Topic 3: Genetics Flashcards
DNA
the genetic blueprint which codes for and determines the characteristics of an organism
Gene
A sequence of DNA that encodes for a specific trait
Locus
The position of a gene on a particular chromosome
Alleles
Alternate forms of a gene that code for different variations of a specific trait
Gene mutations
A change in the nucleotide sequence of section of DNA coding for a specific trait.
What are the causes of sickle-cell anaemia?
The DNA sequence changes from GAG to GTG on the non-transcribed strand and CTC to CAC on the template strand
mRNA sequence changes from GAG to GUG at the 6th codon position
The sixth amino acid for the beta chain of haemoglobin is changed from glutamic acid to valine. This alters the structure of the haemoglobin, forming insoluble strands. This means that it cannot carry oxygen effectively. The shape of the red blood cell changes to a sickle shape. This may form clots within the capillaries, blocking blood supply to vital organs and causing myriad health issues.
Genome
The totality of genetic information of a cell, organism or organelle. Includes all genes as well as non-coding DNA sequences.
The human genome has…
46 chromosomes
3 billion base pairs
21,000 genes
What is the Human Genome Project and what does it do?
It is an international cooperative venture established to sequence the human genome.
Mapping - number, size, location, and sequence of human genes is established.
Screening - the production of specific gene prones to detect sufferers and carriers of genetic diseases.
Medicine - the discovery of new proteins have led to improved treatments.
Ancestry - comparisons with other genomes have provided insight into the origins, evolution, and migratory patterns of man.
Is the number of genes in an organism an indicator of biological complexity?
No
Describe the structure of prokaryotic genes
Prokaryotes do not have a nucleus, instead, genetic material is found free in the cytoplasm in the region of the nucleoid.
Genetic material consists of a single chromosome with a circular DNA molecule.
The DNA is naked, not associated with proteins for additional packaging.
May have plasmids - small, circular DNA molecules containing only a few genes.
Bacterial conjugation
When bacterial cells exchange plasmids via their sex pilli. This allows bacteria to evolve new features within a generation.
Describe the structure of eukaryotic genes
Genetic material consists of multiple linear molecules of DNA associated with histone proteins. This results in a greatly compacted structure, allowing for more efficient storage.
There are different chromosomes that carry different genes.
Centromere
A constriction point in each chromosome which divides the chromosome into two sections or arms.
Homologous chromosomes
Maternal and paternal chromosome pairs. They share the same structural features and the same loci positions.
Diploid
Nuclei possessing pairs of homologous chromosomes. Two gene copies for each trait. All somatic cells in the organism will be diploid, with new diploid cells created via mitosis.
Haploid
Nuclei possessing only one set of chromosomes. A single gene copy for each trait. Present in bacteria and fungi.
Heterosomes
In humans, sex is determined by these. Females possess two copies of a large X chromosome. Males possess one copy of an X chromosome and one copy of a much shorter Y chromosome. The Y chromosome contains the genes for developing male sex characteristics.
Autosomes
The chromosomes in the organism that do not determine sex
Karyotypes
The number and types of chromosomes in a eukaryotic cell. Determined via harvesting cells (usually from a foetus or white blood cells of adults), and chemically induced cell division.
Karyograms
Visual profiles made when chromosomes are stained and photographed. Chromosomes are arranged into homologous pairs according to size.
Down syndrome
A condition whereby the individual has three copies of chromosome 21. This causes mental and physical delays in the way the child develops.
Autoradiography
John Cairns’ technique for measuring the length of DNA molecules. He used it to visualize chromosomes whilst uncoiled, allowing for most accurate indications of length.
Meiosis
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 distinct haploid nuclei.
Interphase
When DNA is replicated to produce two genetically identical copies.
Sister chromatids
Two identical DNA molecules, held together by a single centromere.
Prophase I
Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs.
Metaphase I
Spindle fires from opposing centrosomes connect to bivalents and align them along the middle of the cell.
Anaphase I
Spindle fires contract and split the bivalent, homologous chromosomes move to opposite piles of the cell.
Telophase I
Chromosomes decondense, nuclear membranes may reform, cell divides to form two haploid daughter cells.
Prophase IJ
Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles
Metaphase II
Spindle fibres from opposing centrosomes attach to chromosomes and align them along the cell equator.
Anaphase II
Spindle fibres contract and separate the sister chromatids, chromatids move to opposite poles.
Telophase II
Chromosomes decondense, nuclear membrane reforms, cells divide via cytokinesis to form four haploid daughter cells.
Crossing over
Occurs during prophase I. Homologous chromosomes are held together at points called chiasmata. The crossing over of genetic material between non-sister chromatids can occur at these chiasmata.
Random assortment
The orientation of pairs of homologous chromosomes is random, as is the subsequent assortment of chromosomes into gametes. Random assortment and crossing over both lead to more genetic variation.
Random fertilisation
The fusion of two haploid gametes resulting in the formation of a diploid zygote. The zygote can then divide by mitosis and differentiate to form a developing embryo.
Non-disjunction
Chromosomes failing to separate correctly, resulting in gametes with one extra or one missing, chromosome. This may occur via:
Failure of homologous to separate in anaphase I.
Failure of sister chromatids to separate in anaphase II
Karyotyping
The process by which chromosomes are organized and visualized for inspection. Cells are harvested from the foetus before being chemically induced to undertake cell division.
Methods of karyotyping?
Chorionic villi sampling involves removing a small sample of the chorionic villus via a tube inserted through the cervix
Amniocentesis involves the extraction of a small amount of amniotic fluid with a needle.
Mendel’s experiment
Gregor Mendel developed the principles of inheritance by performing experiments on pea plants. He crossed different varieties of purebred pea plants, and collected the seeds to determine their characteristics. When crossing two purebred varieties, the results were not a blend, only one feature would be expressed.
What are Mendel’s 3 Laws?
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 alleles for one gene occurs independently to that of any other gene.
Principle of dominance - recessive alleles will be masked by dominant alleles.
Genotype
The gene composition for a specific trait. The genotype of a particular gene will be either homozygous or heterozygous.
Phenotype
The observable characteristics of a specific trait
Cystic Fibrosis
An autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7. Individuals produce mucus which is unusually thick and sticky. It clogs the airways and secretary ducts, leading to respiratory failure and pancreatic cycts.
Huntington’s disease
An autosomal disease caused by a mutation to the HTT gene on chromosome 4. Symptoms include uncontrollable sporadic movements and dementia.
Sex linkage
When a gene controlling a characteristic is located on a sex chromosome.
Haemophilia
A genetic disorder whereby the body’s ability to control blood clotting is impaired.
Red-green colour blindness
A genetic disorder whereby an individual fails to discriminate between red and green hues. It is caused by a mutation to the red or green retinal photo receptors, on the X chromosome
Causes of gene mutations
Radiation
Chemicals (e.g. reactive oxygen species, alkylating agents.)
Biological agents (e.g. bacteria, viruses)
Pedigree charts
A chart of the genetic history of a family over several generations. Males are represented as squares, and females as circles. Shaded symbols mean an individual is affected by a condition.
