DNA: The Code of Life - Comprehensive Notes

TOPIC 1: DNA - THE CODE OF LIFE

Topic Outcomes: DNA

  • Location

  • Discovery

  • Structure

  • Function

  • DNA replication

    • When?

    • Where?

    • How?

    • Why?

RNA

  • Location

  • Structure

  • Function

  • Protein synthesis (transcription & translation)

Revision of Cell Structure

  • A cell is the smallest, basic unit of life responsible for all life processes.

  • Each cell contains cytoplasm, a fluid enclosed by a membrane.

  • Cytoplasm contains biomolecules like proteins, nucleic acids, and lipids.

  • Organelles are suspended in the cytoplasm.

Important Structures

Ribosomes

  • Small, round organelles attached to the Endoplasmic Reticulum (ER) or free-floating in the cytoplasm.

  • Found in chloroplasts & mitochondria.

  • Made of protein & RNA.

  • Produce proteins from amino acids during protein synthesis.

  • Site for protein synthesis.

Cytoplasm

  • Fluid part of the cell outside the nucleus and inside the cell membrane.

  • Base substance in which organelles are suspended.

  • Allows metabolic reactions to take place.

Nucleus

  • Double nuclear membrane encloses the nucleus, containing nuclear pores for substance passage.

  • Nucleoplasm: jelly-like fluid within the nucleus.

  • Nucleolus: a dark body in the nucleoplasm containing free nucleotide bases & produces ribosomes.

  • Chromatin network: contains DNA, which forms chromosomes containing the genetic code.

Nucleic Acids

  • Organic compounds responsible for inheritance and transmission of specific characteristics.

  • Contain carbon, hydrogen, oxygen, nitrogen, and phosphorus.

  • Two types in the human body:

    • DNA (deoxyribonucleic acid)

    • RNA (ribonucleic acid)

  • Form the basis of all life on Earth.

Basic Structure of a Nucleotide

  • Nucleic acids consist of monomers called nucleotides.

  • Nucleotides are composed of:

    • A pentose sugar (deoxyribose/ribose)

    • A phosphate ion

    • A nitrogenous base.

DNA: Deoxyribonucleic Acid

A Brief History of the Discovery of the DNA Molecule

  • DNA was first discovered by Friedrich Miescher.

  • Watson and Crick furthered Miescher’s discovery nearly 100 years later.

Timeline of DNA Structure Discovery

  • 1869: Friedrich Miescher identified 'Nuclein'.

  • 1881: Richard Altmann renamed 'nuclein' as a 'nucleic acid'.

  • 1910: Albrecht Kossel isolated adenine (A), cytosine (C), guanine (G), thymine (T) and uracil (U). Nobel Prize in Physiology or Medicine was awarded.

  • 1919: Phoebus Levene proposed that nucleic acids were composed of a series of nucleotides, each containing nitrogen-containing bases, a sugar molecule, and a phosphate group.

  • 1950: Erwin Chargaff concluded that the same nucleotides do not repeat in the same order and that the amount of A=T and G=C (Chargaff's rule).

  • 1952: Raymond Gosling, supervised by Maurice Wilkins and Rosalind Franklin, obtained X-Ray diffraction image of A-DNA (under low humidity) and B-DNA (at higher humidity).

  • 1953: James Watson and Francis Crick created the 3D model of double helical DNA. In 1962, they were awarded the Nobel Prize in Physiology or Medicine along with Maurice Wilkins.

1952 - Rosalind Franklin & Maurice Wilkins

  • Researched the structure of DNA using X-Ray diffraction images.

  • Photo 51 revealed that DNA has a double helix shape.

  • X-ray based fiber diffraction image of a paracrystalline gel composed of DNA fiber taken by Rosalind Franklin and her graduate student Raymond Gosling in May 1952.

  • Franklin was recruited to work on the structure of DNA.

  • The image was tagged "photo 51" because it was the 51st diffraction photograph that Franklin had taken.

  • It was critical evidence in identifying the structure of DNA.

1953 – Watson & Crick

  • Used Franklin's images and created a 3-D double helix model for DNA.

  • Received the Nobel Prize for the discovery of the structure of DNA in 1962.

Location of DNA

  • DNA can be found in three organelles:

    • Nucleus: Nuclear DNA comes in the form of long, linear pieces of DNA called chromosomes.

    • Mitochondrion: Mitochondrial DNA is inherited from the mother and can be used to trace ancestry.

    • Chloroplast: Chloroplast DNA is only found in plants.

Structure of DNA

  • The natural shape of a DNA molecule is a DOUBLE HELIX of two complementary strands.

  • Each strand of the helix is made up of a sequence of DNA nucleotides (monomers).

Nucleotides of a DNA Molecule

  • Phosphate

  • Deoxyribose sugar

  • Nitrogenous base:

    • Weak hydrogen bonds (between nitrogenous bases)

Nitrogenous Bases in DNA

  • Each nucleotide contains one of four different nitrogenous bases.

  • These bases play an important role in storing information from one generation to the next.

  • DNA contains the following nitrogenous bases:

    • Adenine (A)

    • Guanine (G)

    • Cytosine (C)

    • Thymine (T)

Purines & Pyrimidines

  • These bases are divided into TWO groups: Purines & Pyrimidines

  • Purines have a double-ring structure while pyrimidines have a single-ring structure.

  • A specific purine always bonds with a specific pyrimidine!

    • Adenine bonds with Thymine

    • Guanine bonds with Cytosine

Pairing of Nitrogenous Bases in DNA

  • Because nitrogenous bases are complementary, the order of the bases in one strand determines the order of the bases in the other strand.

    • Adenine (A) bonds with Thymine (T)

    • Guanine (G) bonds with Cytosine (C)

  • These bases are held together by weak hydrogen bonds and form a ladder-like structure.

Functions of DNA

  • Responsible for the inheritance of genetic information in all living beings.

  • Plays a crucial role in the production of proteins.

  • Nuclear DNA, contained within the nucleus of every cell in a eukaryotic organism, codes for the majority of the organism’s genomes.

  • Mitochondrial DNA and plastid DNA handle the rest.

  • Most of the DNA strands do not code for anything – Non-coding DNA.

DNA Replication

  • DNA replication is the process through which DNA makes an identical copy of itself.

  • Occurs during Interphase (before mitosis & meiosis) of the cell cycle in the nucleus.

  • Ensures that daughter cells are identical to the parent cell.

  • DNA must be replicated correctly to avoid mutations.

  • It is controlled by the enzyme: DNA helicase.

The Process of DNA Replication

  • Double helix DNA unwinds.

  • Separates into two strands of DNA, forming a ladder-like structure.

  • The weak hydrogen bonds between the nitrogenous bases break.

  • The two DNA strands separate from each other (DNA unzips).

  • Nitrogenous bases are exposed.

  • Each original DNA strand serves as a template to form a new strand.

  • Free-floating DNA nucleotides from the nucleoplasm attach to the original strand.

  • The nucleotides attach to their complementary bases (A to T and G to C).

  • Hydrogen bonds reform between bases.

  • Each DNA molecule now consists of one original strand and one new strand.

  • The result is two genetically identical DNA molecules.

  • DNA rewinds into a double helix.

  • Errors during DNA replication may lead to mutations (a change in the nitrogenous base sequence).

  • If the incorrect nitrogen base attaches to the original strand (i.e., if a nitrogen base is added or deleted), the sequence or order of the bases changes on the new DNA molecule, resulting in a change in the gene structure (gene mutation).

Importance of DNA Replication

  • Ensures that the daughter cells in mitosis will have identical genetic make-up as the parent cell.

  • Ensures chromosome number in each daughter cell is the same number in the parent cell.

  • Ensures genetic properties are transmitted from one generation to the next.

DNA Profiling

  • DNA profiling is a method of identifying an individual based on the sequence of nucleotides in each person's DNA.

  • DNA profile is a pattern produced on an X-ray film.

  • The pattern consists of bars which are of different lengths & thicknesses and in different positions.

  • All individuals have a unique DNA profile except for identical twins.

  • Biological samples: Blood, hair follicles, semen, saliva, nails, cheek cells & skin.

Uses of DNA Profiles

  • Identify crime suspects in forensic investigations

  • Prove paternity (father) & maternity (mother)

  • Determine the probability or causes of genetic defects

  • Establish the compatibility of tissue types for organ transplants

  • Identify relatives

Interpreting DNA Profiles

  • Forensics: All the bands of the DNA sample must match exactly with that of the individual in question.

  • Paternity: Each band of the child must match either that of the mother or of the potential father. If the child has a band that does not match that of either parent, then that excludes that male as the father.

Disadvantages of DNA profiling

  • Human error – Humans interpret the results which means mistakes can be made

  • Small piece of DNA is used, so the profile might not be 100% unique to a particular individual

  • It is expensive, therefore not readily accessible to those who cannot afford it

  • It can be used to frame innocent people

  • Not everybody's DNA has been profiled

RNA: Ribonucleic Acid

Location of RNA

  • Messenger RNA (mRNA): formed in the nucleus but enters the cytoplasm where it attaches to ribosomes.

  • Ribosomal RNA (rRNA): found in the ribosomes in the cytoplasm of the cell.

  • Transfer RNA (tRNA): found freely in the cytoplasm of the cell.

Structure of RNA

  • RNA resembles the same as that of DNA, the only difference being that it has a single strand unlike the DNA which has two strands, and it consists of only a single ribose sugar molecule in it.

Nucleotides of an RNA Molecule

  • Phosphate

  • Ribose sugar

  • Nitrogenous base

    • A, G, C or U

Nitrogenous Bases in RNA

  • Adenine (A)

  • Guanine (G)

  • Cytosine (C)

  • Uracil (U)

Three Types of RNA

  • Messenger RNA (mRNA): Copies the genetic code from the DNA and carries it to the ribosome. It determines the order of the amino acids in the protein.

  • Ribosomal RNA (rRNA): Combines with proteins to form ribosomes.

  • Transfer RNA (tRNA): Anti-codon which determines the amino acid it must transport during protein synthesis.

Functions of RNA

  • It helps in the synthesis of proteins in our body.

  • This nucleic acid is responsible for the production of new cells in the human body.

  • RNA carries genetic information that is translated by ribosomes into various proteins necessary for cellular processes.

  • mRNA, rRNA, and tRNA are the three main types of RNA involved in protein synthesis.

Protein Synthesis

  • Protein synthesis is the creation of proteins by cells that uses DNA, RNA, and various enzymes.

  • There are 20 different amino acids (building blocks of proteins).

  • The number of amino acids and the sequence of the amino acids determine the type of protein that is formed.

  • The bonds between amino acids are known as peptide bonds.

Protein Synthesis Process

  • There are two main processes involved in protein synthesis, namely:

    • Stage 1: Transcription of mRNA from DNA

    • Stage 2: Translation of mRNA to form proteins

  • In preparation for the manufacture of mRNA, a DNA molecule in the nucleus separates into two strands in the region of a gene carrying instructions for a specific protein.

  • Each sequence of three bases in a DNA strand is called a base triplet, which is a code for one of 20 amino acids.

Transcription (Nucleus)

  1. The double helix DNA unwinds

  2. The double-stranded DNA unzips

  3. To form two separate strands.

  4. One strand is used as a template

  5. To form mRNA

  6. Using free-floating RNA nucleotides from the nucleoplasm

  7. The mRNA is complementary to the DNA

  8. mRNA now has the coded message for protein synthesis

mRNA moves from the nucleus to the cytoplasm & attaches to the ribosome

Translation (Cytoplasm on Ribosome)

  1. Each tRNA carries a specific amino acid

  2. When the anticodon on the tRNA

  3. Matches the codon on the mRNA

  4. Then tRNA brings the required amino acid to the ribosome

  5. Amino acids become attached by peptide bonds

  6. To form the required protein

Why is Transcription Important?

  • DNA is too large to leave the nucleus, but a single stranded mRNA is small enough to leave the nucleus to carry a coded message for protein synthesis from the DNA.

  • It is the main point at which the cell regulates which proteins are to be produced and at what rate.

  • It makes the mRNA which will contain the code from the DNA which the ribosome will read to produce new protein molecules.

Genetic Code

  • The genetic code is the order of DNA bases which determines the sequence of amino acids in a protein.

The Effect of Mutation on Protein Structure (DNA Sequence)

  • A mutation is a change in the nitrogenous base sequence of a DNA molecule (or gene).

  • mRNA is copied from the DNA molecule during transcription – Change in codons.

  • Different tRNA molecules carrying different amino acids will be required.

  • The sequence of the amino acid changes resulting in the formation of a different protein.

  • If the same amino acid is coded for there will be no change in the protein structure.

The Effect of Mutation on Protein Structure (DNA Sequence)

  • Mutations create variation within the gene pool (slightly different versions of the same gene)

  • Less favorable mutations can be reduced in the gene pool by natural selection.

  • While more favorable mutations may result in evolutionary changes.

  • Gene mutations –occur when a copy error takes place during replication. One or more base pairs are inserted, replaced or deleted.

  • Chromosome mutations – occur when the chromosome or the chromosome number changes.

Point Mutation

  • Substitution – One base pair (nucleotide) is substituted for another

Frameshift Mutation

  • Deletion – One or more base pairs are lost from the DNA

  • Addition – The insertion of one or more nucleotide base pairs into the DNA sequence

Causes of Mutations

  • Radiation

  • Errors during meiosis

  • Errors during replication

  • Some viruses

Examples of Mutations

  • Sickle cell anemia – change in gene that codes for hemoglobin

  • Cystic fibrosis – Defective gene that makes the body produce abnormally thick & sticky mucus

  • Huntington’s disease – Neurodegenerative (breakdown of nerve cells)

Difference Between DNA & RNA

DNA

  • Deoxyribose

  • It is a double strand of nucleotides

  • helical in shape (helix)

  • Contains thymine, adenine, cytosine, guanine

  • Particular base sequence

  • Bases are paired

RNA

  • Ribose

  • It is a single strand of nucleotides

  • No helical shape

  • Contains adenine, uracil, cytosine, guanine

  • No particular base sequence

  • No pairing of bases occurs