Molecular Basis of Inheritance

DNA is a long thread made of two strands coiled like a ladder twisted into a spiral. This double helix was described by Watson and Crick in 1953 based on X-ray data from Franklin. Each strand has a backbone of sugar phosphate and four bases sticking inside: Adenine A pairs with Thymine T with two hydrogen bonds, Guanine G pairs with Cytosine C with three bonds. Chargaff said A = T and G = C.

One strand runs 5' to 3', other 3' to 5' antiparallel. Length of DNA is huge: human has 2.2 metres in one cell. So it is packed. DNA wraps around 8 histone proteins to form nucleosome, like thread on spool. This beads on string coils further to chromatin, then chromosome. Histone is positive and DNA negative so they attract. Non-histone proteins also help. Heterochromatin dark tightly packed inactive, euchromatin light loosely packed active.

Before cell divides DNA must make exact copy. Replication is semi-conservative proved by Meselson and Stahl using heavy nitrogen N15. They grew E. coli in N15, shifted to N14, after one generation DNA one heavy one light, after two one heavy-light and one light-light, so each new DNA has one old strand one new.

Enzymes work at fork: helicase opens helix, SSB keeps strands separate, primase makes RNA primer, DNA polymerase III adds nucleotides only 5'→3' so one strand continuous leading strand, other discontinuous as small pieces Okazaki fragments which are later joined by ligase. DNA polymerase I removes primer. Replication starts at origin ori. In eukaryotes many origins.

This copy is identical because of base pairing A-T G-C.

Transcription is making RNA from DNA. DNA has promoter where RNA polymerase binds and terminator where it stops. Transcription unit has template strand 3'→5' read by polymerase, coding strand 5'→3' same as RNA except T→U.

In bacteria one RNA polymerase makes all RNA. In eukaryotes three polymerases: Pol I rRNA, Pol II mRNA, Pol III tRNA. Bacteria mRNA does not need processing. Eukaryotic primary mRNA hnRNA has introns non-coding and exons coding. Introns removed by splicing, 5' cap of methyl guanosine added, 3' poly A tail added, then becomes mature mRNA ready to leave nucleus.

RNA is single strand, has U instead of T, more reactive, shorter lived, so better messenger.

Genetic code is triplet of bases coding one amino acid. 64 codons: 61 for amino acids 3 stop UAA UAG UGA no amino acid. AUG is start and codes Methionine. Code is universal almost same in all organisms, degenerate many codons for one amino acid, non-overlapping, commaless, non-ambiguous.

tRNA is adaptor clover leaf structure with anticodon that pairs with codon, and amino acid attachment site. It reads code and brings correct amino acid.

Translation is making protein from mRNA in cytoplasm on ribosome. Ribosome has two subunits. Small reads mRNA, large joins amino acids. Steps: initiation AUG start tRNA Met binds, elongation tRNA brings amino acids chain grows, termination stop codon comes release factor stops protein released. UTR untranslated regions before start and after stop needed for efficient translation. Protein may need folding to become active.

Gene expression is controlled as per need. In bacteria lac operon is example. It has structural genes z y a for enzymes to digest lactose, promoter, operator, and regulator i gene makes repressor. When lactose absent repressor binds operator blocks RNA polymerase no transcription. When lactose present it acts as inducer binds repressor repressor cannot bind operator transcription on enzymes made. This is negative inducible regulation.

Human Genome Project 1990-2003 aimed to sequence 3 billion bases of human DNA, find all 20-25k genes, store data. Findings: 99.9% DNA same in all humans, less than 2% codes protein, many repetitive sequences, 20k genes not 80k as thought.

DNA fingerprinting uses Variable Number Tandem Repeats VNTR which vary person to person. Steps: isolate DNA, cut with restriction enzyme, separate by gel electrophoresis, transfer to membrane Southern blot, hybridize with VNTR probe radioactive, detect bands pattern unique like fingerprint except identical twins. Used in forensics paternity disputes.

Central dogma is DNA→RNA→protein flow of information. Reverse transcription RNA→DNA happens in retrovirus like HIV.