DNA & Protein Synthesis

The Genetic Code: The understanding of how genetic traits are carried and transferred into actual characteristics.

Molecular Genetics

  • Mendel discovered that hereditary factors, now known as genes, are the units of heredity (inheritance). These genes are found within the nuclear structures called chromosomes.

    • Chromosomes exist in various types found within the nucleus of each cell. Typically, there are two copies of each chromosome type (this is called diploid).

    • All somatic (body) cells in an organism have the same number of chromosomes.

    • The number of chromosomes seems to double before a cell divides.

    • Sex cells (sperm and egg) contain exactly half the usual number of chromosomes (haploid).

    • Fertilization results in a diploid cell known as a zygote, which has the same number of chromosomes as the somatic cells of that organism.

  • The Requirement of Genetic Material:

    • It must be able to replicate.

    • It must be able to control expression of traits.

    • It must be able to change in a controlled way.

    • It is universal among biological organisms.

  • Nucleic Acids:

    • Nucleic Acids were first discovered by Friedrich Miescher in 1869, where he referred to the nitrogen and phosphorus-rich material as 'nuclein'. It is recognized as the largest biochemical molecules and is now known as nucleic acid. There are two primary forms of nucleic acids:

      1. Deoxyribonucleic Acid (DNA): This type of nucleic acid is found in the nuclei of both somatic cells and gametes (sperm and egg) and there are variations of DNA present in mitochondria and chloroplasts.

      2. Ribonucleic Acid (RNA): This nucleic acid has various types and functions throughout the cell.

      The structure of DNA was determined to be a double helix by Watson and Crick in 1953.

  • Nucleotides

    • Nucleotides, which are the basic units of DNA, are arranged in long chains linked by sugar and phosphate, creating a sturdy backbone.

    • Each chain of nucleotides serves as a mirror image of the other chain.

    • In the structure of a DNA double helix, the two chains of nucleotides are connected by hydrogen bonds:

      • Adenine (A) bonds with Thymine (T) using two hydrogen bonds.

      • Guanine (G) bonds with Cytosine (C) using three hydrogen bonds.

  • Sugars in DNA or RNA

    • The sugars that contribute to the backbone of nucleotides are pentose sugars. However, the specific type of sugar present depends on the structure of the nucleic acid: deoxyribonucleic acid (DNA) contains deoxyribose, while ribonucleic acid (RNA) contains ribose.

Image result for video nucleotides

  • Nitrogenous Bases

    • DNA contains four nitrogenous bases:

      • Thymine (T)

      • Guanine (G)

      • Cytosine (C)

      • Adenine (A)

    • RNA contains four nitrogenous bases:

      • Uracil (U)

      • Guanine (G)

      • Cytosine (C)

      • Adenine (A)

    • There are two classes of nitrogenous bases:

      1. Purines (double ring structure - larger):

        • Guanine (G)

        • Adenine (A)

      2. Pyrimidines (single ring structure - smaller):

        • Thymine (T)

        • Cytosine (C)

  • Base Pairing

    • Adenine always pairs with Thymine because they form 2 hydrogen bonds.

    • Cytosine always pairs with Guanine due to forming 3 hydrogen bonds.

    • The size of each base and the number of hydrogen bonds influence their pairing, ensuring uniformly sized ‘rungs’ in the DNA structure.

    • Consistent base pairing prevents the DNA molecule from becoming kinked or irregularly shaped.

    • The angles in the DNA backbone contribute to the overall stable twisting helical shape of the DNA.


  • Ribonucleic Acid

    • RNA (Ribonucleic Acid) is a single-stranded molecule composed of shorter segments, unlike the long, continuous structure of DNA. It is present throughout the cell, including both the nucleus and cytoplasm. There are three main types of RNA:

      • Messenger RNA (mRNA): This type serves as a copy of DNA and carries genetic information from the nucleus to the cytoplasm.

      • Transfer RNA (tRNA): The smallest type, tRNA interacts with mRNA to help assemble amino acids into proteins.

      • Ribosomal RNA (rRNA): Found in ribosomes, this type of RNA plays a crucial role in the assembly of polypeptides (proteins).

  • MAJOR DIFFERENCES BETWEEN DNA & RNA

  • DNA; IT IS TOO LONG, THEREFORE, NEEDS TO CONDENSE.

    • DNA winds around histones to form a bead like structure.

    • The bead-like structure coils itself to form tightly packed strands of chromatin.

    • Chromatin will super coil itself in order to create a visible chromosome. This is easier to move in the cell.

  • Central Dogma

    • This dogma forms the backbone of molecular biology and is represented by the stages below.

      • DNA replicates.

      • DNA codes are transcribed in the production of messenger RNA (mRNA)

      • the mRNA migrates from the nucleus to the cytoplasm

      • mRNA carries coded information to ribosomes for use in protein synthesis.

DNA → TRANSCRIPTION → RNA → TRANSLATION → PROTEINS

  • DNA Replication; in a nutshell.

    • The two strands of the double helix are first separated by enzymes. 

    • Other enzymes bind nucleic acids (nucleotides found inside the cytosol) to the individual strands following the rules of complementary base pairing: adenine (A) to thymine (T) and guanine(G) to cytosine(C).

    • Two strands of DNA are obtained from one, each strand identical to one another and to the parent molecule.

  • DNA Replication Theories

    • Conservative replication would leave intact the original DNA molecule and generate a completely new molecule.

    • Dispersive replication would produce two DNA molecules with sections of both old and new DNA interspersed along each strand. 

    • Semi conservative replication would produce molecules with both old and new DNA, but each molecule would be composed of one old strand and one new one. 

  • DNA Replication

    • DNA replication is semi conservative

      • Each strand acts as a template for the synthesis of a new DNA molecule by the sequential addition of complementary base pairs

      • A new DNA strand is generated that is a complementary sequence to the parental DNA.

      • Each newly formed DNA molecule ends up with one of the original strands and one newly synthesized strand.

  • DNA Replication; The 5 Basic Steps

    • The two parent strands are unwound and cleaved with the help of DNA helicases.

    • Single stranded DNA binding proteins (SBPs) attach to the unzipped strands, preventing them from winding back together

    • The strands are held in position, binding easily to DNA polymerase, which catalyzes the elongation of the leading and lagging strands

    • The DNA polymerase on the leading strand operates in a continuous fashion - RNA primer is needed repeatedly on the lagging strand to facilitate synthesis of Okazaki fragments

    • Each new Okazaki fragment is attached to the completed portion of the lagging strand in a reaction catalyzed by DNA ligase.

  • DNA Replication (3 Steps Occur)