DEOXYRIBONUCLEIC ACID(DNA) AND CHROMOSOME

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Hi hivers, I have come with another interesting post on genetics. I will be talking majorly on DNA today but I will like to share a piece of useful information before I proceed.

Do you know why we have more female children than male? Let me tell you if you don't know it.

Male has XY chromosome while female has XX chromosome. The X stands for female while Y is for male. The X chromosome is very weak while Y chromosome on the other hand is very strong. Linking this with the process of fertilization, there is tendency that the Y chromosome will die before X chromosome when competing for the sperm to fertilize the egg thereby, giving chance for more female children than male ones. Now, let's proceed to today's business which is DNA.

What is DNA?

DNA is a double helical structure made of two polynucleotide chains coiled around each other with each chain forming a right-handed helical spiral. The chains run in opposite direction to each other i.e they are anti-parallel.
The three basic components of DNA are;

  • Nitrogenous Base
  • Pentose Sugar
  • Phosphate or Phosphoric Acid

NITROGENOUS BASE
There are two types of nitrogenous bases;
1.) Purine
2.) Pyrimidine

The purine are the Adenine and Guanine. They have two rings each and bigger than pyrimidine.

The pyrimidine are Thymine and Cytosine. They have one ring each, so they are smaller than the purine.

Adenine_group.svg.png

Adenine: sourced from wikimedia.org

Guanine.svg.png

Guanine: sourced from wikimedia.org

558px-Thymine_chemical_structure.png

Thymine: sourced from wikimedia.org

448px-Cytosine_chemical_structure.png

Cytosine: sourced from wikimedia.org

PENTOSE SUGAR
This is a sugar with 5-carbon atoms from which an oxygen molecule had been removed.

PHOSPHATE(PHOSPHORIC ACID)
The phosphate gives the DNA its acidic nature.

THE ARRANGEMENT

The deoxyribose combines with a base and with a phosphate to form a nucleotide. According to Chargaff's Base Pairing Rule, Adenine pairs up with thymine and Guanine pairs up with cytosine. The two strands are held together by hydrogen bond.

images (1).png

image of Nucleotide

Sugar and phosphate form the backbone from which the bases project at right angle. The bases of one chain are held to the other chain at the center. The distance between the two backbone is constant and equal.

CHEMICAL NATURE OF DNA

The DNA molecule is held in one piece by means of some chemical bonds. They are;

  • Hydrogen bonds
  • Covalent bond
  • Hydrophobic interaction.

HYDROGEN BONDS
These are weak bonds found in a DNA duplex between two complementary bases where they bind the two chains together.
They are also found between surface atoms of the DNA and the surrounding water molecule in the cell. This bond confers stability on the DNA.

COVALENT BOND
These are strong bonds that link nucleotide together. Covalent bonds in the phosphodiester backbone of each chain makes the chain relatively resistant to breakage because the chemical bond requires a high energy for the damage.

HYDROPHOBIC INTERACTION
They are found among the nitrogenous bases such that they are stacked like plate on top of each other excluding completely water molecules from the interior of the DNA.

THE STRUCTURE OF THE CHROMOSOME

Analysis of chromosome of eukaryotic cell have shown them to be composed of deoxyribonucleic acid(DNA) and protein with small amount of chromosomal RNA(Ribonucleic Acid) while the chromosomes of prokaryotic cells are composed of DNA only.
The haploid human genome contains approximately 3 billion base pairs of DNA packaged in 23 chromosomes. Most cells in the body( except female ova and male sperm) are diploid with 23 pairs of chromosomes.
The DNA has negative charges along its length and positively charged DNA protein molecules called HISTONE are bound to it. This DNA-protein complex is called chromatin.
The large amount of DNA in cells means that there is a packaging problem. A human cell contains about 2.2meters of DNA distributed among 46chromosomes. Each cell therefore contains about 2.4meters of DNA. The human chromosomes are on the average of 6micrometers long. In order to maintain a high degree of organization when the DNA is folded, the histone protein forms a precise architectural scaffolding for the DNA.
It has been shown that the DNA-helix combines with 8 histones molecules to form a structure called NUCLEOSOME, to give an appearance of beads on a string. These nucleosome and the DNA strands linking them are packed closely together to give 30nm in diameter helix with about 6 nucleosomes per turn. This is known as the SOLENOID FIBRE. Since the DNA must be more tightly packed than the solenoid, then the solenoid themselves must be folded again into loops. This loop may represent the active DNA.

Chromosome.svg.png

chromosome: sourced from wikimedia.org

TYPES OF CHROMOSOME BASED ON THE LOCATION OF CENTROMERE.
Centromere can be located at any point on the lenght of the chromosome.

Centromere_Placement.svg.png

types of chromosomes based on location of centromere: sourced from wikimedia.org

B is the centromere while A and C are the arms.

I is the TELOCENTRIC: Here, the centromere is located at the extreme end where A arm is almost equal to zero.

II is the ACROCENTRIC: If the centromere is located in which there is a distinct long arm or very short arm. It is known as Acrocentric.

III is the SUBMETACENTRIC: Whenever the arms are very close in length but not equal, they are said to be Submetacentric.

IV is the METACENTRIC: It is Metacentric if both arms are equal in length and none is longer than the other.

I will like to stop here for this topic. Hope you've added something new to your previous knowledge? Knowledge is power, get it everyday!

Thanks for reading through and don't forget to stay safe always!

References

Introduction to genetics
Structure of DNA
What is DNA?
What is a chromosome- Genetics Home Reference- NIH



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