DNA - The Code of Life

Deoxyribonucleic Acid (DNA)

Key Questions

  • What is DNA and what does it do?
  • How was the DNA structure discovered?

Key Words

  • DNA: Short for deoxyribonucleic acid, it contains the instructions used in the development and functioning of almost all known living organisms.
  • Chromosomes: Long strands, made of DNA and proteins in the nucleus of a cell, which contain genetic information.
  • Histones: Proteins that form part of the chromosomes.
  • Gene: A section or chemical structure of DNA in a chromosome, which carries a code about a particular characteristic in an organism.
  • Extranuclear DNA: DNA found outside of the nucleus.
  • Double helix: The structure of DNA, which forms a spiral ladder-shaped molecule.
  • Nucleotides: The building blocks of DNA, which consist of a nitrogen-containing purine or pyrimidine base linked to a sugar and phosphate group.
  • DNA replication: The process of making new DNA from existing DNA.
  • Cell cycle: The series of processes that take place in a cell between one cell division and the next.
  • Genetic code: The way DNA controls the structure and functions in a cell.
  • Transcription: The process by which DNA makes RNA.
  • Translation: The process by which the information from DNA carried by mRNA is used to form chains of amino acids that make up proteins with a specific order of nitrogenous bases.
  • Codon: A group of three nucleotide bases on the mRNA strand.
  • Anticodon: A group of three nucleotides that recognises the codons of mRNA; forms a base pair with codons on a strand of mRNA during translation of RNA into protein.
  • Complementary: Match each other
  • DNA fingerprinting or DNA profiling: The analysis of DNA samples to identify individuals or relationships between individuals.

Location of DNA in the Cell, Chromosomes, Genes, and Extranuclear DNA

  • DNA Location:
    • Located in the nucleus of a cell.
    • Forms part of the chromosome structure.
  • Chromosomes:
    • Long strands made of DNA and proteins in the nucleus.
    • Contain genetic information.
    • Formed from DNA and proteins called histones.
    • DNA is tightly wound around histones.
  • DNA, Chromosomes, and Genes:
    • When a cell is not dividing, its chromosomes become long and thin.
    • Genes are sections or chemical structures of DNA along each chromosome.
    • Each gene controls a part of a cell's function and carries a code about a particular characteristic.
    • Genes determine the structure and function of cells or a characteristic (e.g., eye color).
    • DNA carries the code or 'instructions'.
  • Extranuclear DNA:
    • Most DNA is in the nucleus.
    • DNA is also found in the mitochondria of plant and animal cells and chloroplasts of plant cells.
    • DNA outside the nucleus is called extranuclear DNA.
    • Mitochondrial DNA (mtDNA) is passed on only by the female to her offspring.
    • mtDNA is useful in tracing genetic relationships over long periods.

Structure of DNA

  • DNA Structure:
    • A long, twisted molecule made up of two strands twisted into a double helix.
    • A double helix forms a spiral ladder-shaped molecule.
  • Nucleotides:
    • The double helix of DNA is made up of many small units called nucleotides.
    • Nucleotides are the building blocks of DNA.
    • A nucleotide consists of a nitrogen-containing purine or pyrimidine base linked to a sugar and phosphate group.
    • There are about three billion nucleotides in the nucleus of a human cell.
    • A nucleotide is made up of:
      • A sugar called deoxyribose
      • A phosphate molecule
      • A nitrogenous base
  • Nitrogenous Bases:
    • Four different nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C).
    • Divided into two groups: purines (adenine and guanine) and pyrimidines (thymine and cytosine).
    • A purine base always pairs with a pyrimidine base.
  • DNA Molecule Structure:
    • Made up of two parallel strands of nucleotides.
    • Strands are joined together to form the ladder-like structure or DNA double helix.
    • The amount of each nitrogenous base varies in different organisms.
    • Humans have more adenine (A) and thymine (T).
    • Other organisms have more cytosine (C) and guanine (G).
    • There are equal numbers of A and T bases and equal numbers of C and G bases in a DNA molecule.
    • The sequence of the bases in the nucleotides determines the hereditary characteristics.
    • Genes are made up of groups of nucleotides, and a single gene can consist of hundreds of nucleotides.
    • Each sugar molecule is joined to a phosphate to form a chain.
    • The nitrogenous base is attached to the sugar of each nucleotide.
    • Nitrogenous bases are linked by hydrogen bonds to form pairs.
    • A pairs with T, and C pairs with G.
    • The molecule is twisted into a spiral.
  • Discovery of DNA:
    • DNA was first discovered in 1869 by Friedrich Miescher, a Swiss physician and biologist.

Practical Task - Activity 2: Extract DNA from Bananas

  • Materials Needed:
    • 250 ml measuring cup, measuring spoons, 100 ml water, 2.5 ml of salt, chopping board or plate, half a banana, a fork or masher, coffee filter paper or lab filter paper, or a very fine sieve or tea strainer, a filter funnel, a beaker, two test tubes, washing-up liquid, a spoon for stirring, 1 ml of meat tenderiser powder or 1 ml of fresh pineapple juice, 10 ml of ice-cold ethanol or methylated spirits, toothpicks.
  • Instructions:
    1. Pour 100 ml water into a 250 ml measuring cup.
    2. Add 2.5 ml salt and stir to dissolve.
    3. Mash half a banana using a fork or masher and add it to the salt water.
    4. Filter the banana mixture through the filter paper into the beaker.
    5. Place 10 to 15 ml of the filtrate into a test tube. Filtering separates the cell wall from the DNA and proteins, which are now in solution.
    6. Add a few drops of washing-up liquid to the banana filtrate and stir gently.
    7. Leave the mixture to stand for 5 to 10 minutes.
    8. Mix 6 ml of the filtered banana mixture with 1 ml of meat tenderiser or 1 ml of pineapple juice.
    9. Trickle 7 ml of ice-cold ethanol or methylated spirits onto the surface of the banana filtrate.
    10. Use a toothpick to pull out the white stringy substance (banana DNA).
  • Questions:
    1. Why do you think the banana must be mashed?
    2. How would the following actions affect the results of the DNA extraction?
      • 2.1 You put the test tube containing the banana into water heated to 7070 °C. Explain your answer.
      • 2.2 You do not filter the banana mixture.

Discovery of the Structure of DNA by Watson, Crick, Franklin, and Wilkins

  • Early Discoveries:
    • Mid-19th century: Scientists knew genetic material was responsible for inherited characteristics.
    • Mid-20th century: Scientists discovered that the genetic material was DNA.
  • Discoveries by the Early 1950s:
    • DNA consists of molecules of deoxyribose sugar, phosphate molecules, and four nitrogen bases: adenine (A), thymine (T), cytosine (C), and guanine (G)
    • DNA has equal numbers of adenine (A) and thymine (T) bases, and equal numbers of cytosine (C) and guanine (G) bases.
  • Discovery Method
    • By 1953, scientists found a method using X-rays to work out the shape and structure of large molecules such as DNA.
  • Key Figures:
    • Maurice Wilkins and Rosalind Franklin (King's College in London): Used X-ray methods to show that DNA had a regular shape.
    • James Watson and Francis Crick (Cambridge University in England): Started working together on DNA in 1951.
  • Watson and Crick's Work:
    • November 1951: Watson attended a lecture by Rosalind Franklin about her research on DNA structure.
    • Watson and Crick used the information from the lecture to build a model of DNA but were unsuccessful initially.
  • Breakthrough:
    • Watson and Crick studied X-ray photos from Franklin and Wilkins and realized DNA was shaped like a spiral or helix.
    • Watson used cut-out cardboard pieces to represent the shapes of the bases and discovered that adenine-thymine and cytosine-guanine base pairs were the same shape.
    • The base pairs could be packed into the center of the helix.
    • The pairing of bases explained why there were always equal numbers of adenine (A) and thymine (T) bases, and equal numbers of cytosine (C) and guanine (G) bases. Adenine (A) always pairs with thymine (T) and cytosine (C) always pairs with guanine (G).
  • Model Completion:
    • February 1953: Crick took over building the model.
    • By the end of the month, they worked out how all the parts of the DNA molecule fitted together.
    • The sugar and phosphate molecules formed two twisted backbones on the outside of the molecule, making the double helix shape of the DNA.
    • Base pairs were attached between the strands, like steps of a spiral staircase.

Recognition for the Discovery of DNA

  • Publication and Award:
    • Watson and Crick published their famous paper on the structure of DNA in the journal Nature in April 1953.
    • In 1962, Watson, Crick, and Maurice Wilkins were awarded the Nobel Prize for discovering the structure of DNA.
  • Later Careers:
    • Crick studied how DNA functions and how organisms develop. He died in 2004.
    • Watson became a professor of Biology at Harvard University before studying the causes of cancer and became the first director of the Human Genome Project in 1988.
    • Maurice Wilkins taught students, campaigned against nuclear weapons, and wrote his life story. He died in 2004 at age 87.
  • Rosalind Franklin's Contribution:
    • Rosalind Franklin's contributions to the discovery of DNA's structure were not acknowledged.
    • Franklin died of cancer in 1958, before Watson, Crick, and Wilkins received their Nobel Prizes.
    • The Nobel Prize cannot be awarded posthumously.
  • Controversy:
    • Rosalind Franklin gave a lecture suggesting that the DNA structure was a helix or spiral, which Watson attended.
    • Wilkins gave Watson one of Franklin's photographs of DNA without her permission, which helped Watson and Crick determine that DNA could be a double helix.

Activity 3: Revise the Discovery of DNA

  1. Read and discuss the information about the discovery of DNA.
  2. Identify the major steps in the discovery of the structure of DNA.
  3. Do you think Watson and Crick could have worked out the structure on their own? Explain your answer.
  4. Do you think Watson and Crick were ethical in their dealings with Rosalind Franklin? Explain your answer.

Role of DNA - Genes and Non-Coding DNA

  • Proteins and DNA:
    • Proteins are made of long chains of amino acids.
    • DNA contains the instructions for making proteins.
  • Genes and Proteins:
    • Specific sequences of DNA nucleotides (genes) indicate how a single protein is to be made.
    • The order of nucleotides within a gene determines the order and types of amino acids that must be put together to make a protein.
  • Non-Coding DNA:
    • Not all DNA in a cell forms genes that code for proteins.
    • DNA that does not code for proteins is called non-coding DNA.
    • Up to 98% of DNA in the nucleus is non-coding.
    • Much of this DNA has no known biological function and is sometimes called 'junk DNA'.
    • Increasing evidence shows that this DNA has various regulatory roles and influences the behavior of the genes (coding DNA) in important ways.
    • Knowledge about non-coding DNA is still incomplete.

DNA Replication - Cell Cycle and Necessity for Exact Copy

  • DNA Replication:
    • The process of making new DNA from existing DNA.
    • Controlled by specific enzymes.
    • Each DNA double helix is made up of one original DNA strand and one new strand.
    • New strands are built from free molecules of deoxyribose, phosphate, and nitrogen bases present in the cell.
    • The bases pair up correctly (A with T and C with G), so each new helix is exactly the same as the old one.
  • DNA Replication and the Cell Cycle:
    • The cell cycle is the series of processes that take place in a cell between one cell division and the next.
    • DNA replication takes place during interphase of the cell cycle, before cell division begins.
    • The reason for DNA replication is to ensure that the daughter cells that result from cell division each contain the identical genetic material (DNA) that was present in the parent cell.
    • Replication of DNA also ensures that the two daughter cells contain exactly the same amount of DNA as the parent cell.
  • Steps in DNA Replication:
    1. The double helix unwinds.
    2. Hydrogen bonds holding the base pairs together break.
    3. Matching nitrogen bases line up next to the exposed bases in the original strand.
    4. New deoxyribose and phosphate molecules join to form a chain alongside each strand of the old helix.
    5. Hydrogen bonds re-form to join the two matching strands of the new molecule.
    6. Two new identical double strands form.
    7. Each double strand rewinds to form a new helix.
    8. The new double helix winds itself around the histones.
    9. The new DNA molecules contract and become shorter and thicker to form two chromosomes of the new cell.

Ribonucleic Acid (RNA)

  • Central Dogma:
    • DNA makes RNA and RNA makes protein.
  • Types of RNA and Their Location in Cells:
    • RNA is found mainly in the cytoplasm and nucleolus of the cell.
    • Three types of RNA are found in different parts of the cell:
      • Messenger RNA (mRNA) - Nucleolus
      • Ribosomal RNA (rRNA) - Ribosomes in cell cytoplasm
      • Transfer RNA (tRNA) - Cell cytoplasm
    • Each type of RNA has a specific role in the formation of proteins from DNA.
  • Transcription from DNA:
    • The process by which DNA makes RNA is called transcription.
    • mRNA forms during transcription.
    • mRNA carries the encoded message of which protein to make from the DNA to the ribosomes.
    • After mRNA has formed, it moves from the nucleus to the ribosomes of the cell.
  • Process of Transcription
    1. During transcription, a small piece of the DNA molecule unwinds.
    2. The two strands of nucleotides unzip and separate.
    3. New nucleotides line up along one strand of the DNA.
    4. The new nucleotides form pairs with the free nucleotides on the DNA strand.
    5. Instead of thymine bases pairing with adenine, uracil bases pair with adenine.
    6. The DNA rewinds into a double helix.
    7. When the RNA strand has formed, it breaks away from DNA.
    8. The mRNA contains the genetic code from DNA.

Comparison of DNA and RNA

StructureDNARNA
Number of nucleotide strands21
Type of sugar in moleculeDeoxyribose sugarRibose sugar
Nitrogenous bases presentThymineUracil

Translation of RNA into Proteins - Protein Synthesis

  • Translation:

    • Translation is another name for protein synthesis.

  • mRNA:

    • The order of nitrogenous bases in the nucleotides on the mRNA molecule determines which proteins will be formed during translation.

  • Codons:

    • Each group of three nucleotide bases on the mRNA strand is called a codon.
    • Each codon is the code for a specific amino acid.
    • The ribosome builds each protein step by step after reading the order of codons as it moves along the mRNA strand.

  • tRNA:

    • Transfer RNA (tRNA) brings the correct amino acids to the ribosome where they are joined together to make different proteins.
    • The tRNA has a group of three nucleotides called an anticodon.
    • The anticodon recognizes the codons of mRNA.
    • The nucleotides on the anticodon match each other, or are complementary, to the nucleotides on the codon.

  • Steps in Protein Synthesis:

    1. mRNA attaches to ribosomes in the cytoplasm.
    2. The ribosome reads the order of bases on the mRNA.
    3. tRNA brings amino acids to the ribosomes in the correct order.
    4. tRNA links with mRNA. The tRNA has anticodons that match the codons on mRNA.
    5. The amino acids are joined together by enzymes to form proteins.

Genetic Code

  • Genetic Code Definition:
    • The genetic code is the way in which genetic information in DNA controls the synthesis of specific proteins by the cell.
  • Triplets:
    • The code is in the form of a series of three nucleotides, called triplets, on DNA.
    • These triplets determine the mRNA codons.
  • Codons and Amino Acids:
    • There are 64 different codons.
    • All the codons, except for three, encode for one of the 20 amino acids used to form proteins in protein synthesis.
    • Some amino acids are coded for by more than one codon.
  • Start and Stop Codons:
    • The codon ATG is a start codon and is always the first one to be translated into protein. It is also the codon for the amino acid methionine.
    • The codons TGA, TAA, and TAG are stop codons, as tRNA does not have anticodons for these codons.

DNA Fingerprinting or DNA Profiling

  • Definition:
    • DNA fingerprinting or DNA profiling is the analysis of DNA samples to identify individuals or relationships between individuals.
  • Applications:
    • DNA fingerprinting can help to identify criminals, confirm the identity of missing people, or solve disputes about who is the father of a child.
      DNA from related people is similar; unrelated people are unlikely to share the same DNA.

Case Studies

  • Thomas Jefferson - Sally Hemings Affair:
    • DNA analysis of his Y chromosomes has traced the male Jefferson line to descendants of Sally Hemings.
  • The Disappearance Project:
    • The Disappearance Project at the University of the Western Cape's Forensic DNA laboratory uses DNA analysis to help identify victims in human rights cases in South Africa.
    • A recent case was that of ANC struggle hero Solwandle Looksmart Ngudle, who died in police custody in 1963.
    • DNA extracted from bones thought to belong to Ngudle was compared with the DNA of family members.
    • The DNA from the bones matched the DNA of his relatives, bringing closure for the family.

RNA Structure

  • RNA consists of a single strand of nucleotides. The sugar in RNA is ribose, not deoxyribose as in DNA. RNA also contains four different nitrogen bases, but it does not have any thymine bases. Instead of thymine, RNA contains a nitrogen base called uracil. Like thymine, uracil is a pyrimidine base. There are different types of RNA, which carry out different jobs in a cell.