Cell division, DNA and inheritance

Keep chromosomes, chromatids and copies separate

A chromosome contains DNA associated with proteins. Before DNA replication, a chromosome has one DNA molecule; after replication, it consists of two sister chromatids joined at a centromeric region. Chromosome number is conventionally counted by centromeres. Replication therefore doubles the DNA content without immediately doubling the chromosome number. This distinction resolves many apparent contradictions in diagrams. Count the units specified in the question rather than using chromosome and chromatid as interchangeable words.

The cell cycle includes growth and preparation as well as division. DNA replication occurs during S phase of interphase. Mitosis separates replicated chromosomes so that daughter nuclei normally receive equivalent chromosome sets. Cytokinesis divides the cytoplasm. In an ordinary diploid somatic cell, mitosis preserves the diploid number. It supports growth, tissue replacement and some forms of asexual reproduction. A cell does not have to copy DNA again between the separation of chromatids and the completion of cytokinesis.

Meiosis begins after one DNA replication and involves two divisions. Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II. The resulting cells are haploid with respect to chromosome sets. Pairing of homologues and crossing over can exchange DNA between non-sister chromatids during prophase I. Independent assortment also contributes to genetic variation. A homologous pair contains corresponding genes, but the two chromosomes may carry different versions, or alleles, of those genes.

Follow the information rather than changing the molecule's name

DNA replication copies DNA. Transcription produces RNA using a DNA template. Translation uses the sequence of a messenger RNA to build an amino-acid chain at a ribosome. In the usual genetic code, a codon is a sequence of three RNA bases. Transfer RNAs connect codons with amino acids, while stop codons signal termination. Not every RNA molecule is translated into a protein; ribosomal and transfer RNAs have functional roles as RNA.

A mutation changes genetic material. A base substitution can change an amino acid, leave the amino acid unchanged because of code redundancy, or introduce a stop signal. Insertions and deletions in a coding sequence can shift its reading frame when their lengths are not multiples of three. The effect depends on location and context. Do not assume that every mutation causes disease or that every mutation is inherited by offspring. Mutations in a somatic cell affect its descendants within the body; inheritance through sexual reproduction requires involvement of the germ line.

State the model before calculating a cross

At a diploid autosomal locus, an individual usually carries two alleles, one from each parent. A homozygote carries two copies of the same allele; a heterozygote carries different alleles. Under complete dominance, a heterozygote shows the dominant phenotype. Dominant does not mean common, stronger, healthier or destined to increase in frequency. It describes the phenotype of a particular genotype under the stated model.

For a cross between two heterozygotes, each parent produces gametes carrying either allele with probability one half, assuming ordinary segregation. Combine one gamete from each parent. The four equally likely combinations give one homozygous dominant, two heterozygous and one homozygous recessive genotype. With complete dominance, the phenotype ratio is three dominant to one recessive. These probabilities describe each independent conception, not a guarantee about the composition of every family of four children.

Human inheritance can involve autosomal loci, sex-linked loci, multiple genes and environmental effects. For a simple X-linked recessive model, a male with one X chromosome expresses the allele on that X because he lacks a second corresponding allele on another X. Fathers pass their Y chromosome to sons in the standard XX/XY model, so they do not pass an X-linked allele directly to those sons. Build a cross from the specified parental genotypes and the child's specified sex rather than applying an autosomal ratio automatically.