The genetic code is the set of rules by which the sequence of nucleotide bases (codons) in mRNA corresponds to specific amino acids during protein synthesis. It is nearly universal across all life forms on Earth.

Both sickle cell anaemia (GAG→GTG, Glu→Val in β-globin) and phenylketonuria (various point mutations in the phenylalanine hydroxylase gene) are caused by single nucleotide changes (point mutations) in protein-coding genes.

Metacentric chromosomes have their centromere in the middle, so the two arms are roughly equal in length. During cell division they appear V-shaped. Telocentric are rod-shaped (I); submetacentric are L-shaped; acrocentric are J-shaped.

UAA and UGA are both stop (nonsense/termination) codons — they do not encode any amino acid. The three stop codons are UAA (ochre), UAG (amber), and UGA (opal). AUG is the start codon for methionine.

Human cells contain approximately 45 different tRNA species (isoacceptors). Due to wobble base pairing at the third position of the anticodon, 45 tRNAs can recognise all 61 sense codons.

AUG is the universal start codon (initiation codon) that signals the beginning of protein synthesis. It codes for methionine. UAA, UAG, and UGA are stop codons. The ribosome scans mRNA from the 5' cap to find the first AUG.

Translocation is the movement of the ribosome along the mRNA by one codon (three nucleotides) after each peptide bond formation, moving the mRNA 3' direction and shifting the tRNAs from A to P to E sites.

rRNA (ribosomal RNA) forms the structural and catalytic core of ribosomes — the molecular machines where translation occurs and polypeptides are assembled. The large ribosomal subunit rRNA (28S in eukaryotes) is the peptidyl transferase enzyme.

By complementary base pairing (A-T, G-C) and antiparallel orientation: G→C, A→T, G→C, T→A, A→T, C→G, reading in the 3'→5' direction gives CTCATG (which reads 5'→3' as GTACTC). The answer CTCATG is the complement read in reverse.

Phosphodiester bonds link nucleotides in DNA and RNA — they connect the 3'-OH of one nucleotide's sugar to the 5'-phosphate of the next nucleotide, forming the sugar-phosphate backbone of the nucleic acid strand.

Sickle cell anaemia results from substitution of valine for glutamic acid at position 6 of the β-globin chain. The responsible mutation is a single base change (GAG→GTG in the DNA codon), making valine instead of glutamic acid.

The DNA double helix has a diameter of 2 nm (20 Å). The helical pitch (distance per complete turn) is 3.4 nm, with 10 base pairs per turn spaced 0.34 nm apart. These measurements were derived from X-ray crystallography data.

Penicillium (the antibiotic-producing mould) has only 1 pair of chromosomes (haploid n=1, diploid 2n=2) — one of the simplest chromosome numbers among eukaryotes. This makes it a useful organism for genetic studies.

A nucleosome is the fundamental unit of chromatin packaging — approximately 147 bp of DNA wrapped about 1.65 times around a histone octamer (two each of H2A, H2B, H3, H4). Nucleosomes are connected by linker DNA (~20-80 bp).

Chromosomes are approximately 60% protein (mostly histones and some non-histone proteins) and about 40% DNA by mass. The histone proteins organise the DNA into nucleosomes and higher-order chromatin structures.

Chromosomes become visible as distinct thread-like structures in the nucleus during cell division (mitosis/meiosis) when chromatin condenses maximally. Between divisions they exist as dispersed chromatin in the nucleus.

In PKU, accumulated phenylalanine and its metabolites are toxic to the developing brain, causing irreversible intellectual disability (mental retardation) and seizures if dietary phenylalanine is not restricted from birth.

Phenylketonuria (PKU) is caused by a point mutation in the gene encoding phenylalanine hydroxylase. Phenylalanine cannot be converted to tyrosine, accumulates in blood, and causes mental retardation if untreated.

At any given time during translation, one codon of mRNA is exposed at the A (aminoacyl) site of the ribosome. The ribosome has three sites: E (exit), P (peptidyl), and A (aminoacyl) — each accommodating one codon.

AUG is the universal start/initiation codon that signals the beginning of translation in all organisms. It codes for methionine (Met) in eukaryotes and formyl-methionine (fMet) in prokaryotes.

Stop codons (UAA, UAG, UGA) are codons that do not encode any amino acid — they signal the ribosome to terminate translation. They are also called nonsense codons or termination codons.

The genetic code consists of codons — triplets of nucleotides (three consecutive bases) in mRNA that specify a particular amino acid during translation. There are 64 possible codons (4³) encoding 20 amino acids plus start and stop signals.

There are approximately 45 different tRNA molecules in human cells, each recognising specific codons through their anticodon loops. Although there are 61 sense codons, wobble base pairing in the anticodon allows one tRNA to recognise multiple codons.

The promoter is the DNA sequence upstream of a gene where RNA polymerase binds to initiate transcription. Promoters contain specific sequences (like TATA box in eukaryotes) recognised by transcription factors and RNA polymerase.

Transcription is the synthesis of RNA from a DNA template by RNA polymerase. In transcription, the information in a gene's DNA sequence is copied into an mRNA molecule that can then be translated into protein.

Vernon Ingram (1956) discovered the molecular basis of sickle cell anaemia — a single amino acid substitution (glutamic acid → valine) at position 6 of the beta-globin chain caused by a point mutation in the corresponding codon.

Frederick Sanger determined the complete amino acid sequence of insulin (the first protein to be fully sequenced) in 1955, earning him his first Nobel Prize in Chemistry (1958). He later won a second Nobel for DNA sequencing.

Minimal medium for fungi (like Neurospora) contains only inorganic salts, a carbon source (sugar), water, and one vitamin (biotin). Wild-type Neurospora can synthesise all other needed amino acids and vitamins from these minimal components.

DNA polymerase I is present in smaller amounts than DNA polymerase III. It plays a supporting role in replication by removing RNA primers and replacing them with DNA, and participating in DNA repair.

Alkaptonuria is a rare autosomal recessive genetic disorder caused by deficiency of homogentisate oxidase. Homogentisic acid accumulates and is excreted in urine, turning it dark on oxidation. It is the first 'inborn error of metabolism' described by Garrod (1902).

DNA ligase joins (ligates) the Okazaki fragments on the lagging strand by forming phosphodiester bonds between adjacent DNA fragments after DNA polymerase I has replaced the RNA primers with DNA.

Primase synthesises short RNA primers (5-10 nucleotides) that provide a 3'-OH starting point for DNA polymerase III. DNA polymerase cannot start a new strand de novo — it can only extend an existing primer.

The Watson-Crick double helix model suggested that complementary base pairing (A-T, G-C) is the molecular basis of genetic information copying. Each strand serves as a template for a new complementary strand during replication.

The two strands of DNA run antiparallel — one strand runs 5'→3' while the complementary strand runs 3'→5'. This antiparallel orientation is essential for base complementarity and replication.

The DNA double helix has a constant diameter of 2 nm (20 Å). This is because each base pair consists of one purine (large, double ring) paired with one pyrimidine (small, single ring), maintaining uniform width.

Guanine (G) forms three hydrogen bonds with cytosine (C) in a DNA double helix. Adenine (A) forms two hydrogen bonds with thymine (T). G-C base pairs are stronger than A-T pairs, making G-C rich DNA more thermally stable.

Maurice Wilkins (at King's College, London) prepared highly oriented DNA fibres and took X-ray diffraction images. Rosalind Franklin also took famous X-ray photographs (Photo 51) that revealed key structural features used by Watson and Crick.

Purines (adenine and guanine) are double-ringed structures and are larger than pyrimidines (cytosine, thymine, uracil) which are single-ringed. In DNA, a purine always pairs with a pyrimidine, maintaining a constant helix diameter.

Adjacent base pairs in the DNA double helix are stacked 0.34 nm (3.4 Å) apart. There are 10 base pairs per complete helical turn (3.4 nm pitch). The diameter of the helix is 2 nm.

Phoebus Levene determined the basic structure of nucleic acids — identifying the four bases, sugar (ribose in RNA, deoxyribose in DNA), and phosphate groups, and proposing that nucleotides were linked by phosphodiester bonds.

Friedrich Miescher first isolated DNA (which he called 'nuclein') from pus cell nuclei in 1869. He found it was an acidic substance rich in phosphorus located in the nucleus — the first isolation of nucleic acid.

DNase (deoxyribonuclease) is an enzyme that digests (hydrolyses) DNA. Avery used DNase in their transformation experiments — DNase destroyed the transforming activity, confirming DNA (not protein) was the transforming principle.

Avery, MacLeod, and McCarty (1944) identified DNA as the 'transforming principle' in Griffith's experiment. They showed that purified DNA (not protein) from virulent S-strain pneumococci could transform non-virulent R-strain bacteria.

Walter Sutton (1902) first proposed the chromosome theory of heredity — that chromosomes are the carriers of genes. He noted that chromosomes behave during meiosis exactly as Mendel's hereditary factors should behave.

DNA is packaged into progressively higher order structures: nucleosome (DNA + histones) → solenoid (30nm fibre) → loops → supercoils → chromosome. Supercoiling allows the enormous DNA length to fit within the small nucleus.

A typical human chromosome contains about 140 million base pairs (nucleotides). The largest human chromosome (chromosome 1) has about 249 million base pairs; the smallest (chromosome 21) has about 48 million.

Chromosomes are composed of DNA (the genetic material), histone proteins (around which DNA wraps to form nucleosomes), non-histone regulatory proteins, and small amounts of RNA. The complex of DNA and proteins is called chromatin.

Sugarcane (Saccharum officinarum) has approximately 80 chromosomes (40 pairs) in the most common commercial varieties. It is highly polyploid, making it one of the most complex genomes among crop plants.

Genetic linkage is the tendency of genes located on the same chromosome to be inherited together. Linked genes do not assort independently (violating Mendel's law of independent assortment) unless crossing over occurs between them.

Pyrimidines are single-ringed nitrogenous bases: Cytosine (C), Thymine (T in DNA), and Uracil (U in RNA). Purines are double-ringed: Adenine (A) and Guanine (G). In DNA: A pairs with T; G pairs with C.

Translation is protein synthesis — the process by which ribosomes decode the mRNA sequence (codons) and assemble the corresponding amino acids (brought by tRNA) into a polypeptide chain.

mRNA is transcribed from the template (antisense) strand of DNA. The mRNA sequence is complementary to the template strand, using U instead of T. So from template strand A-T-T-G, mRNA is U-A-A-C.

RNA exists in three main types: mRNA (messenger RNA, carries genetic instructions), tRNA (transfer RNA, carries amino acids to ribosome), and rRNA (ribosomal RNA, structural and catalytic component of ribosomes).

By the base pairing rules (A pairs with T, G pairs with C), the complementary DNA strand to A-T-T-G is T-A-A-C. Note that in DNA, uracil (U) is not present — only in RNA does U replace T.

There are 20 standard amino acids used by living organisms to build proteins, encoded by the universal genetic code. These 20 amino acids can be combined in different sequences and numbers to form thousands of different proteins.

Sickle cell anaemia is caused by substitution of valine for glutamic acid at position 6 of the beta-globin chain. This single amino acid change (due to A→T point mutation in codon 6) causes HbS to polymerise when deoxygenated.

Chromosomes are composed of DNA (the genetic material), histone proteins (around which DNA is wrapped), non-histone proteins (for gene regulation), and small amounts of RNA. Together they form chromatin.

Genetic information is encoded in the linear sequence of nucleotide bases (A, T, G, C) along the DNA molecule. This sequence determines the sequence of amino acids in proteins through the genetic code.

Genetic mutations are permanent changes in the nucleotide sequence of DNA. They can involve changes in single base pairs (point mutations) or larger structural changes in chromosomes.

Transcription is the synthesis of an mRNA molecule using DNA as a template by RNA polymerase. The mRNA sequence is complementary to the template (antisense) DNA strand, with U replacing T.

Most mutations are harmful because they disrupt the carefully evolved genetic code. A few are neutral (no effect) and rarely, some are beneficial. The vast majority of non-synonymous mutations impair protein function.

A gene is a specific portion (sequence) of DNA that encodes information for a functional product — usually a protein or functional RNA. Genes are the basic units of heredity and occupy specific positions (loci) on chromosomes.

Human growth hormone (hGH) can now be collected/produced through recombinant DNA technology — the human GH gene is inserted into bacteria which then produce large quantities of the hormone for therapeutic use.

Sickle cell anaemia is a human genetic disorder caused by a single point mutation in the beta-globin gene of haemoglobin. It is most common in people of African, Mediterranean, Middle Eastern, and Indian descent.

Neurospora crassa (pink bread mould) can grow in minimal medium containing only inorganic salts, sugar (carbon source), and the vitamin biotin. Beadle and Tatum used this property to study nutritional mutants.

A chromosomal deletion occurs when a segment of DNA is lost from a chromosome. Deletions can range from a single nucleotide (point mutation) to large chromosomal segments, causing loss of genes and developmental abnormalities.

There are three main types of RNA: mRNA (messenger RNA — carries genetic code from DNA to ribosomes), tRNA (transfer RNA — brings amino acids), and rRNA (ribosomal RNA — structural component of ribosomes).

A nucleoside consists of a deoxyribose (or ribose) sugar covalently bonded to a nitrogenous base. Adding a phosphate group to a nucleoside makes a nucleotide — the monomer of nucleic acids.

Chromosomal duplications (extra copies of chromosome segments) can cause abnormalities of form and function if gene dosage is disturbed. Too many copies of certain genes disrupt normal development.

DNA is a polynucleotide — a long chain of nucleotides linked by phosphodiester bonds. Each nucleotide consists of deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (A, T, G, C).

Genes encode the information for protein synthesis. Haemoglobin is a protein, so genes are ultimately responsible for its synthesis. The one gene-one enzyme hypothesis (Beadle and Tatum) established that genes control protein production.

The head of a bacteriophage consists of an outer protein coat (capsid) made of capsomere proteins surrounding the DNA core. The protein coat protects the viral DNA and helps it attach to and inject into bacterial cells.

Adenine (A) pairs with thymine (T) through two hydrogen bonds in DNA. Guanine (G) pairs with cytosine (C) through three hydrogen bonds. This difference in bonding strength means G-C rich DNA has a higher melting temperature.

DNA is an extremely complex molecule — a double-stranded helix composed of billions of nucleotides arranged in a specific sequence. This complexity allows it to encode all the genetic information of an organism.

Bacteriophages lack the typical cell structure — they have no cell membrane, cytoplasm, or metabolic machinery. They consist only of a protein coat (capsid) enclosing nucleic acid (DNA or RNA).

Genes (hereditary units) are located on chromosomes — linear DNA molecules found in the nucleus. Chromosomes carry genes in a linear order (loci) and are transmitted from parents to offspring during reproduction.

Genes are functional units of heredity made of DNA (deoxyribonucleic acid). Each gene is a specific sequence of nucleotides in DNA that encodes the information needed to produce a functional RNA or protein product.

Each chromosome contains a single, continuous, very long polynucleotide molecule of DNA. In humans, the longest chromosome (chromosome 1) contains about 249 million base pairs in one continuous DNA double helix.