Main Difference – Chromosome vs Gene
DNA carries the genetic blueprint and decides the phenotype of an organism. It is also considered as a book of history, which narrates the evolution of a species. DNA generally exists as a double-helix. A chromosome is considered as the highest organized form of DNA. A gene is a locus on a chromosome which encodes proteins. The main difference between chromosome and gene is that a chromosome is the most packed structure of a DNA with proteins whereas a gene is a segment of DNA which is located on a chromosome. A single chromosome consists of many genes whereas a gene is a locus on a chromosome.
This article studies,
1. What is a Chromosome
– Definition, Characteristics
2. What is a Gene
– Definition, Characteristics
3. What is the difference between Chromosome and Gene
What is a Chromosome
A chromosome is the most condensed structure of a DNA double-helix with proteins. A typical chromosome stores thousands of genes. Prokaryotes consist of a single, circular chromosome packed with histone-like proteins. Eukaryotes consists of large, linear chromosomes. DNA strand of about 145 bp in size is wrapped around a core which is made up of eight histone proteins, forming a structure called nucleosome. The DNA strand at this level is lesser condensed and exhibits a thread-like structure. Nucleosomes fold up and coil to produce a 250 nm fiber called chromatin. The accessibility to the DNA sequence of a particular gene at this level may regulate the expression of that gene. Chromatin is further coiled into a 30 nm fiber, forming the chromosome. This packing provides the structural integrity to DNA double-helix. Humans have 46 chromosomes in their genome: 22 homologous pairs of autosomes plus 2 sex chromosomes. Other than genes, a chromosome contains regions such as origins of replication, centromere and telomeres. Replication origins initiate the DNA replication. The centromere ensures the storage of all f chromosomes within a daughter cell during the chromosomal segregation event. Telomeres protect the genes at the end regions of a chromosome from being truncated. Chromosomes appearing at the metaphase are used to generate karyotypes where the chromosomal abnormalities are analyzed.
Figure 1: Chromosomes
What is a Gene
A gene is a region (locus) of genome sequence which encodes a specific protein. DNA is transcribed into mRNA; mRNA translated into proteins is collectively known as the central dogma of molecular biology. The gene is referred to as the molecular unit of heredity. The modern concept of the gene originates with the studies on the inheritance of characteristics by Gregor Mendel in 1860s.
The majority of organisms utilize DNA as their genetic material. Generally, human genome consists of about 20,000 genes. The structure of a gene is mainly composed of two segments, coding sequence and the regulatory sequence. Coding sequence contains exons and introns. Prokaryotes do not have introns. In eukaryotes, introns are removed at consequent splicing of exons. Multiple proteins can be achieved by alternative splicing from a single gene. Coding sequence also contains untranslated regions of RNA. A regulatory sequence consists of a promoter region, enhancers and inhibitors. In prokaryotes, several genes group to form operons. An operon consists of multiple protein-coding sequences which are transcribed together. Some viruses are completely made up of RNA genomes. Their genes are called RNA genes.
The expression of genes can be regulated at either transcriptional level or translational level. The nucleotide sequence which specifies an amino acid sequence of a protein is referred to as the genetic code. Organisms inherit their complete set of genes through reproduction. Mutations occurring in gene sequence lead to the different variants of the same gene. Variations of a gene are called alleles. Alleles may produce variations of trait inside a population. Alleles are either dominant or recessive. Most alleles undergo Mendelian inheritance.
Figure 2: Chromosome and Gene
Difference Between Chromosome and Gene
Definition
Chromosome: A chromosome is the most condensed structure of a DNA molecule with proteins.
Gene: A gene is a locus on a chromosome.
Genetic Material
Chromosome: A chromosome is always composed of DNA.
Gene: A gene can be composed of either DNA or RNA.
Mutation influence
Chromosome: Chromosomal mutations are relatively large since they occur in homologous recombination.
Gene: Gene mutations are small, sometimes silent. They occur either in DNA replication or consequent DNA damage.
Examples of Mutations
Chromosome: Chromosomal mutations lead to chromosomal abnormalities such as duplication, deletion, rearrangement and inversion of genes.
Gene: Gene mutations include point mutations and frameshift mutations: insertions and deletions.
Conclusion
Chromosomal mutations occur due to the errors in the homologous recombination event. Hence, it changes large regions of a chromosome. For example, duplication of a particular gene may accumulate its gene product, both RNA and the protein. If this over-expression occurs in an oncogene, it could lead to the development of cancer. Therefore, these mutations bring a considerable effect to the organism. However, gene mutations occur in base pair level. Most gene mutations are silent, synonymous or conservative. They have lesser influence in their protein function. A few may be deleterious or lethal. The effectiveness of the mutation also relates to the main difference between chromosome and gene, i.e., the size of the region they occupy in a genome.
Reference:
1. Susman M. “Genes: Definition and Structure.” ENCYCLOPEDIA OF LIFE SCIENCES, Nature Publishing Group, 2001, http://smcg.ccg.unam.mx/enp-unam/03-EstructuraDelGenoma/geneDefinition.pdf. Accessed 09 Feb. 2017
2. Higgins N. P. “Chromosome Structure.” ENCYCLOPEDIA OF LIFE SCIENCES, Macmillan Publishers Ltd, Nature Publishing Group, 2001, http://smcg.ccg.unam.mx/enp-unam/03-EstructuraDelGenoma/cromosomeStructure.pdf. Accessed 09 Feb.2017
3. Schleif R. Genetics and Molecular Biology. 2nd ed., The Johns Hopkins University Press, 1993, pp. 22-47, http://gene.bio.jhu.edu/bm2whole.pdf. Accessed 09 Feb. 2017
Image Courtesy:
1. “Chromosome DNA Gene”By Thomas Shafee – Own work (CC BY 4.0) via Commons Wikimedia
2. “Chromosome en”By File:Chromosome-es.svg: KES47 (talk)derivative work: KES47 – File:Chromosome-es.svg (CC BY 3.0) via Commons Wikimedia
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