitosis, Meiosis, Environment and even the maternal age. If they become pregnant, Older women are at a higher risk of giving birth to abnormal babies than young mothers.
Chromosomes are thread-like structure present in the nucleus that carries hereditary information in the form of genes which is passed from parents to offspring. Every species has a characteristic structure and number of chromosome present. Due to certain irregularities at the time of cell division, alteration in the structure or number of chromosomes may happen. Even the slightest alteration can lead to various abnormalities.
Changes in one chromosome parts, whole chromosome or chromosomal sets are known as “chromosomal aberrations”
Chromosomal Disorders in Humans
Each human cell contains 46 (2n) chromosomes present as 23 pairs (n pairs), out of which 22 are autosomes and 1 pair of sex chromosomes.
Chromosomal disorders result from structural changes or numerical changes in chromosomes.
A. Chromosomal disorders due to numerical abnormalities
Chromosomal disorders are due to the change in the number of chromosomes present. This can be categorised into various types:
- Aneuploidy: loss or gain of a chromosome. This happens due to non-disjunction of chromatids when chromatids fail to separate during cell division. This results in one gamete having two copies of one chromosome and the other having no chromosome.
- Trisomy: The cell has one extra chromosome (2n+1)
- Monosomy: The cell has one chromosome less (2n-1)
Aneuploidy can be due to nondisjunction of autosomes i.e. chromosomes 1-22 or sex chromosomes.Karyotype of Down’s syndrome fig. 1Klinefelter’s syndrome fig. 2Turner’s syndrome fig. 3
Chromosomal disorders due to aneuploidy: This is the cause of most of the genetically inherited disorders and abortion during pregnancy
|Genetic Disorder||Defect in||Genotype||No. of Chromosomes||Phenotypic effect|
|Down’s syndrome||Autosomes||Trisomy of 21st chromosome||47||1:700 live birthsShort statured with a small round headFurrowed tongue with partially open mouthBroad palm with palm creaseCalled as mongolism or Mongolian idiocySusceptible to heart disease and respiratory problemsShorter life spanPhysical, psychomotor and mental development is retarded|
|Patau syndrome||Trisomy of 13th chromosome||47||1:5000 live birthsRarely live for more than a few monthsSloping forehead, hair clip and cleft palateSerious defects in the eyes, brain, kidney or circulatory systemMentally retarded|
|Edward’s syndrome||Trisomy of 18th chromosome||47||1:10000 live birthsMultiple malformations like in lower jaw, fingers, skull, face and feetCardiac malformationsCan’t live for more than 3-4 months|
|Klinefelter’s syndrome||Sex chromosome||XXY||47||These males have tall and masculine stature with feminine characteristicsDevelopment of breasts (gynecomastia)SterileSmall testicles, high-pitched voice and sparse body hairsMental retardation|
|Turner’s syndrome||Monosomy- XO||45||1:5000 live birthsOnly viable monosomy in humansSterile females with short statureBreasts and ovaries are underdeveloped, thin and less pubic hairs|
|Triple X syndrome||XXX||47||1:1000 live birthsThey are called super femalesMild development delays and menstrual irregularities|
|XYY syndrome||XYY||47||Males with an unusual heightSevere acneBelow normal intelligence|
- Euploidy: Loss or gain of the whole set of chromosome. Mostly occurs in plants.
- Haploid: Loss of one set of the chromosomes, i.e. ‘n’ number of chromosomes
- Polyploid: Addition of one or more set of chromosomes, e.g. ‘3n (triploid)’, ‘6n (hexaploid)’ etc.
B. Chromosomal disorders due to structural abnormalities
This happens when a large set of genes are deleted, duplicated or rearranged causing structural changes in the chromosome. Structural abnormalities can be due to:
1) Deletion: A portion of the chromosome is lost during cell division. A portion of chromosome without the centromere lags during anaphase movement and are lost from reorganising nuclei or digested by nucleases. The resulting chromosome lacks certain genes, that gets inherited to offspring. This condition is usually lethal due to missing genes.
- Deletion can be terminal, where a terminal portion of a chromosome breaks resulting in one break
- Intercalary deletion, where an intermediate portion is lost resulting from two breaks, which results in 3 pieces. The middle piece is lost and the other two parts rejoin
⇒ Example of disorder due to deletion:
Cri du chat (cry of the cat): Deletion of a small portion of 5th chromosome. Children with this disease have a small head with unusual facial features, severe mental retardation and make a sound like a cat while crying.
2) Duplication: The presence of part of a chromosome in excess is known as duplication. If the duplication is present only in one of the homologous pair of a chromosome, the duplicated part makes a loop to maximise juxtaposition of homologous regions during pairing. The extra segment can be arranged in many ways:
- Tandem duplication, where the duplicated region is present side by side (ABCDEF→ABCDEDEF)
- Reverse tandem, here duplicated region is just reverse of the normal sequence (ABCDEF→ABCDEEDF)
- Displaced duplication, here duplicated region is not situated adjacent to the normal sequence
- Transposed duplication, duplicated part becomes attached to a non-homologous chromosome
- Extra-chromosomal duplication, here duplicated part acts as an independent chromosome in the presence of centromere
⇒ Example of disorder due to duplication:
Fragile X: Affects 1:1500 males and 1:2500 females. This is the most common form of mental retardation, where the CGS segment is repeated more than 200 times.
3. Inversion: inversion results from breakage and reunion of a part of the chromosome rotating by 180° on its own axis. So there occurs a rearrangement of genes. Its effects are not as severe as in other structural defects
4. Translocation: The shifting or transfer of a set of genes or part of a chromosome to a non-homologous one is known as translocation. There is no addition or loss of genes, only the rearrangement occurs. This rearrangement may lead to phenotype changes pertaining to the new environment. It can cause difficulties in the development of egg, sperm or zygote. These often result in miscarriages and children born with disabilities.
- Reciprocal translocation, here segment of two chromosomes gets interchanged
- Robertsonian translocation, here an entire chromosome attaches to another chromosome
⇒ Example of disorder due to translocation:
Acute Myelogenous Leukemia: In this type of cancer, bone marrow and cells derived from it show the presence of a short chromosome named as “Philadelphia (Ph1) chromosome”. The 22nd chromosome loses a part of its arm which gets translocated to the distal end of the 9th chromosome. It is not transmitted to the offspring.
This was in brief about chromosomal disorders. Learn in detail about each chromosomal disorder only with BYJU’S.