DNA, Genes & Genome

Follow DNA from paired bases to protein-associated packaging, then distinguish a gene, a chromosome and a genome. Molecular views below use experimental coordinates; they are not photographs of a living human cell.

A nucleotide combines a phosphate group, deoxyribose sugar and a base. The backbone joins successive nucleotides; complementary bases belong to opposing strands.

This selected adenine–thymine pair comes from the same crystal model as the duplex. Covalent sticks show modeled connections within each nucleotide; hydrogen atoms and hydrogen-bond lines are not reconstructed.

The two DNA strands are antiparallel. A written sequence needs an orientation: 5′ and 3′ refer to the sugar–phosphate chemistry, not the top and bottom of a page.

This reconstituted nucleosome model shows DNA associated with a histone assembly. It uses Xenopus histones and a deposited DNA construct; it is not a native human chromosome specimen.

DNA and associated proteins form chromatin. Local organisation can change with cellular work. A single nucleosome model does not establish a universal sequence of evenly spaced beads, a compulsory 30-nm fibre or one fixed chromosome shape.

A nuclear chromosome contains DNA organised with proteins. After replication, sister chromatids contain the copied DNA; during division they can condense into the familiar paired form. Before copying, a chromosome is not two identical sister chromatids.

Gene expression uses a DNA sequence to produce RNA. Regulatory sequences and associated factors help determine when and how much RNA is made. A protein-coding region is not the whole genome, and noncoding does not automatically mean useless.

RNA uses ribose and usually uses uracil where DNA uses thymine. RNA is often single-stranded, but it can fold and form paired regions. DNA is retained while selected information is transcribed into RNA.

RNA polymerase uses a DNA template to make a complementary RNA in the 5′→3′ direction. For this example, the template is read 3′→5′. The opposite coding strand has the corresponding sequence with T where RNA has U.

For many human protein-coding genes, an initial transcript is processed: introns are removed and retained exon sequences are joined. Typical mature mRNAs also carry end modifications. These are not universal descriptions of every RNA molecule.

An exon is retained in the mature RNA. Protein-coding transcripts can include untranslated regions within exons, as well as a coding region. Alternative splicing can retain different combinations of exons; it does not randomly rearrange the DNA genome.

During translation, a ribosome reads an mRNA coding region in codons. tRNAs connect codons with amino acids; the growing chain becomes a protein product. The ribosome translates RNA, not the DNA strand.

A new amino-acid chain must acquire a useful structure; many proteins also undergo processing or interact with partners. Delivery depends on the protein. The ER/Golgi route is not the destination of every protein.

rRNA contributes to ribosomal structure and function. tRNA carries amino acids into translation. Other RNAs participate in regulation or processing. An RNA product is not merely a temporary unused copy of DNA.

Many nucleated somatic cells share much of their nuclear genome but express different combinations and amounts of genes. Signals, chromatin, RNA handling and protein regulation contribute to changing cell states.

Replication copies DNA before many forms of cell division. Each original strand can serve as a template for a new strand. DNA synthesis proceeds 5′→3′; fork geometry makes continuous and fragment-based synthesis necessary on different strands.

A representative cycling cell passes through G1, S and G2 before mitosis and cell separation. DNA is copied in S phase. Cells do not all cycle continuously or at the same speed; some remain outside the cycling sequence.

Most human nucleated somatic cells have a diploid nuclear chromosome complement. Gametes differ, and a mature human erythrocyte has no nucleus. Mitochondria carry a separate small genome; mitochondrial function also depends on many nuclear genes.

A karyotype can show chromosomes isolated from a cell and arranged by features such as size and banding. It is not their position inside a living nucleus and does not reveal every base in a DNA sequence.

Differentiation involves changes in gene expression and cellular organisation. Stem-cell populations differ in their renewal and differentiation potential. A tissue stem cell should not be shown as a universal source of every cell in the body.

Sequence analysis records base order. A karyotype compares chromosomes. Structural studies model molecules; microscopy examines cellular organisation. None alone gives a complete account of a cell’s current activity.

Connect cell products and regulation to specialised tissues. These links add an explanation layer rather than replacing anatomy with a sequence diagram.

This chapter covers selected DNA structure, chromatin, RNA/protein expression and genome concepts. It does not provide genetic diagnosis, treatment, a complete catalogue of genes or a deterministic explanation of human traits.