DNA Notes

NUCLEUS

• Surrounded by double membrane
• Pores in membrane: passage between nucleoplasm and cytoplasm
• Nucleolus suspended in nucleoplasm
• Mass of thread-like structures occur in nucleus: chromatin network
• Chromatin network coils and thickens into thread-like structures (chromosomes) during cell division
• Functions:
  • Chromosomes carry genetic material
  • Nucleus regulate the structure and metabolism of the cell

NUCLEIC ACIDS

• Organic molecules
• Control the synthesis of proteins in all living cells
• Store and transfer genetic information
• Two types:
  • DNA: deoxyribonucleic acid
  • RNA: ribonucleic acid

DNA: DEOXYRIBONUCLEIC ACID

• Occur mainly in nucleus
• Forms part of chromatin network: chromosomal DNA
• Small amount of DNA found in mitochondria and chloroplasts: extra-nuclear DNA
• NOTE: Mitochondrial DNA (mtDNA) is passed from mother to child. Used to trace the maternal lines.

CHROMOSOMES AND GENES

Chromosomes

• Long
• thin
• thread-like structures
• composed of DNA
• wrapped around proteins (histones)

Genes

• short segment of DNA
• codes for a particular protein (carries the info for the synthesis of a particular protein)

Proteins:

• Determine the characteristics (structure and functioning) of an organism

STRUCTURE OF DNA

• DNA is:
  • giant molecule
  • consists of two strands
  • strands are twisted to form a double helix
  • looks like ladder when unwound
  • is a polymer (made up of large numbers of similar units called monomers)
• The monomer is called a nucleotide

NUCLEOTIDE

• Each nucleotide consists of three parts:
  • Sugar molecule – deoxyribose
  • Phosphate group
  • Nitrogenous base

NITROGENOUS BASE

• Four different types:
  • Adenine (A)
  • Thymine (T)
  • Guanine (G)
  • Cytosine (C)
• The bases are grouped according to size:
  • Purine bases: large molecules
    • Adenine and Guanine
  • Pyrimidine bases: smaller molecules
    • Cytosine and Thymine

FORMATION OF A NUCLEOTIDE

• Deoxyribose combines with a phosphate
• Deoxyribose combines with a nitrogenous base
• There are four different types of nucleotides in a DNA molecule

FORMATION OF DNA

• Deoxyribose of one nucleotide forms a bond with the phosphate of another nucleotide
• Two long strands are formed: ladder
• Each rung is formed by linking two nitrogenous bases
• The two bases are called base pairs
• Nitrogenous bases are joined by weak hydrogen bonds: easily broken by enzymes

FORMATION OF DNA…. CONTINUES

• Nitrogenous bases always combine:
  • A = T
  • G = C
• Two hydrogen bonds between A and T
• Three hydrogen bonds between G and C
• Following base pairs are formed:
  • AT
  • GC
  • TA
  • CG
• The base pairs occur in any sequence: sequence important for coding
• One DNA strand is the complement of the other strand

INTERESTING FACTS

• 2 anti-parallel strands
• Ladder twist through 360^{\circ} every 10 bases
• The order of the bases varies between organisms
• This sequence is the code that controls the development of organisms: code of life or genetic code
• Total length of DNA in the nucleus of a human cell is approximately 2 metres

ROLE OF DNA

Genes

• DNA carries the code for the synthesis of proteins
• A gene is a short segment of DNA
• A gene contains a specific sequence of nitrogenous bases
• The sequence of nitrogenous bases determines the sequence and type of amino acids to from a particular protein
• DNA can replicate to transfer the genetic code

Non-coding DNA

• ± 2% of DNA codes for proteins
• The rest does not carry information to produce proteins
• Referred to as “junk DNA”
• Non-coding varies between individuals
• Used in DNA fingerprinting/profiling

Functions of non-coding DNA

• Plays a role in regulation and control of the expression of genes in coding DNA
• Protects genes from mutations