6.1 aslevel bio

• Nucleic acids


  DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are macromolecules (giant molecules)
• Polymers


  Made up of many similar, smaller molecules (known as subunits or monomers) joined into a long chain
• Nucleotides


  The subunits that make up DNA and RNA
• Components of nucleotides


  • Nitrogen-containing base (also known as a nitrogenous base)
  • Pentose sugar (containing 5 carbon atoms)
  • Phosphate group
• The bases adenine and guanine are purines – they have a double ring structure The bases cytosine, thymine and uracil are pyrimidines – they have a single ring structure
• The nucleic acid DNA is a polynucleotide – itis made up of many nucleotides bonded together in a long chain
• DNA molecules are made up of two polynucleotide strands lying side by side, running in opposite directions – the strands are said to be antiparallel
• These bonds are covalent bonds known as phosphodiester bonds
• The phosphodiester bonds link the 5-carbon of one deoxyribose sugar molecule to the phosphate group from the same nucleotide, which is itselflinked by another phosphodiester bond to the 3-carbon of the deoxyribose sugar molecule of the next nucleotide in the strand
• Each DNA polynucleotide strand is said to have a 3’ end and a 5’ end , one is known as the 5’ to 3’ strand and the other is known as the 3’ to 5’ strand
• The two antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by hydrogen bonds between the nitrogenous bases
• These hydrogen bonds always occur between the same pairs of bases: The purine adenine always pairs with the pyrimidine thymine – two hydrogen bonds are formed between these bases
• The purine guanine always pairs with the pyrimidine cytosine – three hydrogen bonds are formed between these bases
• This is known as complementary base pairing These pairs are known as DNA base pairs
• DNA replication occurs in preparation for mitosis, when a parent cell divides to produce two genetically identical daughter cells
• DNA replication occurs during the S phase ofthe cell cycle
• The hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands are broken This ‘unzips’ or unwinds the DNA double helix to form two single polynucleotide DNA strands
• Each ofthese single polynucleotide DNA strands acts as a template forthe formation of a new strand – the original strand and the new strand then join togetherto form a new DNA molecule
• This method ofreplicating DNA is known as semi-conservative replication because half of the original DNA molecule is conserved in each ofthe two new DNA molecules
• In the nucleus,there are free nucleotides to which two extra phosphates have been added (these free nucleotides with three phosphate groups are known as nucleoside triphosphates or‘activated nucleotides’)
• The extra phosphates activate the nucleotides, enabling them to take partin DNA replication
• The bases ofthe free nucleoside triphosphates align with their complementary bases on each ofthe template DNA strands
• The enzyme DNA polymerase synthesises new DNA strands from the two template strands It does this by catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands
• DNA polymerase breaks off the two extra phosphates and uses the energy released to create the phosphodiester bonds
• Hydrogen bonds then form between the complementary base pairs ofthe template and new DNA strands
• DNA polymerase can only build the new strand in one direction (5’ to 3’ direction) As DNA is unzipped from the 3’towards the 5’ end
• , DNA polymerase will attach to the 3’ end of the original strand and move towards the replication fork
• DNA polymerase enzyme can synthesise the leading strand continuously This template strand thatthe DNA polymerase attaches to is known as the leading strand The othertemplate strand created during DNA replication is known as the lagging strand On this strand, DNA polymerase moves away from the replication fork (from the 5’ end to the 3’ end)
• This means the DNA polymerase enzyme can only synthesise the lagging DNA strand in short segments (called Okazakifragments)
• DNA ligase is needed to join these lagging strand segments togetherto form a continuous complementary DNA strand
• DNA ligase does this by catalysing the formation of phosphodiester bonds between the segments to create a continuous sugar-phosphate backbone
• Like DNA,the nucleic acid RNA is a polynucleotide
• RNA nucleotides contain the nitrogenous bases adenine , guanine and cytosine Unlike DNA, RNA nucleotides never contain the nitrogenous base thymine
• – in place ofthis they contain the nitrogenous base uracil Unlike DNA, RNA nucleotides contain the pentose sugar ribose