IMMS Flashcards
Describe the structure of DNA
Double helix structure made up of complimentary base pairs (Adenine-Thymine, Cytosine-Guanine) bonded together by hydrogen bonds.
In the cell DNA is wrapped around histone proteins to form nucleosomes, which supercoil to form chromosomes.
Karyotype of human cell?
46 chromosomes arranged in pairs, in size order from biggest to smallest. Sex chromosomes number 23.
22 pairs of autosomes.
Final fair sex chromosomes (XY=male, XX=female)
A human chromosomes constitutes…
a continuous DNA duplex of around 10^7 base pairs and several hundred genes.
A long arm (q) and a short arm (p)
How many genes do humans have?
around 22000
Role of mitosis?
Produces 2 daughter cells, genetically identical to the parent cells. For growth and replacment of dead cells.
How does meiosis differ to mitosis?
Only in gametes.
Recombination genetic material for genetic diversity.
2 cell divisions occur.
4 haploid (23 chromosomes) cells produced, genetically different from each other and parent cell.
NOT a cycle.
Describe meiosis step by step.
Meoisis 1: chromosome number halves.
- Prophase 1: crossing over between non-sister chromatids, genes sort independently of each other, resulting in genetic diversity.
- Metaphase 1: random assortment of chromosomes on the metaphase plate relates in genetic diversity.
Meiosis 2:
sister chromatids separate, haploid cells produced
Describe gametogenesis in males
Proliferation of primordial germ cells (undifferentiated developing gametes) by mitosis.
Some mitosis occurs in embryonic stages to produce primary spermatocytes present at birth.
Mitosis really begins during puberty, continues throughout life.
Cytoplasm divides evenly - after meiosis 2 there are 4 equal sized gametes.
Millions of mature sperm continually produces, process taking 60-65 days.
100-200million sperm per ejaculate.
Describe gametogenesis in females
Primordial germ cell undergoes 30 mitoses to become oogonia.
OOgonia enters prophase 1 of meiosis 1 by 8th month of intrauterine life. Process is then suspended and cells enter ovulation 11ish years letter until 5ish.
Meiosis 1 is completed at ovulation, then cells remain in suspended animation (1 big 1 small cell, each with diploid DNA).
Meiosis 2 only completed if fertilisation occurs - big and small cell’s Cytoplasm divides unequally to form 1 egg and 3 polar bodies which apoptose.
Describe non-disjunction and the problems this can cause.
failure of chromosome pairs to separate in meiosis 1 or of sister chromatids to separate in meiosis 2.
This can result in Down’s Syndrome, where non-disjunction of chromosome 21 results in trisomy 21 (extra chromosome).
Can also result in monosomy - loss of chromosome - e.g Turner’s syndrome, where there is 1 X chromosome.
Describe gonadal mosaicism and the problems this can cause
Occurs when the precursor germline cells to ova or spermatozoa are a mixture of 2 or more genetically different cell lines due to errors in mitosis - 1 cell line is normal, the other is mutated.
Incidence increases with advancing paternal age.
Since the genetic change occurs only in the germline and all the other cells are unaffected, the parent is healthy but the foetus may have genetic diseases.
Can be observed with any inheritance pattern, but most commonly autosomal dominant and X-linked.
More common in males.
Observed in a number of conditions including osteogenesis imperfect and duchenne muscular dystrophy.
Causes/spectrum of disease - categories and examples.
Genetic:
Individually rare, but cumulatively enough to have regional genetic services.
Down’s syndrome, cystic fibrosis, huntingdon’s disease, haemophilia.
Multifactorial: main cause of disease in developed countries, has genetic and environmental roles. Spina bifida, cleft lip, diabetes, schizophrenia.
Environmental: main causes of disease in the 3rd world or in A&E. Genetics play small role.
Poor diet, infection, drugs, accidents.
Causes/spectrum of disease - categories and examples.
Genetic:
Individually rare, but cumulatively enough to have regional genetic services.
Down’s syndrome, cystic fibrosis, huntingdon’s disease, haemophilia.
Multifactorial: main cause of disease in developed countries, has genetic and environmental roles. Spina bifida, cleft lip, diabetes, schizophrenia.
Environmental: main causes of disease in the 3rd world or in A&E. Genetics play small role.
Poor diet, infection, drugs, accidents.
What is a locus in genetics
The position of a gene on the genetic map
Define:
Genotype
Phenotype
Allele
Genotype - genetic constitution of an individual
Phenotype - appearance of an individual resulting from the interaction of the environment and the genotype
Allele - one of several alternative forms of a gene at a specific locus. Normal allele referred to as wild type whereas disease allele carries the pathogenic mutation.
Define polymorphism
Frequent hereditary variations at a locus. Don’t cause problems but can be more or less efficient or make you more or less susceptible to disease. e.g. blood types, sex
Define consanguinity
Reproductive union between relatives
Define
homozygous
heterozygous
hemizygous
homozygous - same alleles at a locus
heterozygous - different alleles at a locus
hemizygous - genes that are carried on an unpaired chromosome - refers to a locus on an X chromosome in a male
Define penetrance -
complete penetrance vs. incomplete penetrance?
Proportion of people with a specific gene/genotype who show the expected phenotype.
Complete - clinical symptoms are present in all individuals who have the disease-causing mutation.
Incomplete - some individuals express the associated trait while others do not even though they carry the disease-causing gene.
Define variable expression?
Sex limitation?
Variable expression - variation in clinical features (type&severity) of a genetic disorder between individuals with the same gene alteration.
sex limitation -
what is the telomere of a chromosome?
region at the end of the chromosome which protects the chromosome from deteriorating or binding to other chromosomes
steps of interphase
G1 : mRNA and proteins synthesised
S : DNA replication and centrosome replication
G2 : cell forms the material that makes up the spindle fibres
basic occurrences in prophase
- chromatin condenses into chromosomes
- centrosomes produce microtubules
- centrosomes move to opposite poles of nucleus
order of steps of mitosis
Prophase Prometaphase Metaphase Anaphase Telophase
Events in prometaphase
- nuclear membrane breaks down
- microtubules invade nuclear space
- chromosomes attach to microtubules
events in metaphase
chromosomes line up along equatorial plane
describe events in anaphase
sister chromatids separate and move to opposite poles of th ecell
describe events of telophase
- nuclear membranes reform
- chromosomes unfold back into chromatin
- cytokinesis begins
what are primordial germ cells
cells in the embryo which will later develop into gametes
(male) what does lots of mitotic divisions of primordial germ cells form before puberty?
spermatogonia
(male) after puberty, what happens to the spermatogonia?
meiotic divisions commence, producing 4 equal gametes after meiosis 2
(female) what does lots of mitotic divisions of primordial germ cells form before puberty?
oogonia
what is gonadal mosaicism?
Genetic mutations in the germ line cells (cells that produce the gamete)
One population of cells in the gonad contains the usual genetic complement. One population contains DNA mutations
The parent is healthy.
If a child is formed from mutant germ line cell then the child may develop a genetic disease
