Molecular Basis Of Inheritance

Question 1

Describe the experimental evidence to prove that DNA contains 32P

Answer 1

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

Question 2

What is DNA composed of?

Answer 2

DNA is a polymer of nucleotides each consisting of Deoxyribose sugar, Phosphate, Nitrogenous bases

Question 3

What is the difference between deoxyribose and ribose

Answer 3

Deoxyribose: H at the 2’ position on the sugar ring
RIbose: in the ribonucleic acid OH at the 2’ position on the sugar ring

Question 4

Purine

Answer 4

9 carbons- guanine and adenine

Question 5

Pyrimidines

Answer 5

6 carbons - cytosine and thymine

Question 6

where does the nitrogenous base attach to the sugar

Answer 6

At the 1’ carbon

Question 7

Describe the formation of a phosphodiester bond

Answer 7

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

Question 8

Describe the polarity

Answer 8

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

Question 9

What criteria did the structure of DNA have to meet

Answer 9

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

Question 10

What were Charagaffs rules

Answer 10

%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

Question 11

Describe the structure of DNA

Answer 11

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

Question 12

How much of the genome actually encodes for the protein

Answer 12

1.1%

Question 13

Contrast the prokaryotic and eukaryotic genes

Answer 13

Prokaryotic gene-
PROMOTER- CODING SEQUENCE- TRANSCRIPTION TERMINATOR

Eukaryotic gene- split by intervening regions (23% of human genome)
Not like prokaryotic

Question 14

What is the composition of the human genome

Answer 14

protein coding genes

Regulatory regions- promotors and transcription terminators

Other uniques sequence including introns

Transposon based repeats

Heterochromatin

Question 15

Describe the Transposon Based Repeats

Answer 15

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

Question 16

Describe the hetereo chromatin (highly condensed DNA)

Answer 16

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

Question 17

Dosage compensation

Answer 17

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

Question 18

Name the X linked recessive disorders that reveal inactivation in humans

Answer 18

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

Question 19

Explain a role in which ncRNA’s play an important role

Answer 19

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

Question 20

State how miRNAs can be important in the formation of cancers

Answer 20

Many miRNAs are unregulated or down regulated in cancer to contribute to the progression of the disease

Question 21

Describe the mitchondrial DNA

Answer 21

Contain their own circular genome - mtDNA- 13 polypeptides, rRNA and tRNA

Mitochondria exclusively inherited from mother

Question 22

Name two mitochondrial cytopathies

Answer 22

MELAS - Myopathy Encephelopathy Lactic Acidosis Stroke like episodes

LHON- Lebers Heridtary Optic Neuropathy

Question 23

What has been done to prevent the inheritance of damaged mitochondria from mother

Answer 23

IVF- using healthy mitochondria from other another female