Where is DNA found? What is the structure of DNA?
Nucleus and mitochondria. Double helix with strands in opposite directions. Complementary base pairs between strands. A-T = 2 bonds, C-G = 3 bonds
How is DNA packaged into a chromosome?
DNA coils around histones to form nucleosomes, coils again into supercoils + again into chromosomes
How many chromosomes are there to each cell? What are the two chromosome arms?
46 chromosomes (23 pairs). Long arm (q) + short arm (p(etite)) separated by centromere
Which two dyes are used to stain chromosomes?
- Giemsa = G banding - Quinacrine = Q banding, each band = 6-8 million base pairs
What is the purpose of mitosis?
- To produce 2 genetically identical daughter cells - For growth + to replace dead cells
What stage must a cell be in for mitosis to start? What happens in the synthesis stage for the cell cycle?
Cell must be in interphase. S phase = DNA and centrosome replication
What happens in the first stage of mitosis? Remember IPPMAT
Prophase: - Chromatin condenses into chromosomes - Centrosomes nucleate microtubules + move to opposite poles of nucleus
What happens in the second stage of mitosis?
Prometaphase. - Nuclear membrane breaks down - Spindle fibres begin to form
What are cells called that are permanently in the cell cycle? How about cells that never enter the cell cycle?
Permanently in = labiles. Never in = myocytes
What is the third stage of mitosis?
Metaphase. - Chromosomes line up along equatorial plane
What is the fourth stage of mitosis?
Anaphase. - Sister chromatids separate + pushed to opposite poles of cell
What is the last stage of mitosis?
Telophase. - Nuclear membranes reform - Chromosomes unfold into chromatin - Cytokinesis begins
What is the clinical relevance of studying cells undergoing mitosis?
- Can detect chromosomal abnormalities - Can categorise tumours as benign or malignant - Can grade malignant tumours - lots of mitosis = high grade
Which drugs can be used to disrupt mitosis?
To break mitotic spindle: - Taxol -Vinca alkaloids, e.g. vinblastine To break spindle poles: - Ispinesib To stop anaphase: - Colchicine-like drugs
What is meiosis? How is different to mitosis?
Only occurs in gametes, has 2 cell divisions to create 4 genetically varied haploid daughter cells. It is also not a cycle
What is crossing over? Where in meiosis does it take place?
Crossing over = sorting genes independently. Takes place in Prophase 1. - 1: Interphase G1, Interphase S, Prophase 1, Metaphase 1, Anaphase 1, Telophase 1 - 2: Prophase 2, Metaphase 2, Anaphase 2, Telophase 2, Cytokinesis
What is the difference between sperm and egg production?
Sperm: cytoplasm divides evenly, 4 equal gametes after meiosis 2 Egg: cytoplasm divided unequally to give 1 egg and 3 polar bodies (that apoptose). Meiosis completed at ovulation, meiosis 2 only completed if fertilisation occurs
What is non-disjunction?
The failure of chromosome pairs to separate in meiosis 1 or sister chromatids to separate properly in meiosis 2, e.g. Down syndrome = extra chromosome on 21
What is gonadal mosaicism?
All or part of parental germline is affected by disease mutation, but the parental somatic cells are not affected. Parent = healthy, foetus may have genetic disease. Risk increases with parent age
What are the 3 categories of disease causation?
- Genetic, e.g. Down’s syndrome - Multifactorial, e.g. Spina bifida - Environmental, e.g. scurvy from poor diet
What do phenotype and genotype mean?
- Genotype = genetic constitution of an individual - Phenotype = appearance of an individual = result of genotype + environment
What do the terms allele and polymorphism mean?
- Allele = one of several forms of a gene - Polymorphism = frequent hereditary variations at a locus
What do homozygous, heterozygous and hemizygous mean?
- Homozygous = both alleles the same at a locus - Heterozygous = alleles different at a locus - Hemizygous = only one allele refers to a locus on an X chromosome in a male
What are the 3 categories of genetic disease?
- Chromosomal - Mendelian, e.g. autosomal dominant/recessive, X-linked - Non-traditional, e.g. mitochondrial, imprinting (one allele active, other inactive) + mosaicism (error in cell division, same cells have different genetic makeup)
What are acrocentric chromosomes?
Chromosomes with a short arm
What is autosomal dominant inheritance? How does this compare to autosomal recessive inheritance?
- Autosomal dominant inheritance = disease which is manifest in the heterozygous state. Only one defected gene needed. Affects multiple generations. Both parents can sometimes be unaffected, e.g. gonadal mosaicism, mother has reduced penetrance, mother has variable expression. IF THE TRAIT IS DOMINANT, ONE OF THE PARENTS MUST HAVE THE DISEASE. Example = Huntington’s disease - Autosomal recessive inheritance = disease manifest in homozygous state. Requires 2 defected genes. Affected individuals are in single generation. Chance of having disease = 25%, chance of being carrier = 50%. If a child has an affected sibling, their chance of being a carrier increased to 2/3 (66%), as one possibility removed. Example = Cystic fibrosis
What is X-Linked inheritance?
Caused by mutation in a gene on X chromosome. Never male to male - sons always get X chromosome from mother. Transmitted usually through unaffected female. All daughters from affected males are carriers. Can be recessive (Duchenne’s muscular dystrophy) or dominant (Alpert’s syndrome)
Why can’t males pass on mitochondrial diseases?
Mitochondria = bulky, so can’t fit into sperm
What are amino acids?
- Building blocks of proteins - Side chain often determines polarity (hydrophilicity) or non-polarity (hydrophobicity) - Charge determined by all 3 components (NH2, COOH and R) - 20 different R groups
What are the bonds between amino acids called? How strong are they?
Peptide bonds. Very stable. Cleaved by proteolytic enzymes.