Chromosome Structure and Organization

DNA is not freely floating in the nucleus; instead, it is coiled to save space. DNA strands wrap around proteins called histones, forming nucleosomes. These nucleosomes then undergo further supercoiling to create the familiar shape of chromosomes. Human diploid cells contain two versions of each chromosome (maternal and paternal), totaling 46 chromosomes (23 pairs).

Each chromosome duplicates through DNA replication, resulting in two identical sister chromatids. Homologous pairs of chromosomes contain the same genes but may have different alleles. Sister chromatids, however, are identical and are separated during mitosis to ensure each daughter cell receives a complete set of chromosomes.

The chromosome theory of inheritance emerged from observations of chromosomes during mitosis, validating Mendel's abstract concept of genes. Genes are long stretches of DNA located at specific positions on chromosomes. The separation of homologous chromosomes during meiosis accounts for Mendel's law of segregation, and their random arrangement explains the law of independent assortment.

Genes constitute only a small percentage of the genome; the majority is noncoding DNA. This noncoding DNA serves various functions, such as transcribing non-mRNA RNAs, acting as origins of replication, regulating gene expression, and forming centromeres and telomeres. Telomeres, found at the ends of chromosomes, protect genetic information from erosion during DNA replication. Telomerase can extend telomeres in some cells. Transposons are noncoding sequences that can change positions within the genome.

Humans have a pair of sex chromosomes: XX for females and XY for males. The Y chromosome is much smaller and lacks many genes present on the X chromosome. This leads to X-linked genes, where males only have one allele for certain genes, making them more susceptible to expressing recessive X-linked disorders like color-blindness and hemophilia. In females, one of the two X chromosomes is largely inactivated in each cell, chosen randomly, which can result in mosaic phenotypes.

A replicated chromosome, consisting of two sister chromatids, is a complex structure of looped domains wrapped around a scaffold. These looped domains can be unwound to reveal a fiber of nucleosomes (DNA wrapped around histones), which can be further zoomed in to show the double helical DNA, its nitrogenous bases, and individual atoms. This intricate structure houses genes that produce all the proteins in the body.