Higher Biology- Unit 1

• DNA has a double stranded antiparallel structure
• DNA nucleotides are composed of

  a deoxyribose sugar, phosphate and a base
• The deoxyribose sugar and the phosphate of neighbouring nucleotides form a sugar-phosphate backbone
• Bases form pairs (adenine + thymine and cytosine + Guanine) by hydrogen bonds
• The deoxyribose sugar is at the 3' end of the strand and the phosphate is at the 5' end of the strand
• The base sequence of DNA forms the genetic code
• Prokaryotes have a singular, circular chromosome and smaller circular plasmids
• Eukaryotes all have linear chromosomes, in the nucleus, which are tightly coiled around and packaged with associated proteins
• The associated proteins are called Histones
• Eukaryotes also contain circular chromosomes in their mitochondria and chloroplasts
• Yeast is a special example of a Eukaryote as it also has plasmids
• Prior to cell division, DNA is replicated by a DNA polymerase
• DNA polymerase needs a primer to start replication
• A primer is a short strand of nucleotides which binds to the 3' end of the template DNA strand allowing polymerase to add DNA nucleotides
• DNA is unwound and hydrogen bonds between bases are broken to form two template strands
• DNA polymerase adds DNA nucleotides to the deoxyribose 3' end of the strand that is newly forming
• DNA polymerase adds DNA nucleotides using complementary base pairing
• DNA polymerase can only add nucleotides in one direction resulting in the leading strand being replicated continuously and the lagging strand replicated in fragments
• Fragments of DNA are joined together by Ligase
• Polymerase Chain Reaction (PCR) amplifies DNA using complementary primers for specific target sequences
• In PCR, primers are short strands of nucleotides which are complementary to specific target sequences at the two ends of the region of DNA to be amplified
• Repeated cycles of heating and cooling amplify the target region of DNA
• DNA is heated to between 92 and 98 in order to separate the strands
• Its then cooled to between 50 and 65 to allow primers tol bind to target sequences
• Its then heated to between 70 and 80 degrees Celsius for the heat- tolerant DNA polymerase to replicate the region of DNA
• PCR can amplify DNA to help solve crimes, settle paternity suits and diagnose genetic disorders
• Only a fraction of the genes in a cell are expressed
• gene expression involves the transcription and the translation of DNA sequences
• Transcription and Translation involve three types of RNA:

  1. mRNA
  2. tRNA
  3. rRNA
• RNA is single stranded
• RNA nucleotides are composed of nucleotides containing

  1. Ribose sugar
  2. phosphate
  3. base
• The four bases in RNA are adenine, cytosine, guanine, and uracil
• Uracil is complementary to adenine
• mRNA is transcribed from DNA in the nucleus and translated into proteins at the ribosome in the cytoplasm
• Each triplet of bases on the mRNA molecule is called a codon
• codons code for a specific amino acid
• RNA polymerase moves along the DNA unwinding the double helix and breaking he hydrogen bonds between the bases
• RNA polymerase synthesises a primary transcript of mRNA from RNA nucleotides by complementary base pairing
• RNA splicing forms a mature mRNA transcript
• The introns of the primary transcript are noncoding regions of DNA