VCE biology summary notes

Biology study design dot points
UNIT 3 AOS 1
The relationship between nucleic acids and proteins
- nucleic acids as information molecules that encode instructions
for the synthesis of proteins: the structure of DNA, the three main
forms of RNA (mRNA, rRNA and tRNA) and a comparison of their
respective nucleotides.
o Nucleic acid:
 Deoxyribonucleic acid (DNA):
 located in chromosomes in the nucleus
 genetic material that contains hereditary information
 transmitted from generation to generation
 double-stranded helix
 Ribonucleic acid (RNA)
 Formed against a template strand of DNA
 Both are made up of nitrogenous bases and a sugar-
phosphate backbones
 Both polymers made up of repeated subunit monomers called
nucleotides.
o Nucleotide structure:




o
o Synthesis of nucleotides: condensation polymerisation
 One nucleotide is joined to the next by a covalent
phosphodiester bond between the P group on the 5’ C of one
nucleotide and the hydroxyl group (OH) 3’ C of the other
nucleotide
o DNA (deoxyribonucleic acid)
 Nucleotides joined together form a polynucleotide chain
running from 5’ to 3’
 5’ end has a free phosphate group
 3’ end has a free hydroxyl group
 Structure of DNA
 Two polynucleotide chain
 The strands run antiparallel
 Two polynucleotide strands spiral around each other
forming a double helix

,  Hydrogen bonds between pairs of nitrogenous bases
stabilise the secondary structure of the DNA forming
the double helix
 Sugar:
 Made up of a deoxyribose sugar
 Deoxy sugar means that it is derived from the sugar
ribose by loss of an oxygen atom
 Nucleotides in DNA
 Adenine (a): purine. Pairs with thymine (T) – pyrimidine
o Two (weak) hydrogen bonds
 Guanine (g)- a purine. Pairs with cytosine (c) - a
pyrimidine
o Three (weak) hydrogen bonds
 Nucleosomes
 In Eukaryotes the DNA is coiled around small proteins
called histones, forming a structural unit called a
nucleosome
o Nucleosome consists of a length of DNA coiled
around a core of histones
o RNA (ribonucleic acid)
 Structure of RNA
 Singled stranded
 Uracil replaces thymine
 Sugars:
 Made of a ribose sugar
 Nucleotides in RNA
 Adenine (A) pairs with uracil (u)
 Guanine (G) pairs with cytosine (c)
o Main forms of RNA
 Messenger RNA (mRNA)
 Carries genetic information from nucleus to the
ribosomes for protein synthesis
 Transcription results in pre-mRNA, pre-mRNA is
processed forming mature mRNA which goes into the
cytosol and binds to ribosome for translation
 Transfer RNA (tRNA)
 Delivers specific amino acids to the ribosome after
recognising specific nucleotide sequences on mRNA
 61 different tRNA that combines with 1 particular
amino acid.
o Does not recognise the 3 stop codons, so
translation is terminated
 tRNA binds to 3 places: exit site, peptidyl site, and the
aminoacyl
 on the other side of the tRNA there is a sequence of
nucleotides known as the anticodon
 anticodon recognises a particular sequence of
nucleotides in the mRNA
o allows for an amino acid to be positioned in the
correct space on a protein.

,  Ribosomal RNA (rRNA)
 Serves as the main structural component of ribosomes
within cells
 Together with proteins, rRNA forms the organelle
ribosome. Ribosomes are the sites of translation:
where mRNA is translated into a chain of amino acids




- the genetic code as a universal triplet code that is degenerate
and the steps in gene expression, including transcription, RNA
processing in eukaryotic cells and translation by ribosomes.
o DNA
 Stores and transmits hereditary information
 Store as sequence of nucleotides
 Order of nucleotides determines the products synthesised
o Genes
 A section of DNA that codes for a protein
 Store information
o DNA to protein
 Transcription and RNA processing
 DNA sequence of gene is copied into RNA nucleotides
to form mRNA.
 Translation
 mRNA is then decoded during translation to specify the
sequence of amino acids required for the polypeptide
chain
 Start and stop regions
 DNA has a start triplet (TAC)
o On the mRNA strand, this codon will be AUG
o This codes for the amino acid methionine
 Stop (termination) signal (ATT or ATC or ACT)
o The stop triplet does NOT code for an amino acid
o On the mRNA strand, the stop codons will be
UAA, UAG, and UGA.
 Template strand
 Only one of the two chains contains the information
present in a particular gene, and this is called the
template strand
 The complementary chain is sometimes called the non-
template strand, or the coding strand
 Gene sequencing

,  The order of the nucleotides in a gene is identified, the
gene is said to be “sequenced”
o Genetic code
 The genetic code is a series of rules that define how genetic
information stored within nucleotides is transcribed and
translated into functional proteins
 Stored as a 3-letter code of nucleotides in DNA: called a
triplet
 When DNA is transcribed into mature mRNA, the triplet is
then called a codon
 Each triplet or codon codes for one amino acid or may
provide specific instructions such as “start translation”
or “stop translation”
 Pieces of information in the genetic code consist of triplets or
3-base sequences
 Non-overlapping: each triplet or codon is read independently,
without overlapping from adjacent triplets or codons
 Universal: the rules are the same for all organisms on Earth.
The code is essentially the same in bacteria, in plants and in
animal. The same codons/triplets code for the same amino
acids in all living things
 Degenerate: more than one triplet of bases codes for one
amino acid
 Benefit: buffer against mutations; a single mutation
may not necessarily lead to a change in amino acid
coded for (and thus the protein produced)
 Unambiguous: each codon is only capable of coding for one
specific amino acid




o Gene expression