Mendel's Laws, Genetics 4/5/6

Cystic Fibrosis

defective chloride channels, chloride ion build up outside the cell

recessive disorder

tay sachs disease

defective lipase enzyme leads to accumulation of lipids in the brain

recessive disorder

sickle cell disease

faulty hemoglobin

recessive disorder

dominant disorders

achondroplasia - causes dwarfism

huntingtons’s disease - lethal nervous system disorder, caused by dominant allele on chromosome 4

non invasive genetic testing

cfDNA testing for fetal genetic disorders

cfDNA testing

cfDNA are short DNA fragments

cfDNA in maternal blood comes from mother and fetus

in trisomy 21, the amt of cfDNA for chromosome 21 is higher than normal

chromosomal theory of inheritance

mendelian genes have a specific loci on chromosomes, which undergo segregation and independant assortment

T.H Morgan fruit fly experiment

showed sex-linked genes are located on sex chromosomes

eg white eye mutation in fruit flies appeared only in males

linked genes

genes located on the same chromosome tend to be inherited togethergene

genetic recombination

offspring with new trait combinations due to crossing over

recombination frequencies

used to map gene distances in centimorgans

what does crossing over do

allows gene exchange and creation of recombinant offspirng

sex chromsomes

sperm usually determine sex

variations exist among species

sex linked disorders

colour blindness

duchenne msucular dystrophy - dystophin protein is largely absent, leads to degeneration of muscles

hemophilia - one or more proteins needed for blood clotting are missing

chromosomal abnormaltieies

non disjunction - leads to aneuploidy

down syndrome - caused by trisomy 21, linked to maternal age

sex chromosome aneuploidy

klinefelter syndrome (XXY) - male with some female characteristics

turned syndrome (XO) - female,

XYY syndrome - taller than avg males

XXX syndrome - females generally normal

barr-body

condensed inactive X chromosome found in the somatic cells of female mammals

sex linked traits then become a mosaic, like sweat glands in humans

aneuploidy

offspring with abnormal chromosome number

trisomic

(2n+1)

monosomic

(2n - 1)

polyploidy

more than one complete set of chromosomes

triploidy

3n

common in plants

tetraploidy

4n

known in fished and amphibians

chromosomal deletion

removes a chromosomal segment

chromosomal duplication

repeats a segment

chromosomal inversion

reverses a segment within a chromosome

chromosomal translocation

moves a segment from one chromosome to another, non-homologous one

cri du chat

deletion on chromosome 5

severe mental retardation and early death

chronic myelogenous leukemia

cancer of the cells that produce white blood cells

reciprocal translocation of chromosome 9 and 22

mitochondrial DNA

passed maternally

affects metabolic functions

genomic imprinting

expression depends on the allele’s parental origin

happens in gamete formation where one allele is silenced

transformation

a change in genotype and phenotype due to the assimilation of external DNA by a cell

bacteriophages

viruses that infect bacteria

chargaff’s rule

A=T

G=C

DNA replication

involves unwinding (helicase), priming (primase) and elongation (DNA polymerase)

helicases

enzyme that untwists and seperates DNA helix

single strand binding proteins

bind to seperate strands and hold them apart

leading strand

synthesized continuously

replicated towards the replica

lagging strand

synthesized in okazaki fragments, joined by DNA ligase

replicated away from the replication fork

okazaki fragments

100-200 nucleotide pieces joined together by DNA ligase into a single strand

another DNA polymerase replaces primer with DNA before ligase can join the fragments

primer

the start of a new DNA chains which is made of RNA

~10 base pairs long

leading strand only needs 1 primer, but each okazaki fragments needs a primer

primase

an enzyme which joins RNA nucleotides into a primer

can initiate the process from scratch

telomeres

protect chromosome ends, maintained by telomerase (active in germ cells)

conservative model

the parental double helix remains intact and an all-ne copy is made

semiconservative model

the two stands of the parental molecule seperate

each functions as a template for synthesis of a new complementary strand

dispersive model

each strand of both daughter molecules contains a mixture of old and newly synthesized parts

meselson-stahl experiment

demonstrated semiconservatice DNA replication

origins of replication

sites where proteins attach and seperate the strands of DNA

DNA polymerase

an enzyme that adds new nucleotides to the replication fork at rate of 50-500 per second

add only to the 3’ end of a growing strand

nucleoside triphosphate

energy and substrate for polymerisation

DNA proofreading and repair

thymine dimer distorts DNA molecule

nuclease enzyme cuts damaged DNA strand at 2 points

repair synthesis by DNA polymerase fills the gap

DNA ligase seals the remaining nick

mismatch repair

incorrectly paired nucletoides are fixed by an enzyme

nuclease

enzyme which cuts DNA

nucleotide excision repair

excised DNA section filled in by a polymerase and ligase

5’ problem

polymerase can only add nucleotides to 3’ end, so there’s no way to complete the 5’ end

leads to chromosome shortening

how to fix the 5’ problem

prokaryotes have circular DNA which avoids the problem

eukaryotes have telomeres

telomeres

100-1000 repeated short sequences of DNA such as TTAGGG in humans

telomerase

enzyme containing RNA which further lengthens the 3’ end to allow the completion of the 5’ end