Summary:

• The DNA and histones are packaged into a structure called chromosomes. These are the thread-like structures inside a cell containing genetic information.
• Chromosomes are not usually visible inside the cell. It is only visible under a light microscope when the cell undergoes cell division.
• The constriction site of each chromosome is known as the centromere which divides the structure into two arms: a short arm or a ‘p arm’ and a long arm or a ‘q arm’.
• Chromosomes have a major role in the proper functioning, growth, and development of a cell.
• Mutations in chromosomes can result in structural changes and changes in the number of chromosomes resulting in serious disorders.

Introduction:

A chromosome is a structure that contains all or some of the genetic information of an organism in the form of a DNA molecule present in each cell. A chromosome is structured as a thread-like structure made up of a DNA molecule that is tightly coiled with other proteins to support its structure. Chromosomes appear as a dark-stained rod-shaped body when stained with the basic dyes and observed under a light microscope during the metaphase of cell division.

The word chromosome comes from a Greek word, chroma meaning colour, and soma i.e., the body which refers to its strong staining ability by dyes. The term chromosome was first developed by German anatomist Waldeyer in 1888.

History:

Till the middle of the nineteenth century, the physical nature of genetic information which is transferred from cell to cell and ultimately from one individual to another remained unknown. Using different microscopy techniques, German anatomist Walther Flemming studied the fibrous network present inside the nucleus which he termed as chromatin ‘a stainable material’. Based on many observations at different stages of the cell cycle, Fleming described the mechanism for precise distribution of chromosomes during cell division which was later confirmed with the help of modern techniques. Fleming’s observations paved the way for the discovery of hereditary mechanisms. Later with further discoveries, the Chromosomal theory of inheritance was proposed by Boveri and Sutton which confirmed chromosomes as genetic material responsible for Mendelian inheritance.

Chromosomes and How They Behave in Cell Division:

To understand the structure and functioning of chromosomes, we first need to understand the DNA and genome of an organism.

DNA and genome: DNA (deoxyribonucleic acid) is the genetic material of living organisms. In humans, DNA is found in almost all cells. A cell’s DNA provides all the basic information for the survival, growth, and proper functioning of a body. When a cell divides, its DNA replicates and passes its copy to the newly formed daughter cell. Similarly, DNA is passed at an organismic level when the DNA from the sperm cell and egg cell combine to form a new individual with genetic information from both parents.

A DNA’s structure is made up of a long strand of paired nucleotides that are abbreviated as A, T, G, and C. These nucleotides organize in a form that carries genetic information and is called a gene. Genes give instructions for protein formation which later helps in a cell’s functioning. DNA can be divided into different types based on its location in a cell. In eukaryotes, plants, and animals, the DNA is present inside the nucleus thus called Nuclear DNA. The DNA of mitochondria, the powerhouse of a cell, is called Mitochondrial DNA. Similarly, chloroplasts that are involved in photosynthesis in plants have their DNA called Chloroplast DNA. While in prokaryotes, DNA is present only at the centre of the cell called the nucleoid.

The whole set of DNA in a cell or an organism is referred to as its genome. While mitochondrial and chloroplast’s DNA is separate then the organismic genome.

Figure 1 DNA in a cell

Chromatin: In a cell, DNA exists in association with other interacting proteins to make its structure more compact. In eukaryotes, these proteins are positively charged histone proteins that wrap themselves around a long thin strip of DNA molecule and stabilises its structure. Moreover, histone proteins also help to identify the active and inactive genes in a DNA strand that are involved in cellular mechanisms. This complex of histones and DNA strand is called Chromatin.

Figure 2 DNA wrapped with the histone proteins (Clark, Choi and Douglas, 2018)

Chromatins spend most of their lifespan in a decondensed state i.e., in the form of thin strings that cannot be seen in the microscope. Chromatin becomes condensed only when the cell begins to divide. This condensation makes the DNA appear as a single long linear string of a chromosome under the light microscope.

Chromatins are further classified as euchromatin and heterochromatin based on the level of condensation and compaction.  Euchromatin is a less compact structure condensed up to 11nm fibre which is described as ‘beads on the string’ where beads refer to the nucleosomes and string to the DNA. Whereas heterochromatin is more compact and is condensed up to 30nm fibre.  Heterochromatin is further categorised into constitutive heterochromatin and facultative heterochromatin. Constitutive heterochromatin stays condensed throughout the cell cycle and does not actively participate. While facultative heterochromatin can uncoil to form euchromatin. This heterochromatin is more active and responds to cellular changes during the cell cycle. They contain genetic information which can be transcribed during cell division.

Figure 3 Heterochromatin and Euchromatin.

Chromosome and cell division: A chromosome is made up of identical chromatids which are referred to as sister chromatids. These sister chromatids are attached with the help of proteins called cohesins. The centromere is the site where the sister chromatids are joined. This region of DNA is important in the disjunction of chromatids at a late stage of the cell cycle. If the sister chromatids are connected through the centromere, they are considered a single chromosome. Whereas when they have pulled apart during the cell cycle, each one is considered a separate single chromosome.

Figure 4 Chromosomes after cell division.

The cells make their chromosomes undergo replication, condensation, and separation processes so that each cell contain an equal number of chromosomes after cell division.

Structure of Eukaryotic Chromosomes:

In eukaryotes, the chromosomes are present inside the nucleus in the form of large linear strands. These chromosomes mainly consist of two arms that are joined at the centromere. Each chromosome is structurally divided into three components: Pellicle, matrix and chromonemata.

• Pellicle: is an envelope that surrounds the material of chromosome.
• Matrix: is the actual substance a chromosome is made up of.
• Chromonemata: it is the substance present in the matrix of the chromosome. Each Chromonemata is made up of two spiral thread-like structures that are formed by a double helix of DNA.

Let us now discuss the structural features of chromosomes that appear during metaphase and can be seen through the light microscope. The main components of chromosome include centromere, secondary constriction, and telomeres.

• Centromere: It is a small constriction that is considered permanent in the structure of a chromosome. Another name for centromere is primary constriction which divides the structure of the chromosome into two arms; the short arm is called a ‘p arm’ and a long arm is called a ‘q arm’. Each centromere’s position is constant in a particular class of chromosomes and thus helps in identification. Furthermore, during the cell cycle, the chromosomes are attached to spindle fibres with the help of centromeres. Chromosomes can be further divided into different types based on location and number of centromeres.

Figure 5 A Sub-metacentric chromosome

• Secondary constriction: It is a type of constriction present on the chromosome’s structure other than primary constriction. The difference in primary and secondary constriction can be seen during the anaphase of the cell cycle. These constrictions also help in the identification of sets of chromosomes. These are also called nucleolar organiser as these constrictions are associated with the nucleolus of the cell.
• Telomeres: The repeated DNA sequences at the end of the chromosomes are called telomeres. They give the ends of chromosomes a particular polarity which prevents the other chromosomes to fuse with it. It is interesting to know that telomeres have a role in the life expectancy of an individual. Telomeres shorten with age. Shorter the telomeres, less in the lifespan of that individual. 

Structure of Prokaryotic Chromosomes:

• Prokaryotes do not have membrane-bound nucleus thus their chromosome is found in the cytoplasmic region called the nucleoid. A prokaryotic chromosome is single, circular DNA. Some of the prokaryotes for example Vibrio Cholerae have two circular chromosomes.
• Prokaryotic chromosomes lack histone proteins. Nucleoid associated proteins (NAPS) present in the nucleoid region of the cell play role in the folding of prokaryotic DNA. NAPs further help the prokaryotic cells in performing cellular mechanisms.
• Prokaryotic chromosomes lack homologous pairs. They are found in haploid form (1n, without paired chromosome).
• Prokaryotic cells differ in size and shape as compared to eukaryotic chromosomes.
• Prokaryotes also have an additional chromosome in their cells which is called a plasmid. A plasmid is a small, circular DNA that contains very few numbers of genes.  

Types of Chromosomes:

There can be different criteria for categorising different types of chromosomes.

• Autosomal and Sex Chromosome:

There are mainly two types of chromosomes. The sex chromosomes contain information regarding the sex of a person while all other genetic information is present in autosomal chromosomes. Humans have 22 pairs are autosomal chromosomes and one pair is of sex chromosomes which play their role in sex determination of an individual as well. Thus, a total of 46 chromosomes are present in each cell.

• Based on a different number of centromeres:
  • Monocentric: A chromosome with only one centromere.
  • Dicentric: A chromosome with two centromeres
  • Polycentric: A chromosome with more than two centromeres.
  • Acentric: A chromosomes with no centromere. These types of chromosomes refer to the newly broken segments of chromosomes that are unstable and are degraded.
  • Diffused or non-located: A chromosome that has diffused centromeres throughout its length.

• Based on different location of centromeres:
  • Telocentric: A chromosome that has a centromere at the end of its length thus making only one arm in the structure of the chromosome.
  • Acrocentric: A chromosome in which the centromere is occupying a sub-terminal position. This type of chromosome has one arm which is very long and another very short arm.
  • Sub-metacentric: A chromosome having centromere a little distant from the centre making both the arms unequal in length.
  • Metacentric: A chromosome having centromere at the centre making two arms of almost equal length.

Figure 6 Types of Chromosomes based on different location of centromere

Chromosome Number:

Chromosome number is different in different species. For example, a fruit fly has 8 chromosomes, humans have 46, a pea plant 14, a dog 39 and many others. It is not the number of chromosomes that makes a species unique rather it is the genetic information encoded in the DNA that mainly distinct the species. A full set of chromosomes of an organism can be viewed through photographs captured during the cell division. These images are termed karyotypes. Karyotypes help in studying the structure and an abnormal number of chromosomes in an individual.   

Function and Importance of Chromosomes:

Chromosome number is the same in a particular specie which helps in the taxonomic and phylogenetic categorization of organisms.

Role in Cell division: Chromosomes directs the successful cell division during mitosis. The parent chromosomes ensure that the transfer of correct information to the daughter cells which is important for a cell’s proper growth and survival.

Storage of Genetic Code: Chromosomes are made up of genes that contain genetic information required for a cell’s functioning. These units of genes on DNA encode for specific proteins required by a cell.

Protein formation and Storage: Chromosomes contain genetic code which is transcribed and further translated into proteins. Thus, ensures the proper functioning of a cell by making the required protein according to a cell’s need. Furthermore, a chromosome has proteins in its structure that are stored and help in the packaging of the DNA.

Sex Determination: Chromosomes play important role in the sex determination of an individual. There are many sex determination systems while XY sex-determination system is found in mammals, reptiles, and plants. Humans have a pair of chromosomes XY out of 46 chromosomes which are involved in sex determination. Human females are represented by two X chromosomes (XX) while human males are represented by XY chromosomes. In humans, sex determination is done by the chromosome contributed by the male. If the X chromosome is given out of XY, the individual will be female, and if the Y chromosome is given, the individual will be male.

Sex chromosome abnormalities: Abnormalities in sex chromosomes can result in gender-specific conditions. Here we will be discussing a few very common diseases:

• Turner’s Syndrome: This is a monosomy X condition in which a female has only one X chromosome instead of two in normal genotype. This results in various developmental and medical problems.
• Trisomy X Syndrome: This is a condition in which there are three X chromosomes instead of two. This can cause symptoms but most of the females also remain asymptomatic throughout their lives.
• Klinefelter Syndrome: This condition is of trisomy in which males have XXY chromosomes which is the presence of an extra X chromosome.

Chromosomal Mutations:

There are mainly two types of chromosomal mutations: mutation causing structural changes and mutations causing a change in chromosomal number.

Structural chromosomal Abnormalities: results from the incorrect joining or breakage of a chromosome that cause changes in nucleic acid sequence e.g., duplication or deletion or inversion of the gene sequence.

Mutation in Chromosomal Number: The mutations during the meiosis stage of the cell cycle results in an abnormal number of chromosomes i.e., either greater or less than the actual set of chromosomes. For example, an extra chromosome on autosome 21 causes down syndrome.

Interesting Facts:

• All organisms do not have sex chromosomes for example organisms like wasps, bees etc. In such organisms, sex is determined by the fertilization of the egg. A fertilised egg develops into a male and unfertilised eggs are developed into females. This type of reproduction is a form of parthenogenesis.
• The y chromosome is smaller than the X chromosome. The size of the Y chromosomes is almost one-third of that of the X chromosome. Thus, the X chromosomes express 5% of the cell’s DNA while the Y chromosomes depict only 2% of the overall genetic material of the cell.
• Males have enhanced X chromosome activity because of having only one X chromosome. Thus, with the help of different protein complexes X chromosome’s expression is increased.
• Chromosomal damage is not repaired during the cell cycle because the cell does not recognise the damaged DNA at that stage. Repairing the damaged DNA during mitosis may cause telomeres to fuse which is result in cell death.
• Shortening of the telomeres of chromosomes is linked with ageing and cancer cell development.
• Humans have almost 30,000 genes in just 46 chromosomes.
• Some organisms have a lot of DNA but most of it is blank which is called junk DNA. The clear function of junk DNA is still unknown.

Reference list

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