Download BIOL10005: Genetics and the Evolution of Life

BIOL10005: Genetics and the Evolution of Life
Lecture 1: Genetics: Variation and the Genome
Genetics is the study of the variation between and among living things, and how it is inherited.
Variation may result from:
• environmental factors
• genetic factors
• a combination of genetic and environmental factors
DNA is the hereditary material
Genes, the units of biological information, are composed of DNA
Word
Gene
Genotype
Phenotype
Genome
Definition
The fundamental physical and functional unit of heredity, which carries information
from one generation to the next; a segment of DNA
The specific allele constitution of an organism, either of the entire organism or for a
certain set of genes or a certain gene
The morphological, biochemical and behavioural properties of an organisms resulting
from a specific genotype and its interaction with the environment
The genetic complement in a set of chromosomes
Lecture 2: Structure and Replication of DNA
Semi-conservative replication of DNA in prokaryotes and eukaryotes
The replication fork, polarity of DNA
Differences between replication origins in prokaryotes and eukaryotes
Enzymes involved
Lagging strand and leading strand replication
Replication of telomeres
Polymerase Chain Reaction (PCR) – a technique that mimics replication
Word
Semi-conservative
replication
Complementary
base paring
Purine
Polynucleotide
Phosphodiester
bond
5’ (PO4) end
3’ (OH) end
Leading/lagging
strand
Definition
The method of DNA replication where the new molecule of DNA has one strand
which comes from the parent molecule and one strand which is newly synthesised
Nucleotides or nucleotide sequences that are able to base pair, for example G and C
are complementary, as are A and T
One of the two types of nitrogenous base compounds found in nucleotides. The purine
bases in DNA are adenine and guanine
A polymer consisting of nucleotide subunits
The chemical bond that links the adjacent nucleotides in a polynucleotide
The end of the polynucleotide that terminates with a phosphate attached to the 5’carbon of the sugar
The end of the polynucleotide which terminates with a hydroxyl group attached to the
3’-carbon of the sugar
The leading strand is that which is synthesised continuously, whereas the lagging
strand is synthesised in a discontinuous fashion during DNA replication
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Okazaki fragment One of the RNA-primed short segments of DNA synthesised during replication of the
lagging strand of the double helix
DNA helicase
An enzyme that uses energy from ATP to unwind the DNA
Topoisomerase
An enzyme which can relax DNA by cutting and reforming the polynucleotide
(gyrase)
backbones
Single-strand
One of the proteins that attaches to single-stranded regions of DNA and prevents the
binding protein
reformation of the base pairs, particularly in the region of the replication fork
Primase
An enzyme which synthesises the RNA primer
DNA Polymerase An enzyme which adds nucleotides to gaps when primers are removed
I
DNA Polymerase The main polymerase which adds nucleotides to the template in a complementary way
III
RNA primer
Sequence of RNA to make a stretch of double-stranded DNA on which the DNA
polymerase III can work
Ligase
An enzyme which joins sequences of DNA together
Lecture 3: Genes, Alleles and Chromosomes
The one gene – one polypeptide hypothesis
The products of genes are polypeptides and RNA
A polypeptide is a chain of amino acids linked by peptide bonds
Genes are defined by mutations; the concept of alleles
Genes are mostly located in chromosomes
Chromosomes consist of DNA and protein:
• Chromatin, nucleosomes – histones
Behaviour of chromosomes – non disjunction
Word
Mutation
Locus
Haploid
Diploid
Aneuploidy
Euploidy
Autosome
Sex Chromosome
Chromatin
Nucleosome
Definition
A change in the genetic material which can be passed from cell to cell or parent to
offspring. A somatic mutation will occur in a somatic cell and hence will not be
passed on to the next generation, compared to a germline mutation which can be
passed to offspring
The position of a gene on a chromosome
One copy of each chromosome
Two of each type of chromosome; the two similar chromosomes are described as
homologous
Addition or deletion of a single chromosome or sometimes extra single chromosomes
Addition of deletion of a complete haploid set/s of chromosomes
A chromosome not involved in determining the sex of the organism
A chromosome involved in determining the sex of the organism
DNA and protein (histones)
Basic unit of chromatin where DNA is wrapped around histone proteins
Lecture 4: Behaviour of Chromosomes and Alleles; Segregation of Alleles
Meiosis includes 3 processes which are important to patterns of inheritance:
• Segregation of alleles
• Independent assortment of chromosomes
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• Crossing over and recombination of alleles
A single factor cross and the principle of segregation:
• P, F1 and F2 generations, phenotype/genotype
• Dominance/recessiveness
• Homozygote/heterozygote
• Punnett square
• Complete dominance
• Incomplete dominance, e.g. snapdragons
• Codominance, e.g. haemoglobin and blood groups
• Allelic notation
• The F2 monohybrid ratio of 3:1
• The reciprocal cross
• The test cross
• The backcross
Word
Dominant phenotype
Recessive phenotype
Monohybrid cross
Complete
dominance
Codominance
Incomplete
dominance
Definition
The phenotype expressed in the heterozygote
The phenotype not expressed in the heterozygote
A crossing involving one gene where on parent is homozygous recessive, the other
homozygous dominant, the F1 is heterozygote and the F2 is in a ratio 1:2:1 or 3:1
The homozygous dominant and the heterozygote have the same phenotype
The effect of each allele can be seen in the heterozygote, e.g. sickle cell anaemia
The heterozygote is a blended phenotype, e.g. flower colour
Lecture 5: Multiple Alleles and other Extensions to Mendel
Multiple alleles, e.g. clover, tabbying in cats, ABO blood group
Pleiotropy, e.g. sickle cell anaemia, albinism
Phenocopy, e.g. amelia-meromelia/thalidomide, vitamin D deficiency/vitamin D resistant crickets
Variable expression, e.g. polydactyly
Penetrance, expressivity, e.g. eyeless in Drosophila, polydactyly
Lethality, e.g. yellow locus in mice, achondroplasia in humans, Manx locus in cats (all of these are recessive
lethals)
Changes to the monohybrid ratio resulting from some of these effects
Word
Pleiotropy
Phenocopy
Lethal allele, lethal
genotype, lethal
mutation
Recessive lethal
Dominant lethal
Definition
A single mutation which simultaneously affects several apparently unrelated aspects
of the phenotype
A phenotype that results from an environmental factor that mimics a genetically
caused phenotype mutation
An allele, genotype or mutation which results in death (may be lethal at different
stages of development)
Results in death only in the homozygous state
Results in death when heterozygous or homozygous
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Lecture 6: Two Genes and Independent Assortment
One of the processes of meiosis which affects the outcome of a genetic cross is independent assortment of
chromosomes. This is relevant when there is more than one pair of chromosomes
Independent assortment in the context of meiosis – homologous, non-homologous chromosomes, the
consequences for segregation of alleles
Gamete formation for two or more loci
Punnett square: F2 genotypic ratio, F2 phenotypic ratio 9:3:3:1
The use of a test cross: one locus; two loci
Lecture 7: Gene Interaction
A reminder of the 9:3:3:1 phenotypic ratio and how it is generated
Reminder of the action of a single gene in a biosynthetic pathway
Genes do not operate in isolation – the phenotype results from the interaction of the gene products from
more than one locus
Gene interaction where the ratio in the F2 of a dihybrid cross is 9:3:3:1, e.g. the dense/dilute pigment locus in
dogs and cats interacting with the black/brown pigment locus, comb shape in fowl
Gene interaction, e.g. recessive epistasis 9:3:4 – an explanation of the interaction which results in the ratio:
• Illustration of how a 9:3:4 phenotypic ratio can be explained using a biosynthetic pathway
Gene interaction, e.g. dominant epistasis 12:3:1 – an explanation of the interaction which results in this ratio
The genetic explanation for a 9:7, 13:3, 9:6:1 and 15:1 ratio
Word
Recessive epistasis
Dominant epistasis
Definition
Interaction between two genes affecting the same phenotypic trait, where the
homozygous recessive genotype at one locus masks the effect of the alleles at the
other locus, e.g. coat colour in mice
Interaction between two genes affecting the same phenotypic trait, where the allele
for the dominant trait at one locus masks the effect of the alleles at the other locus,
e.g. colour in plants (green/yellow/white)
Lecture 8: Sex Determination
Sex determining mechanisms:
• Environmental
• Genes, e.g. androgen insensitivity in humans (TDF locus), tra in Drosophila
• Haplo/diploidy in hymenoptera (bees and wasps)
• Balance of autosomes to sex chromosome, e.g. Drosophila
• The presence of a sex determining chromosome, e.g. Y in humans
• Aneuploidies of sex chromosomes, how do they arise
Lecture 9: Inheritance Relation to Sex and X Inactivation
The human X and Y chromosomes:
• Pseudoautosomal region
The pattern of inheritance of a trait on the X chromosome (brief comparison with Z or birds)
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