Chromosomes and DNA replication: clear guide
Cell biology • Cell division
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Chromosome structure in the nucleus
Chromosomes consist of a single long DNA molecule wound around histone proteins to form chromatin; further folding produces the compact structure visible under a microscope during division. The compacted chromosome shows distinctive regions such as the centromere and two arms; chromosomes are numbered by size in many organisms and occur in matched pairs in diploid nuclei. The matched pair arrangement provides two copies of each gene, called alleles, one inherited from each parent . Chromosomes remain less condensed during most of interphase so DNA is accessible for transcription and replication. Condensation into X-shaped forms occurs after DNA replication when each chromosome consists of two identical sister chromatids joined at the centromere; the X-shape improves mechanical separation of copies during mitosis .
DNA structure and specific base pairing
DNA is a double helix formed by two antiparallel strands of nucleotides. Each nucleotide contains a sugar, phosphate and a base (adenine, thymine, cytosine or guanine). Complementary base pairing (A with T, C with G) holds the two strands together and preserves the genetic code because each base pairs only with its specific partner . Complementary base pairing provides a reliable template for copying. Each original strand acts as a template for a new complementary strand so that two DNA molecules form with identical base sequences. The chemical specificity of the bases enforces correct pairing during replication and reduces copying errors, although occasional mutations can still occur .
Mechanism of DNA replication
DNA replication proceeds by separating the two original strands and synthesising a complementary strand alongside each template using base-pairing rules; the result is two double helices each containing one original and one newly made strand. The template-driven pairing causes exact copying of the sequence of bases so that each daughter DNA molecule carries the same genetic information as the parent molecule . Replication of every chromosome produces two sister chromatids prior to cell division. The chromatids remain joined at the centromere until mitosis, so each chromosome effectively has its copy attached and ready for equal distribution to daughter cells during the cell cycle .
Cell cycle: doubling and division of genetic material
The cell cycle includes a synthesis phase (S) in which the genome is duplicated, followed by a mitotic phase (M) that separates the duplicated chromosomes into two nuclei and cytokinesis that divides the cytoplasm. Genome duplication during S phase ensures that each daughter cell receives a full diploid set of chromosomes. The nucleus dissolves and chromosomes shorten and thicken before mitosis to allow accurate movement of chromatids . Mitosis segregates one copy of each chromosome to opposite poles: duplicated chromosomes line up at the cell equator, sister chromatids separate and move to opposite ends, and new nuclear envelopes form around the two sets. Cytokinesis then splits the cell membrane to produce two genetically identical diploid daughter cells, each containing one complete copy of the original genome .
Errors and limiting factors during replication and division
Replication fidelity depends on accurate base pairing and proofreading by repair enzymes; failure of proofreading or exposure to mutagens can produce mutations such as base substitutions, insertions or deletions. Mutations can alter gene function and are a limiting factor in producing identical copies of DNA every cycle . Physical problems during mitosis, such as failure of spindle fibres or improper centromere function, can cause unequal chromosome distribution (aneuploidy). Proper chromosome condensation, spindle formation and checkpoint controls reduce such errors and ensure reliable doubling and equal division of genetic material .
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