Molecular Basis Of Inheritance Flashcards
Describe the experimental evidence to prove that DNA contains 32P
A bacteriophage ( a virus that infects bacteria) -composed of DNA ( which can be labelled with 32P as there is no phosphorus in protein) and 32S ( as there is no sulphur in DNA)
A) phage with capsid 35S attaches to bacteria
Dislodges the phage
Centrifuged
The phage separates from the bacteria
No label in bacteria but label is only found in dislodged phage coat
B) phage with DNA labelled with 32P
Dislodges the phage
Centrifuged
The phage appears in bacteria
Label appears in bacteria and also progeny phage
What is DNA composed of?
DNA is a polymer of nucleotides each consisting of Deoxyribose sugar, Phosphate, Nitrogenous bases
What is the difference between deoxyribose and ribose
Deoxyribose: H at the 2’ position on the sugar ring
RIbose: in the ribonucleic acid OH at the 2’ position on the sugar ring
Purine
9 carbons- guanine and adenine
Pyrimidines
6 carbons - cytosine and thymine
where does the nitrogenous base attach to the sugar
At the 1’ carbon
Describe the formation of a phosphodiester bond
Nucleotides at the 3’ can be attached to the phosphate group attached at the 5’ of another nucleotide in IN A CONDENSATION REACTION.
Hence why it is called 3’- 5’ phoshodiester bond
Catalysed by DNA polymerase
Describe the polarity
As the direction is 5’ - 3’
5’ phosphate end
3’ hydroxyl end
As these are different groups- one end has different polarity than the other end
What criteria did the structure of DNA have to meet
Must incorporate a feature that determines its role as a store of information
There must be suggested replication mechanism that permits its passage across generations of progeny
Must be capable of undergoing mutation- such that information is carried is altered in a heritable manner
What were Charagaffs rules
%G = %C
%A = %T
This implied that A/T and C/G are pairs
The composition of DNA varies from species in terms of ATCG- gives hint that there is diversity molecularly that this inherent in DNA
Describe the structure of DNA
DOUBLE HELIX- 2 polynucleotides chains coiled around each other
RH double helix
BASE PAIRING- A=T and C=G
Stabilised by base pairing through H bonds
ANTIPARRALEL - 5’-3’ one strand
3’-5’ other strand
BASES PERP TO HELICAL AXIS
MEETS THE CRITERIA- information is stored as a sequence of bases and specific base pairing suggests a copying mechanism
How much of the genome actually encodes for the protein
1.1%
Contrast the prokaryotic and eukaryotic genes
Prokaryotic gene-
PROMOTER- CODING SEQUENCE- TRANSCRIPTION TERMINATOR
Eukaryotic gene- split by intervening regions (23% of human genome)
Not like prokaryotic
What is the composition of the human genome
protein coding genes
Regulatory regions- promotors and transcription terminators
Other uniques sequence including introns
Transposon based repeats
Heterochromatin
Describe the Transposon Based Repeats
Repeated sequences moving to randomly selected new location ( via copy and paste mechanism RNA intermediate using RNA intermediate)
A) LINES- long interposed elements
L10 6500 BP and 500,000 copies contains its own reverse transcriptase
I) Most are inactive II) approx 100 are still actively transposing- this causes disease eg some cases of haemophilia A due to the L1 Trans position into Factor VIII gene on X
B) SINES- Short interposed elements
Alu repeat 160 BP 1.2 million
Describe the hetereo chromatin (highly condensed DNA)
Tends to be transcriptionally inactive
- Composed of long arrays of high copy number tandemly repeated DNA sequence- satellite DNA
Ranges from 4-171 BP
Since the number of repeat varies between individuals varies- used for paternity testing and forensic science
-The genome specifies ncRNA required for exon splitting and decoding mRNA
A) message processing- sncRNAs (upto 360 nucleotides ) form complexes with proteins to form complexes with proteins to form snRNPs required for splicing pre- mRNAs
B) decoding mRNA- there are 3000+ genes that specify for ncRNAs eg Xist 19.3 kb RNA controls mammalian X inactivation ( in the sense that females have twice as many X genes as males but twice as much is not transcripted- not observed)
SEE DOSAGE COMPENSATION
Dosage compensation
One of the X chromosomes is transcriptionally silent
1) the X inactivation centre- Xic contains the Xist gene- transcribes into ncRNAs
2) Heterochromatin spreading from the inactivation centre
Occurs in early embryogenesis- random which one
Once it has occurred all the cells are descended from initial cell will exhibit inactivation of the same chromosome
Name the X linked recessive disorders that reveal inactivation in humans
Antibiotic ectodermal dysphasia- defective sweat glands
Red-green colour blindness- Males are fully colour blind if they carry the mutant allele- heterozygous females have mosaic retinas
Explain a role in which ncRNA’s play an important role
MiRNA formed - 1) Transcription from miRNA gene to give a hairpin stem-loop precursor miRNA
2) Export into the cytoplasm and processing by nuclear energy
3) This leaves a single strand of RNA that base pairs with coding mRNA
MiRNA then causes translational repression and deadenylation
State how miRNAs can be important in the formation of cancers
Many miRNAs are unregulated or down regulated in cancer to contribute to the progression of the disease
Describe the mitchondrial DNA
Contain their own circular genome - mtDNA- 13 polypeptides, rRNA and tRNA
Mitochondria exclusively inherited from mother
Name two mitochondrial cytopathies
MELAS - Myopathy Encephelopathy Lactic Acidosis Stroke like episodes
LHON- Lebers Heridtary Optic Neuropathy
What has been done to prevent the inheritance of damaged mitochondria from mother
IVF- using healthy mitochondria from other another female
