by Negash Alamin

A chromosome is one of the components of the cell inside the nucleus which codes for proteins and controls over all phenotypic traits of the organism. In Genetics there are two different terminologies you probably heard being used several times: Genotype and phenotype. In Genotype what is meant to say is that the organism’s genetic configuration; but, phenotype denotes the amino acids produced by the genetic codes of the organism that we can see on the outside. The chromosome is actually a wrapped and compacted strip of an important element called DNA (deoxyribonucleic acid). In humans we have 23 haploid and 46 diploid chromosomes. There are 22 autosomal chromosomes in a haploid cell starting from the first chromosomes on the ideogram to the 22nd and the last 23rd is called sex chromosome because, it determines the sex of the organism. Male is XY and female is XX.


The DNA which you can think of as unraveled chromosome is a ladder type structure made from nucleotides, phosphates and sugars; these components organize together to form the DNA molecule. Now, let us see important terms that will help you make sense of the overall field of molecular biology.



Important terms to Remember in molecular biology

Nucleotide: a molecule composed of Pentose sugar, a phosphate group and a nitrogenous base.

Base: any one of the organic molecules adenine, thymine or uracil in RNA, guanine, and cytosine.


Gene: a specific genetic code of an organism. Just like a computer has a one and zero code; the DNA also has its own code A-T and G-C. These nucleotides adenine and thymine as well as Guanine and cytosine ALWAYS go together.

Genome: a complete genetic material of an organism.

Amino acid: The elementary component of a protein that makes up our body like skin, hair, and other organs.

Helicase: an important enzyme that unwinds the Double helix or double stranded DNA preparing it for transcription into messenger RNA.

Primase: also another important enzyme that provides a short complementary RNA to the DNA template strand so that it can start transcription process.

DNA polymerase III: An enzyme which adds complementary nucleotides from the surrounding to the back bone on the DNA template in the transcription process creating the Okazaki fragments.

RNA: a different form of DNA material with only a single strand which has Uracil in place of thymine and has and extra oxygen at its sugar back bone unlike DNA. Types of RNA include messenger RNA, transfer RNA and ribosomal RNA.

Transcription: The process of the conversion of genetic data that we talked about above in chromosomes from template DNA into messenger RNA. It is performed in three steps: initiation elongation and termination.

Allele:  A variation in DNA sequence in a locus on the chromosome. Alleles can be identical/ homozygous or different/ heterozygous and if only one allele is present it is called hemizygous and hemizygousity determines maleness or femaleness.

Kinetochores: Is the overall attachment point of the sister chromatids, by sister chromatids I mean for example on the first chromosome you can see two separate treads labeled as 1; these are sister chromatids; but, the constriction in the chromosome in very center―where the attachment occurs, is called centromere.

Metacentric: The centromere of a chromosome is located in the middle of the chromosome.

Telocentric: The centromere is located at the end of chromosome.

Acrocentric: The centromere is at very end of the chromosome.


Locus: Simply means a position on the chromosome where genes are found.

Gene expression: refers to the process of transcription of DNA into messenger RNA, then translation of this code to amino acid sequence or proteins.


Once, you understand these terms as a beginner in cellular biology and genetics then you can move on to advanced courses and topics like chromosomal abnormality, mutations, interspecies genetics study or what is called evolutionary genetics and so forth; but, in order to accomplish this you need to understand the basic concepts and terminologies of overall genetics which is invaluable in the overall understanding of life processes. I did not write too much text because I believed the pictures will explain more than words; so, I have inserted several diagrams and pictures into this article in a hope that the reader will get a complete knowledge of elementary genetics which is really a super interesting and fascinating subject on the natural world.


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