DNA Structure

• DNA

  • Deoxyribonucleic Acid (DNA) is a macromolecule found in all living cells.
  • Believed to be formed 3 billion years ago.
  • Is the chemical which controls everything that happens in cells.
  • Was discovered in 1869 by Friedrich Miescher who isolated DNA from the cells nucleus, which contained phosphates and was acidic.
  • This gave the term “Nucleic acid”.
• DNA

  • Macromolecules are large molecules (made up of thousands of atoms) found in living organisms.
• DNA

  • James Watson and Francis Crick were two scientists who discovered the double helix structure in the 1940’s.
  • Rosalind Franklin’s work with X-ray diffractions made this possible.
• DNA

  • To account for the enormous variation that DNA provides across life, it must do two functions:
  • Make identical copies of itself – pass on DNA from one cell to the next (DNA Replication).
  • Provide a code that is used by cells to manufacture proteins molecules.
  • DNA functions the same in all living organisms.
  • Made up of repeating nucleotides.
• DNA Structure (Nucleotide)

  • Has a phosphate, sugar, organic base, connected by covalent bonds
• Prokaryotic and Eukaryotic

  • Prokaryotic cells have a singular circular chromosome attached to the cell membrane
  • Chromosomes
  • Attachment site
  • Binding proteins
  • DNA
  • Eukaryotic cells contain linear chromosomes within a nucleus
  • Chromosomes
  • Nucleus
  • Chromatin
  • Histones
  • DNA
  • Circular chromosomes are also found in mitochondria and chloroplast organelles in a eukaryotic cells
• DNA Structure (Nucleotide)

  • Contains a sugar-phosphate backbone.
  • Deoxyribose (5-carbon) sugar.
  • A phosphate group.
  • One of four organic bases:
  • Adenine
  • Thymine
  • Cytosine
  • Guanine
• DNA

  • Adenine binds to thymine and has two hydrogen bonds
  • Cytosine binds to guanine and has three hydrogen bonds
  • This is through weak hydrogen bonds, which can be easily broken and re-formed (important for DNA replication).
  • DNA analysis shows an equal amount of adenine and thymine, and equal amounts of cytosine and guanine.
  • Start of DNA strand is 5’ (phosphate), end of strand is 3’ (sugar) Flows 5’ to 3’
• DNA Structure

  • Phosphate group is the beginning of the DNA strand.
  • This is called the 5’ end (5 prime)
  • Sugar group is the end of the DNA strand.
  • This is called the 3’ end (3 prime)
  • Therefore, DNA is written a 5’ to 3’ direction.
  • DNA consists of two complementary strands, which are read in opposite directions (5’ to 3’ direction).
• Genes

  • Genes are a sequence of bases on one strand of DNA.
  • The sequence (order) of bases varies, and is what gives DNA its versatility.
• Chromosomes - Prokaryotes

  • DNA is located in cytosol of prokaryotic cells, called the nucleoid region.
  • DNA is circular (a complete loop). Consists of one chromosome which is double stranded.
  • Nucleoid region also contains RNA and proteins.
  • Prokaryotic cells also contain separate circular DNA called plasmids.
• Chromosomes - Eukaryotes

  • Chromosomes are condensed linear strands of DNA, combined with proteins called histones.
  • Human somatic (body) cells consist of 46 chromosomes, organised into 23 pairs.
  • 22 pairs of autosomal (non-sex) chromosomes, and one pair of sex chromosomes (XX In females, XY in males).
  • A set of human chromosomes are called a karyotype.
• Chromosomes - Eukaryotes

  • The ends of chromosomes have short lengths of DNA called telomeres.
  • This prevents chromosomes from breaking down and binding to each other.
  • Decondensed chromosomes are called chromatins.
  • This allows DNA to be read for DNA Replication, Protein Synthesis and RNA Synthesis.
• • two types of eukaryotic membrane-bound organelles also contain DNA, mitochondria (mtDNA) and chloroplasts (cpDNA).
  • mtDNA and cpDNA are both double stranded, circular and not bound to histone proteins (similar to prokaryotic cells)
  • There are theories for the existence of DNA in these organelles. including the fact that mitochondria and chloroplasts were once 'free living' unicellular organisms that were capable of their own independent existence
• DNA Replication

  • Process of copying DNA of organism to pass onto daughter cell.
  • Occurs in both prokaryotic and eukaryotic cells.
  • Occurs in nucleoid region (prokaryotes) and nucleus (eukaryotes).
  • DNA is semi-conservative as each new strand of DNA consists of one old DNA strand, and one newly built one.
  • DNA strand is split open by helicase (enzyme).
  • DNA nucleotides are added in opposite directions.
  • DNA is synthesised in a 5’ to 3’ direction by DNA polymerase.
  • Therefore, DNA nucleotides are added in the 3’ end first.
  • This occurs to the “old” strands (original parent strands).
• DNA replication

  1. Helicase breaks hydrogen bonds between complementary bases

  2. Each strand serves as template for new complementary strand

  3. DNA polymerase adds free DNA nucleotides to exposed bases according to complementary base pairing

  4. New DNA synthesised in 5' to 3' direction

  5. New double-stranded DNA molecule rewinds into double helix and joins centromere
• Helicase


  Enzyme that breaks the hydrogen bonds between the complementary bases joining the two strands, unwinding, and exposing their bases
• DNA polymerase


  Enzymes that add free DNA nucleotides to the exposed corresponding bases on the separate strands, according to complementary base pairing rule
• Complementary base pairing rule


  A binds with T and C binds with G
• The new DNA is synthesised in the 5' to 3' direction
• Each new double-stranded DNA molecule rewinds into a double helix and are joined to the centromere
• DNA Replication 


  Following DNA replication, the DNA is double and all chromosomes in the cell consist of two identical sister chromatids. DNA can therefore be see in three forms:

  • decondensed chromatin
  • As a condensed chromosome
  • after DNA replication as a chromosome consisting of two identical sister chromatids

  DNA Replication is limited to one replication per cell cycle (life of cell), as each time it occurs, the telomere (end of chromosome) shortens
• State the differences between linear and circular chromosomes, giving examples of the types of species in which each might be found.

  Linear chromosomes are generally in pairs and are found in the nucleus of eukaryotic cells and they have distinct ends. Circular chromosomes are joined at the ends and prokaryotic organisms only have one which is located in the nucleoid region.
• Describe why chromosomes are not visible in cells that are not dividing.

  When cells are not dividing, chromatin is present, which is a very long decondensed version of chromosomes, that cannot be seen under a light microscope. Only condensed chromosomes during cell division can be seen.
• Structures in DNA

  • Complementary base
  • Deoxyribose sugar
  • Phosphate group
  • Weak hydrogen bonds
• Explain why DNA replication is called 'semi-conservative'

  DNA replication is the process during which the DNA molecule on a chromosome makes an identical copy of itself. It is called semi conservative because the original DNA is made up of 2 complementary strands and the 2 new DNA molecules will each have one original strand and one new strand.
• State how DNA stores information

  Information is stored in DNA in the number, sequence, and type of organic bases (A,T, G, or C)
• State how DNA transmits information from parent cells to daughter cells.

  DNA replication makes an identical copy of the original DNA molecule, When a cell divides, DNA is passed on to the new cells that are formed. If the division is mitosis or binary fission, each daughter cell has the same DNA as the parent.
• Name two organelles, other than the nucleus, in which DNA might be found in a eukaryotic cell. Suggest one possible reason for the presence of this DNA.

  Chloroplasts and mitochondria. It is theorised that they were both originally independent cells. Therefore, they both likely contain codes for molecules required by the organelle.
• Discuss the differences between a chromosomes and a sister chromatid.

  A chromosome refers to the structural unit, made up of DNA and protein, that contains the hereditary information. When DNA divides during replication, a chromosome makes another copy of its DNA molecule and, for a brief moment, the chromosome is made up of 2 identical DNA molecules called sister chromatids.
• Explain the significance of different species having different number and types of chromosomes.

  The chromosomes are made up of genetic information. Different species have different features and characteristics and therefore have different numbers and types of chromosomes, each coding for the characteristics specific for that species.
• Discuss the likely reason that chromosomes are often referred to as 'structural units of information in organisms'.

  Just as a cell could be described as a structural unit that builds a multicellular organisms, a chromosome is often described similarly because it is a visible structure that contains information in the form of bases (A, T, G, C) that will provide a code for genetic information.
• Explain why DNA replication is essential for the production of new cells.

  When cells reproduce, they usually produce daughter cells that are identical to the parent cell (sex cell production is an exception). If the cells are to be identical, they must have equal and identical copies of the chromosomes, hence DNA replication.