DNA Structure and Replication

Stage 2 Biology: 1.1 DNA Structure

• DNA stores and transmits genetic information and functions the same way across all living things
• It is a helical double-stranded molecule
• In prokaryotes, DNA is unbound in and circular in the cytosol
• In eukaryotes, DNA is unbound in and circular in the mitochondria and chloroplasts

Content Checklist

• Compare chromosomes in prokaryotes and eukaryotes
  • Replication of DNA allows for genetic information to be inherited
  • Base-pairing rules and the method of DNA replication are universal
• Describe the structural properties of the DNA molecule
  • Nucleotide composition and pairing
  • The weak bonds between strands of DNA that allow for replication
• Explain the importance of complementary base pairing (A-T, C-G)
• DNA strands direction are read 5' to 3'
• Describe and represent the process of semi-conservative replication in DNA

Nucleotides

• Nucleotides consist of a phosphate group, deoxyribose or ribose sugar, and a nitrogenous base
• Nucleotides are monomers held together by covalent bonds
• Nucleotides form polynucleotides resulting in the nucleic acids, DNA or RNA

Nucleic Acids

• Nucleic acids are macromolecules, storing and transmitting genetic information important for protein structure and function
• They are chains of smaller, repeating units known as nucleotides

Types of Macromolecules

• Amino acids link to form proteins
• Fatty acids link to form lipids
• Glycerol and monosaccharides link to form carbohydrates
• Nucleobases link to form DNA/RNA

Proteins

• Proteins are macromolecules that carry out life processes, such as hormones and enzymes
• DNA stores genetic information
• RNA builds proteins based on the stored genetic information
• Proteins are functional units that perform life processes

DNA in Prokaryotes and Eukaryotes

• DNA is a macromolecule in all living cells
• In eukaryotes, DNA is found in the nucleus
• In prokaryotes, DNA is located in the cytosol, also known as the nucleoid region
• Chloroplasts and mitochondria also contain DNA
• They were once free-living unicellular organisms

DNA - Prokaryotes and Eukaryotes Comparison

• Prokaryotic DNA
  • Number of DNA strands: Two
  • Number of chromosomes: One
  • Shape of chromosomes: Circular
  • Location in cell: Nucleoid region
• Mitochondria/Chloroplast DNA
  • Number of DNA strands: Two
  • Number of chromosomes: One
  • Shape of chromosomes: Circular
  • Location in cell: Mitochondria matrix, stroma
• Eukaryotic DNA
  • Number of DNA strands: Two
  • Number of chromosomes: 2 or more in pairs
  • Shape of chromosomes: Linear
  • Location in cell: Nucleus

DNA - Structure and Function

• DNA is a double-stranded molecule of repeating nucleotides, forming a double helix
• DNA stores and transmits genetic information, allowing for:
  • Identical copies to be made and inherited
  • A code to manufacture protein molecules

DNA - Structure and Function (cont.)

• Nucleotides are attached by the phosphate of one nucleotide attached to the sugar of the next, results in DNA 'backbone' of alternating phosphates and sugars
• One strand of DNA ends with a phosphate (5')
• The opposite strand of DNA ends with a sugar molecule with a hydroxyl group (3')
• DNA reads from 5' to 3'
• Phosphate gives DNA an overall negative charge that is helpful in separating DNA molecules

DNA - Structure and Function (cont.)

• Each base forms weak hydrogen bonds with its complementary base on the opposing strand
• Weak bonds are beneficial as they are easy to break and reform.
• Breaking these bonds is critical in DNA replication
• Adenine (A) always binds with Thymine (T) - 2 hydrogen bonds
• Cytosine (C) always binds with Guanine (G) - 3 hydrogen bonds
• This is complementary base pairing

Chromosomes

• DNA in cells is found in structures called chromosomes
• Chromosomes are the structural unit storing and transmitting genetic information in cells
• The number of centromeres indicates the number of chromosomes

Chromosome vs Chromatid

• ​Chromosomes are a structural unit that store DNA and proteins (hereditary information)
• When DNA replicates, the chromosome makes an identical copy, made up of two identical DNA molecules, called sister chromatids

Chromosomes and DNA in Prokaryotes

• Prokaryotes are unspecialized cells without membrane-bound organelles
  • DNA is located in the cytosol
  • It is circular, double-stranded with no ends
  • The cytosol contains proteins and RNA
  • Plasmids within contain small, circular pieces of DNA separate from the cell's chromosomal DNA
  • Circular DNA lacks telomeres

Chromosomes and DNA in Eukaryotes

• In eukaryotes, double-stranded DNA molecules are:
  • Bound to histone proteins to form condensed linear chromosomes in the nucleus
  • Each chromosome has two ends
  • DNA 'cap' the tip of each end of chromosome with telomeres to protect from breaking or binding
• Chromosomes
  • Are only visible under a light microscope during cell division
  • Coiled around histone proteins (condensed

Chromosomes and DNA in Eukaryotes (cont.)

• When chromosomes are relaxed it is referred to as chromatin, which is not visible under a light microscope
• Chromatin is present in the nucleus throughout the cell cycle, with the exception of cell division.
• Chromatin is critical as it makes the genetic code accessible during growth and development
• Tightly coiled chromosomes protect genetic code to ensure exact copies are made during cell division

Chromosomes - Mitochondria and Chloroplasts

• Two membrane-bound organelles in eukaryotic cells:
  • Mitochondria
    • mtDNA
  • Chloroplasts
    • cpDNA
• DNA is double-stranded, circular and not bound to histone proteins

Chromosomes - Genes

• Chromosomes carry genetic information in the form of genes
• Genes:
  • Consist of unique segments of DNA with a specific sequence of bases that code for an RNA molecule or protein
  • Are inheritable factors that control specific characteristics (e.g., blood type)
  • Reside specifically on a chromosome (locus)
  • Have alternative expressions (allele)

Summary

• DNA and chromosomes are universal in all living organisms, where the structure of DNA allows cells to:
  • Make identical copies of itself
  • Provide a code that can be used by cells to synthesise proteins
• DNA stores and transmits genetic information
• The number of chromosomes, the length of DNA molecules, the number of bases, and the base sequences of the DNA stores genetic information and differs between species:
  • Humans have 46 chromosomes
  • Koalas have 16 chromosomes
  • Sunflowers have 34 chromosomes

DNA Replication

• Genetic information is inherited by daughter cells and descendants through DNA replication
• DNA can produce an identical copy of itself because each single strand stores the information needed to make a complementary strand using the base pairing rule
• DNA Replication location
  • Prokaryotes: nucleoid region
  • Eukaryotes: nucleus
• DNA replication is semi-conservative, as each new DNA double strand has one old (parent DNA) strand and a new strand

DNA Replication - Enzymes Involved

• Helicases: unwind the DNA double helix
• Primases: initiate replication
• Polymerases: catalyse the synthesis of new complementary DNA strands by adding new nucleotides to the growing DNA strand
  • DNA polymerase cannot initiate - only add - and on the 5' to 3' direction
• Ligases: join DNA fragments together

DNA - How Replication Occurs

• Leading Strand
  • Strand synthesized in the same direction as the movement of the replication fork (towards)
  • Synthesised continuously (5' → 3')
  • Only one primer is required to initiate replication
  • DNA ligase is not required
• Lagging Strand
  • Strand synthesized in the opposite direction as the movement of the replication fork (away from)
  • Synthesised discontinuously - with fragments called Okazaki fragments (5' → 3')
  • Each fragment requires a separate RNA primer to initiate
  • DNA ligase is required to join the fragments

DNA Replication - Aftermath

• Following DNA replication, the DNA is doubled and all chromosomes in the cell have two sister chromatids.
• DNA can be in three forms:
  • Decondensed chromatin
  • Condensed chromosome
  • As two identical sister chromatids after DNA replication
• The limiting factor in the number of times replication can occur is the telomeres shorten so that they are too short
• DNA replication occurs prior to cell division (mitosis or binary fission)

Karyotype and Genome

• A genome describes the total number of genes of an organism
• The human genome was mapped in 2001 after ten years. Every gene of all 46 chromosomes of humans was sequenced.
• Humans carry roughly 21,000 genes.

Explore the structure, function, and replication of DNA. This quiz covers base pairing rules, enzyme functions, and differences between species in DNA composition. Key concepts include hydrogen bonds, DNA polarity, and eukaryotic DNA characteristics.