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DNA Replication
Conservative replication – One new double helix that consisted of 100% new
material but still identical to original double helix is made
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DNA
polymerase collects free-floating nitrogen bases in the nucleoplasm of the nucleus
and aligns them opposite their complementary bases
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DNA ligase forms hydrogen bonds between the complementary bases (joining them
together) and then coils the DNA strands to form 2 identical double helix strands of
DNA
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Meselson and Stahl’s DNA replication using Nitrogen and Bacteria
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coli bacteria in a medium where their only source of Nitrogen
was the radioactive isotope of N15
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2
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coli
bacteria that were subject to N14
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3
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N14 is lighter then N15 and hence produced a
band high up the test tube
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4
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Why is DNA a triplet codon?
There are 20 amino acids that combine to make proteins in the body
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Due to this DNA is a triplet codon of bases to produce the
20 amino acids in the body
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Protein Synthesis
Transcription – DNA helicase uncoils the DNA double helix strand and breaks the
weak hydrogen bonds between the complementary bases
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The
RNA polymerase proofreads the mRNA opposite the DNA antisense strand and
checks to see whether the bases complement each other
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Phosphate groups and ribose sugars form between the RNA
bases
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Translation – The mRNA travels to the ribosome and attaches to a specific site on
the ribosome
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Only 2 triplet codon
bases of the mRNA are allowed on the ribosome specific site at any one time
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Each tRNA carries an amino acid
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The tRNA anti-codon aligns alongside the
mRNA strand pairing up the complementary bases
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Once this is done the mRNA is moved along so that the
next 2-triplet codon can be coded
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The polypeptide chain detaches from the tRNA and
move through the cytoplasm to where they are needed
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It can be caused by insertion, deletion, inversion or duplication of a single
base unit in a DNA strand
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A change in the gene itself
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Whole-chromosome mutations – An entire chromosome is lost during meiosis or
duplicated in one cell by errors in the process
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Down syndrome

Mutations
Advantages
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Disadvantages

A new and superior protein that
can lead to reproductive
advantage
Natural selection

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Cancer where there a tumour is
formed
Gamete mutations are passed
onto offspring
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X – rays

Enzymes
Enzymes are biological catalysts that speed up the rate of chemical reaction without
themselves being altered
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They have oneentry specific active sites that are highly specific to the enzyme and can only
accommodate a specific substrate; this means that they have a high-specificity
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As the temperature starts to increase above the optimum temperature of the
enzyme, the enzyme tertiary and quaternary structure starts to unravel and it loses
its high specificity
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By increasing the pH of the solution above the optimum pH of the enzyme, the
higher pH interferes with the enzymes ability to form hydrogen bonds and sulfur
bridges, which leads to the tertiary and the quaternary-coiled structure not being
able to form
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The dominant alleles are predominantly expressed in the phenotype of
the organism, and the recessive allele is only portrayed if both the alleles are
recessive
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Locus - the position that a gene is found that codes for a particular characteristic
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Homologous – chromosomes carry the same gene
Dominant allele – the characteristic that it codes for is expressed in the phenotype
of the organism
Recessive allele – the characteristic that it codes for is only expressed in the
phenotype when homozygous recessive is present (no dominant alleles)
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Genetic experiments – The organisms used should be:
1
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3
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5
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The law of independent assortment: Mendel’s second law
Different traits are inherited independently of each other

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