Molecular Basis of Inheritance Flashcards
Human DNA
3.3 x 10^9 bp of nucleotides in the DNA of haploid chromosomes
Nucleotides
N-glycosidic linkage- Nitrogenous base is attached to OH of 1’C of deoxyribose or ribose sugar
Phosophoester linkage- Phosphate group attached to OH of 5’C of deoxyribose or ribose sugar
Polynucleotide
Two nucleotides are joined by 3’-5’ phosphodiester linkage. OH of 3’C of pentose sugar of one nucleotide is attached to the phosphate group of 5’C of another nucleotide
5’ end of polynucleotides
Free Phosphate group at 5’C of the pentose sugar
3’ end of polynucleotides
Free OH group at 3’C of the pentose sugar
Friedrich Miescher
Identified DNA in 1869
Named it ‘Nuclein’
Present in the nucleus and acidic
Maurice Wilkins and Rosalind Franklin
Obtained DNA images from X-ray crystallography
James Watson and Francis Crick
Proposed the double-helix model of DNA in 1953
Erwin Chargaff
Elucidated that the DNA contains equal amounts of adenine and thymine and in the same way guanine and cytosine
DNA double helix
Antiparallel strands, coiled in a right-handed fashion
Backbone- sugar-phosphate
Bases are bonded by hydrogen bonds and projected inside
Adenine bonds to thymine by two hydrogen bonds
Guanine bonds to cytosine by three hydrogen bonds
Pitch- 3.4 nm and 10 bp in each turn
Central Dogma
Proposed by Francis Crick in 1958
The flow of genetic information from DNA → RNA → Protein
Nucleoid
Found in prokaryotes
The region where DNA is concentrated within a cell associated with positively charged proteins
Histones
Positively charged basic proteins, rich in lysine and arginine
Eight molecules of histones associated with DNA to form a nucleosome
Nucleosome
DNA associated with histones octamer
200 bp of DNA in each nucleosome
Appear as beads on chromatin threads
Euchromatin
Loosely packed and lightly stained part of chromatin
Shows active transcription
Heterochromatin
Densely packed and darkly stained part of chromatin
Inactive for transcription
Transforming principle
Frederick Griffith in 1928
He concluded that there was some transforming principle in the heat-killed S-strain, which transformed R strain Streptococcus pneumoniae to virulent
Avery, MacLeod and McCarty
Demonstrated that the DNA caused the bacterial transformation
They showed that proteases and RNAases did not affect transformation, whereas DNAase inhibit transformation
Alfred Hershey and Martha Chase
Unequivocal proof of DNA as genetic material in 1952
They worked on bacteriophages
Radioactive 35S labelled- protein capsule
Radioactive 32P labelled- DNA
RNA as a genetic material
Present in many viruses, known as a retrovirus
E.g. TMV, QB bacteriophage, etc.
ds RNA- reovirus, etc.
Semiconservative DNA replication
Proposed by Watson and Crick
Experimentally proved by Matthew Meselson and Franklin Stahl in 1958 using 15N isotope in the growing medium for E.coli
Taylor and colleagues worked on Vicia faba using radioactive thymidine
DNA-dependent DNA polymerase
Main enzyme for replication
Polymerisation in 5’→3’ direction
DNA ligase
Join the DNA strand, which is synthesised discontinuously during replication
DNA helicase
Unwind DNA at the origin of replication forming replication fork
Transcription
Synthesis of RNA from DNA
Template strand- 3’→5’ DNA
Coding strand- 5’→3’ DNA, has the same sequence as newly synthesised RNA strand
DNA-dependent RNA polymerase
Catalyses transcription
Polymerisation in 5’→3’ direction
Transcription unit
Reference point with respect to the coding strand (5’→3’, DNA)
Promoter- present at 5’ end of a structural gene, RNA polymerase binds here
Terminator- at 3’ end of the coding strand
Cistron
A segment of DNA coding for polypeptide
Monocistronic- eukaryotes
Polycistronic- prokaryotes
Exons
Coding sequence of DNA expressed in mature and processed RNA
Introns
Interrupted sequence which are spliced in mature RNA
Translation
Formation of polypeptide from RNA
mRNA- template for protein synthesis
tRNA- adapter molecule, brings amino acids and reads codon on mRNA
rRNA- structural and catalytic role
Eukaryotic transcription
RNA polymerase I – 28S, 18S, 5.8S rRNA
RNA polymerase II – hnRNA (mRNA precursor)
RNA polymerase III – tRNA, 5S rRNA, snRNA
hnRNA processing
Splicing – removal of introns and joining together of exons
Capping – addition of methyl guanosine triphosphate at 5’ end
Tailing – addition of adenylate residues (200-300) at 3’ end
Marshall Nirenberg
Deciphered first the 64 triplet codons for 20 amino acids present in protein
Start codon
AUG
Codes for Methionine
Stop codons
UAA, UAG, UGA
They do not code for any amino acids
tRNA
Structure- 2° structure like clover leaf, 3° like inverted L
Anticodon loop- complementary mRNA codon for specific codon
Acceptor arm- binds to amino acid
DHU loop, TψC loop and variable loop
Function- Adapter molecule, brings amino acid for translation and reads codon
Translation
Occurs in cytoplasm
The first step is charging of tRNA, aminoacylation or binding of tRNA to amino acid
Peptide bond formation takes place between the amino acid of growing chain and newly brought amino acid in two sites present in the large subunit of ribosomes
Ribozyme
Catalytic RNA
28S rRNA in bacteria
Untranslated regions (UTR)
Regions on mRNA, which are not translated
Present on both 5’ and 3’ ends
Required for efficient translation
Francois Jacob and Jacque Monod
First elucidated transcriptionally regulated system in lac operon
Operon
Present in prokaryotes
Contains multiple genes, which are regulated by a promoter and an operator
E.g. lac operon, val operon, his, ara, trp, operon, etc.
lac operon
i gene- repressor (negative regulation)
Structural genes-
z – beta-galactosidase (β-gal)
y – permease
a – transacetylase
Inducer – lactose or allolactose
Human genome project (HGP)
Started in 1990, Completed in 2003
3 x 109 base pairs (haploid)
20,000-25,000 genes
Largest gene- Dystrophin gene of the X chromosome (2.4 million bp)
Smallest gene- SRF/TDY gene of the Y chromosome (14 bp)
SNPs
Single nucleotide polymorphism
Single base differences exist at
1.4 million places in the human genome
Sequence annotation
Assigning different regions of genome according to function
Expressed Sequence Tags (ESTs)
Identifying all the genes, which are expressed as RNAs
DNA Fingerprinting
Developed by Sir Alec Jeffreys
Involves identifying of repeated DNA sequences
Satellite DNA forms smaller peaks and shows a high degree of polymorphism
VNTR
Variable number of tandem repeats
Mini-satellite
Number and position differs from person to person
Flanked by restriction sites
Southern blotting
Transfer of electrophoresis separated DNA to nitrocellulose or nylon membrane and detection by probe hybridisation
Used to detect DNA sample in blood or tissue
