Lesson 1: Genetic Materials and The Structure and Functions of DNA and RNA.

Video Lesson:

Competences

After the successful completion of this section, the student will be able to:

• Define genetics, genes, chromosomes, DNA, and RNA.
• Describe the structure and functions of DNA and RNA.

Brainstorming Questions:

1. Before starting this section, ask yourself this question: “What do I know about genetic material and what do I want to learn from
   this section?”

Key terms:

Genetic Materials, DNA, and RNA

• The branch of biology concerned with the study of the genetic materials of organisms and how traits are passed from one generation to the next generation through genes is called genetics. The genetic material of an organism refers to material that carries genetic information and passes it from one generation to the next generation to perpetuate life. The genetic material in almost all organisms is DNA. RNA is also a genetic material in some viruses like HIV, COVID-19.
• DNA (deoxyribonucleic acid) is the hereditary material in humans and other organisms. It exists in a double helix formed by base pairs attached to a sugar-phosphate backbone. RNA (ribonucleic acid) serves as the genetic codes in some viruses. It is involved in protein synthesis in cells.

  The structure and function of DNA and RNA

• Both DNA and RNA have their own structures and important role in determining the characteristics of organisms.

                          The Structure and function of DNA

                          The structure of DNA

• The structure of DNA is a ladder-like double helix twisted into a spiral shape, in which the sugar and phosphate groups form the two vertical ladder and the nitrogenous bases form the ladder’s rungs. It consists of two long chains of chemicals called polynucleotide that twist around each other to form a double helix. Nucleotides are the basic building blocks of a DNA molecule.

Each nucleotide is composed of a sugar, phosphate group and a nitrogenous base. There are four types of nitrogenous bases. These are: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). The nitrogen bases belong to the two large chemical families called purine and pyrimidine. The A and G are purines and the C and T are pyrimidine’s. A pairs with T and C pairs with G to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule to form a nucleotide, the building blocks of the DNA called nucleotide.

In the DNA structure, the sugar (Deoxyribose) and phosphate form the backbone of the DNA molecule and the nitrogenous bases form hydrogen bonds between the two strands (backbone), to form a ladder-like structure. Each strand of DNA shows polarity (two ends are different). The one is referred as 5’ end and the other is 3’ end. The two strands of DNA run in opposite directions. The strands are helically twisted where each strand forms a right-handed coil. James Watson and Francis Crick discovered the double helix structure of a DNA molecule. The following diagram explains the DNA structure that represents its different part.

1.1 Genes and chromosomes

Sections of the DNA structure that contain the set of instructions that determine the characteristics of an organism are called genes. Genes are the basic structural and functional units of inheritance in nature. Genes pass from parents to offspring during both sexual and asexual reproduction through cell division. Genes are located on chromosomes. Chromosomes are threadlike structures made of a protein called histone and DNA molecule. Each chromosome may contain hundreds to thousands of genes that are arranged linearly along the length of each chromosome (like beads on a string), with each gene having its own unique position on to chromosomes called locus or loci.

Chromosomes exist in pair in diploid organisms in which one chromosome is always inherited from the mother and the other from the father. For example a human cell contains 46 chromosomes which exist in 23 pairs of chromosomes. Human Karyotype, male and female looks like:

1.2 The Function of DNA

• The function of DNA is to store all of the genetic information that an organism needs to grow, develop, reproduce, control the cell and survive. While DNA determines the characteristics of an organism, it is also responsible for carrying and transmitting the hereditary materials or the genetic instructions from parents to the offspring. The transmission of this information from the mother to daughter cells occurs through the process of DNA replication during cell division.

 1.2.1 DNA replication

• DNA replication is the process by which DNA makes a copy of itself during cell division. DNA has a unique property of replication or production of carbon copies. This is essential for transfer of genetic information from one cell to its daughters and from one generation to the next. DNA gives rise to RNAs through the process of transcription.
• DNA replication is a semiconservative, which means that each strand in the DNA double helix acts as a template for the synthesis of a new, complementary strand. In other words, the two original DNA strands separate during replication; each strand then serves as a template for a new DNA strand. Each newly synthesized double helix is a combination of one old and one new DNA Strand. DNA replication involves the following enzymes.
• Table that shows enzymes and their functions is:

• Replication fork: A structure that forms within the long helical DNA during DNA replication is called replication fork. It is the point formed due to unwinding and separations of two strands appear like Y –shaped fork is called replicating fork.
• Leading strand: the strand of new DNA, which is synthesized in the same direction as the growing replication fork.
• Lagging strand– the strand of new DNA whose direction of synthesis is opposite to the direction of the growing replication fork.
• There are three stages in DNA replication. These are:
• Stage one – the DNA helix structure is unwound and unzipped, hydrogen bonds between bases, which are holding the two strands together break and the double helix structure of the DNA molecule separate in to two strands.
• Stage two – The two separated strands will act as templates for making the new strands of DNA. DNA polymerase will add the free DNA nucleotides using complementary base pairing (A-T and CG). One of the strands is synthesized in the same direction as the growing replication fork (leading strand). DNA polymerase adds nucleotides to the deoxyribose (3’) ended strand in a 5’ to 3’ direction. The other strand synthesizes opposite to the direction of the growing replication fork (lagging strand).
• Stage three– The two new strands twist to form a double helix. Each is identical to the original strand.

1.2.2 The structure and function of RNA

The structure of RNA RNA has single strand structure. RNA contains the sugar ribose, phosphates, and the nitrogenous bases adenine (A), guanine (G), cytosine (C) and uracil (U) which replaces thymine in DNA. There are three most well-known types of RNA in all organisms. These are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA). All types of RNAs are formed on DNA strands by transcription process. In the next section, we shall see the role of each type of RNA in protein synthesis.

1.2.3 The function of RNA

• RNA is the most important molecule in all lives. RNA is involved in a variety of functions within the cell and is found in all living organisms. RNA functions in protein synthesis and used as a storage of genetic information in some viruses. RNA facilitates the translation of the DNA into different proteins required by organisms. For example, it serves as a messenger in conveying instructions between the DNA and the ribosome during proteins synthesis.