gene and the position of a gene
A gene is a sequence of DNA that encodes for a specific trait (traits may also be influenced by multiple genes)
The position of a gene on a particular chromosome is called the locus (plural = loci)
alleles
Alleles are alternative forms of a gene that code for the different variations of a specific trait
For example, the gene for eye colour has alleles that encode different shades / pigments
How do alleles differ?
As alleles are alternative forms of the one gene, they possess very similar gene sequences
Alleles only differ from each other by one or a few bases
gene mutation
A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait
New alleles are formed by mutation
Gene mutations can be beneficial, detrimental or neutral
Beneficial mutations change the gene sequence (missense muta
What is an example of a disorder caused by gene mutation?
Sickle cell anaemia is an example of a disorder caused by a gene mutation
The disease allele arose from a base substitution mutation - where a single base was changed in the gene sequence
Cause of Sickle Cell Anemia
Sickle cell anaemia results from a change to the 6th codon for the beta chain of haemoglobin
DNA: The DNA sequence changes from GAG to GTG on the non-coding strand (CTC to CAC on the coding strand)
mRNA: The mRNA sequence changes from GAG to GUG at the 6t
Consequence of Sickle cell anemia
The amino acid change (Glu ? Val) alters the structure of haemoglobin, causing it to form insoluble fibrous strands
The insoluble haemoglobin cannot carry oxygen as effectively, causing the individual to feel constantly tired
The formation of fibrous haem
genome
The genome is the totality of genetic information of a cell, organism or organelle
This includes all genes as well as non-coding DNA sequences (e.g. introns, promoters, short tandem repeats, etc.)
What does a human genome consist of?
The human genome consists of:
46 chromosomes (barring aneuploidy)
~3 billion base pairs
~21,000 genes
Human Genome Project
The Human Genome Project (HGP) was an international cooperative venture established to sequence the human genome
The HGP showed that humans share the majority of their sequence, with short nucleotide polymorphisms contributing diversity
Prokaryote genetics
Prokaryotes do not possess a nucleus - instead genetic material is found free in the cytoplasm in a region called the nucleoid
The genetic material of a prokaryote consists of a single chromosome consisting of a circular DNA molecule (genophore)
The DNA o
What do prokaryotes have that eukaryotes do not?
Plasmids are small, circular DNA molecules that contain only a few genes and are capable of self-replication
Plasmids are present in some prokaryotic cells, but are not naturally present in eukaryotic cells
Bacterial cells may exchange plasmids via their
Eukaryote genetics
The genetic material of eukaryotic cells consist of multiple linear molecules of DNA that are associated with histone proteins
The packaging of DNA with histone proteins results in a greatly compacted structure, allowing for more efficient storage
summarise the organization of eukaryotic chromosomes
Organisation of eukaryotic chromosomes can be summarised as follows:
DNA is complexed with eight histone proteins (an octamer) to form a complex called a nucleosome
Nucleosomes are linked by an additional histone protein (H1 histone) to form a string of c
Eukaryotic chromosomes
Eukaryotic chromosomes are linear molecules of DNA that are compacted during cell division (mitosis or meiosis)
Homologous chromosomes
Homologous chromosomes are chromosomes that share:
The same structural features (e.g. same size, same banding patterns, same centromere positions)
The same genes at the same loci positions (while the genes are the same, alleles may be different)
Homologou
Where do sexually reproducing organisms receive genetic material from?
As sexually reproducing organisms receive genetic material from both parents, they have two sets of chromosomes (diploid)
To reproduce in turn, these organisms must create sex cells (gametes) with half the number of chromosomes (haploid)
When two haploid
Diploid vs Haploid
Diploid
Nuclei possessing pairs of homologous chromosomes are diploid (symbolised by 2n)
These nuclei will possess two gene copies (alleles) for each trait
All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosi
In humans, how is sex determined?
In humans, sex is determined by a pair of chromosomes called the sex chromosomes (or heterosomes)
Females possess two copies of a large X chromosome (XX)
Males possess one copy of an X chromosome and one copy of a much shorter Y chromosome (XY)
what are the remaining chromosomes called?
The remaining chromosomes in the organism are called autosomes (they do not determine sex)
karyotypes
Karyotypes are the number and types of chromosomes in a eukaryotic cell - they are determined via a process that involves:
Harvesting cells (usually from a foetus or white blood cells of adults)
Chemically inducing cell division, then arresting mitosis wh
karyogram
The chromosomes are stained and photographed to generate a visual profile that is known as a karyogram
The chromosomes of an organism are arranged into homologous pairs according to size (with sex chromosomes shown last)
What is karyotyping used to do?
Karyotyping will typically occur prenatally and is used to:
Determine the gender of the unborn child (via identification of the sex chromosomes)
Test for chromosomal abnormalities (e.g. aneuploidies or translocations)
chromosome length
John Cairns pioneered a technique for measuring the length of DNA molecules by autoradiography
Previously, chromosome length could only be measured while condensed during mitosis (very inaccurate due to supercoiling)
Cairns used autoradiography to visuali
chromosome number
Chromosome number is a characteristic feature of members of a particular species
Organisms with different diploid numbers are unlikely to be able to interbreed (cannot form homologous pairs in zygotes)
In cases where different species do interbreed, offsp
genome size
Genome size can vary greatly between organisms and is not a valid indicator of genetic complexity
The largest known genome is possessed by the canopy plant Paris japonica - 150 billion base pairs
The smallest known genome is possessed by the bacterium Car
Meiosis
Meiosis is the process by which sex cells (gametes) are made in the reproductive organs
It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei
Process of meiosis
The process of meiosis consists of two cellular divisions:
The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid ? haploid)
The second meiotic division separates sister chromatids (created by the repl
Sister chromatids
Meiosis is preceded by interphase, during which DNA is replicated (in the S phase) to produce two genetically identical copies
The two identical DNA molecules are identified as sister chromatids, and are held together by a single centromere
The sister chr
First meiotic divison
Meiosis I
The first meiotic division is a reduction division (diploid ? haploid) in which homologous chromosomes are separated
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
M-I: Spindle
Second meiotic division
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle f
Final outcome of meiosis
The final outcome of meiosis is the production of four haploid daughter cells
These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)
crossing over of genetic material between non sister chromatids
In prophase I, homologous chromosomes undergo a process called synapsis, whereby they pair up to form a bivalent (or tetrad)
The homologous chromosomes are held together at points called chiasmata (singular: chiasma)
Crossing over of genetic material betw
Random assortment
During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements:
Maternal copy left / paternal copy right OR paternal copy left / maternal copy right
This orientation of pairs of homologous chromosomes is random,
Sexual life cycle
Fertilisation of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote that can grow via mitosis
If chromosome number was not halved in gametes, total chromosome numbers would double each generation (polyploidy)
What are the 3 main sources of genetic variation arising from sexual reproduction?
The three main sources of genetic variation arising from sexual reproduction are:
Crossing over (in prophase I)
Random assortment of chromosomes (in metaphase I)
Random fusion of gametes from different parents
Random fertilisation
The fusion of two haploid gametes results in the formation of a diploid zygote
This zygote can then divide by mitosis and differentiate to form a developing embryo
As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm w
non disjunction
Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)
The failure of chromosomes to separate may occur via:
Failure of homologues to separate in Anaphase I (re
chromosomal abnormalities
If a zygote is formed from a gamete that has experienced a non-disjunction event, the resulting offspring will have extra or missing chromosomes in every cell of their body
Conditions that arise from non-disjunction events include:
Patau's Syndrome (triso
Down syndrome
Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)
One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
The other parental gamete was normal and had a single copy of chromosome 21
When the
What did Gregor Mendel develop?
Gregor Mendel was an Austrian monk who developed the principles of inheritance by performing experiments on pea plants
First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics
Next,
What did Mendel discover?
As a result of these experiments, Mendel discovered the following things:
When he crossed two different purebred varieties together the results were not a blend - only one feature would be expressed
E.g. When purebred tall and short pea plants were crosse
Mendel's laws
While there are caveats to Mendel's conclusions, certain rules can be established:
Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene
Law of Independent Assortment: The segregation of all
Haploid Gametes
Gametes are haploid sex cells formed by the process of meiosis - males produce sperm and females produce ova
During meiosis I, homologous chromosomes are separated into different nuclei prior to cell division
As homologous chromosomes carry the same genes
Types of Zygosity
For any given gene, the combination of alleles can be categorised as follows:
If the maternal and paternal alleles are the same, the offspring is said to be homozygous for that gene
If the maternal and paternal alleles are different, the offspring is said
genotype
The gene composition (i.e. allele combination) for a specific trait is referred to as the genotype
The genotype of a particular gene will typically be either homozygous or heterozygous
phenotype
The observable characteristics of a specific trait (i.e. the physical expression) is referred to as the phenotype
The phenotype is determined by both the genotype and environmental influences
When does codominance occur?
Co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual
Heterozygotes therefore have an altered phenotype as the alleles are having a joint effect
When representing alleles, the convention is to u
How are genetic diseases caused?
Genetic diseases are caused when mutations to a gene (or genes) abrogate normal cellular function, leading to the development of a disease phenotype
Genetic diseases can be caused by recessive, dominant or co-dominant alleles
What's an example of an autosomal recessive disorder?
Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7
Individuals with cystic fibrosis produce mucus which is unusally thick and sticky
This mucus clogs the airways and secretory ducts of the digestive sy
Sex linkage
Sex linkage refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)
The Y chromosome is much shorter than the X chromosome and contains only a few genes (50 million bp; 78 genes)
The X chromosome is longer and contains m
sex linked inheritance patterns vs autosomal patterns
Sex-linked inheritance patterns differ from autosomal patterns due to the fact that the chromosomes aren't paired in males (XY)
This leads to the expression of sex-linked traits being predominantly associated with a particularly gender
Examples of X-linked recessive conditions
Red-green colour blindness and haemophilia are both examples of X-linked recessive conditions
Consequently, they are both far more common in males than in females (males cannot mask the trait as a carrier)
When assigning alleles for a sex-linked trait, th
Haemophilia
Haemophilia is a genetic disorder whereby the body's ability to control blood clotting (and hence stop bleeding) is impaired
The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome
When o
Red green colour blindness
Red-green colour blindness is a genetic disorder whereby an individual fails to discriminate between red and green hues
This condition is caused by a mutation to the red or green retinal photoreceptors, which are located on the X chromosome
Red-green colo
gene mutation
A gene mutation is a change to the base sequence of a gene that can affect the structure and function of the protein it encodes
Mutations can be spontaneous (caused by copying errors during DNA replication) or induced by exposure to external elements
Examples of factors which can induce mutations
Radiation - e.g. UV radiation from the sun, gamma radiation from radioisotopes, X-rays from medical equipment
Chemical - e.g. reactive oxygen species (found in pollutants), alkylating agents (found in cigarettes)
Biological Agents - e.g. bacteria (such as
pedigree
A pedigree is a chart of the genetic history of a family over several generations
Males are represented as squares, while females are represented as circles
Shaded symbols mean an individual is affected by a condition, while an unshaded symbol means they
determining autosomal inheritance
Dominant and recessive disease conditions may be identified only if certain patterns occur (otherwise it cannot be confirmed)
autosomal dominant
If both parents are affected and an offspring is unaffected, the trait must be dominant (parents are both heterozygous)
All affected individuals must have at least one affected parent
If both parents are unaffected, all offspring must be unaffected (homoz
autosomal recessive
If both parents are unaffected and an offspring is affected, the trait must be recessive (parents are heterozygous carriers)
If both parents show a trait, all offspring must also exhibit the trait (homozygous recessive)
x linked dominant
If a male shows a trait, so too must all daughters as well as his mother
An unaffected mother cannot have affected sons (or an affected father)
X-linked dominant traits tend to be more common in females (this is not sufficient evidence though)
x linked recessive
If a female shows a trait, so too must all sons as well as her father
An unaffected mother can have affected sons if she is a carrier (heterozygous)
X-linked recessive traits tend to be more common in males (this is not sufficient evidence though)
gel electrophoresis
Gel electrophoresis is a laboratory technique used to separate and isolate proteins or DNA fragments based on mass / size
Samples are placed in a block of gel and an electric current is applied which causes the samples to move through the gel
Smaller samp
DNA separation during gel electrophoresis
DNA may be cut into fragments using restriction endonuclease - different DNA samples will generate different fragment lengths
Fragments separate because DNA is negatively charged due to the presence of a phosphate group (PO43-) on each nucleotide
DNA samp
protein separation during gel electrophoresis
Proteins may be folded into a variety of shapes (affecting size) and have positive and negative regions (no clear charge)
Proteins must first be treated with an anionic detergent (SDS) in order to linearise and impart a uniform negative charge
Protein sam
DNA profiling
DNA profiling is a technique by which individuals can be identified and compared via their respective DNA profiles
Within the non-coding regions of an individual's genome there exists satellite DNA - long stretches of DNA made up of repeating elements cal
gene modification
The transfer of genes between species is called gene modification, and the new organism created is called a transgenic
Summarise the process of gene transfer
The process of gene transfer can be summarised in four key steps:
Isolation of gene and vector (by PCR)
Digestion of gene and vector (by restriction endonuclease)
Ligation of gene and vector (by DNA ligase)
Selection and expression of transgenic construct
Genetically modified organisms
Genetically modified organisms (GMOs) are used in agriculture to improve crop yields and reduce farming costs
However the use of GM crops is a contentious issue, as economic benefits must be weighed against environmental risks
GM crops and human health
GM crops can be used to improve human nutritional standards, by incorporating genes for certain proteins, vitamin or vaccines
Additionally, GM crops can be manufactured that lack common natural allergens or toxins
However, the inclusion or removal of cert
GM crops and economic consequences
GM crops can include genes to enable them to grow in a wider range of environments (e.g. drought / frost / salinity resistance)
GM crops can be manufactured to produce greater yields (crops can potentially grow larger and faster)
GM crops can include gene
GM crops and environmental issues
The ability to farm a wider range of environments with GM crops will potentially reduce the need for associated deforestation
Also, the generation of pest-resistant crops means that less chemical insecticides will be released into the environment
However,
clones
Clones are groups of genetically identical organisms or a group of cells derived from a single original parent cell
Organisms that reproduce asexually will produce genetically identical clones
Additionally, mechanisms exist whereby sexually reproducing or
somatic cell nuclear transfer
Somatic cell nuclear transfer is a method by which cloned embryos can be produced using differentiated adult cells
Somatic cells are removed from the adult donor and cultured (these cells are diploid and contain the entire genome)
An unfertilised egg is r
natural cloning
Many species can reproduce asexually and hence possess natural methods of cloning
All bacteria, the majority of fungi and many species of protists reproduce asexually to produce genetic clones
While most plants reproduce sexually, they also possess method
What are some animal cloning methods?
Binary Fission
The parent organism divides equally in two, so as to produce two genetically identical daughter organisms
This method of cloning occurs in Planaria (flatworms) but is also common to bacteria and protists (e.g. euglena, amoeba)
Budding
Cells
what are some plant cloning methods?
Plants have the capacity for vegetative propagation, whereby small pieces can be induced to grow independently
This is because adult plants possess meristematic tissue capable of cellular differentiation (totipotent)
Virtually all types of roots and shoot
what are some human cloning methods?
Human Cloning Methods
Even human beings are capable of creating genetic clones through natural means
Identical twins (monozygotic) are created when a fertilised egg (zygote) splits into two identical cells, each forming an embryo
Non-identical twins (dizy
what's one reliable method of artificial cloning?
A second and more reliable method of artificial cloning involves somatic cell nuclear transfer (SCNT)
This involves replacing the haploid nucleus of an unfertilised egg with a diploid nucleus from an adult donor
The advantage of this technique is that it
stem cutting
A stem cutting is a separated portion of plant stem that can regrow into a new independent clone via vegetative propagation
All stems possess nodes, from which a leaf, branch or aerial root may grow - the region between nodes are called internodes
Stem cu
There are a variety of factors that will influence successful rooting of a stem cutting, including:
Cutting position (whether cutting occurs above or below a node, as well as the relative proximity of the cut to the node)
Length of cutting (including how many nodes remain on the cutting)
Growth medium (whether left in soil, water, potting mix, compost o
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