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High school biology

Course: High school biology > Unit 4

Lesson 2: Chromosome structure and numbers
Science
High school biology
Reproduction and cell division
Chromosome structure and numbers
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Chromosome structure and numbers review

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Key Terms

TermMeaning
GenomeThe full set of genetic information an organism has in its DNA
ChromosomeThreadlike structure of DNA and protein containing genetic information
Homologous chromosomesSet of chromosomes (one from each parent), that are very similar to one another and have the same size/shape
Diploid (2n)Cell that contains two sets of homologous chromosomes
Haploid (n)Cell that contains only a single set of genes
Sex chromosomeOne of two chromosomes (X or Y) that determines an organism's sex
AutosomeChromosome that is not a sex chromosome
KaryotypeMicrograph image of diploid set of chromosomes, grouped in pairs

DNA and chromosomes

A cell’s set of DNA is called its genome. We can refer to the genome of an organism or of a species, since the members of a species typically have similar genomes.
Most prokaryotes contain a single circular DNA chromosome. This genetic information is found in the cytoplasm.
Eukaryotic DNA is highly structured and organized and contain multiple chromosomes. These chromosomes are packaged and stored within the nucleus.

Chromosome number

Different species have different numbers of chromosomes. For example, humans are diploid (2n) and have 46 chromosomes in their normal body cells. These 46 chromosomes are organized into 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes.
The sex cells of a human are haploid (n), containing only one homologous chromosome from each pair. This is so that when the sex cells fuse together during fertilization, a complete diploid set is formed.
An organism's full set of chromosomes can be viewed by taking photographs of cells dividing. These images are then arranged to form a karyotype.
Image of the karyotype of a human male, with chromosomes from the mother and father false-colored purple and green, respectively.
Image modified from "Karyotype," by the National Institutes of Health (public domain).

Common mistakes and misconceptions

  • Eukaryotic chromosomes consist of two sister chromatids. The sister chromatids are identical to one another and are attached at a compressed region called the centromere. This is important for chromosomal separation during cell division. When the sister chromatids are connected at the centromere, they are still considered to be one chromosome (X-shaped). However, as soon as they are pulled apart during cell division, each is considered a separate chromosome.
What happens to a chromosome as a cell prepares to divide.
  1. The chromosome consists of a single chromatid and is decondensed (long and string-like).
  2. The DNA is copied. The chromosome now consists of two sister chromatids, which are connected by proteins called cohesins.
  3. The chromosome condenses. It is still made up of two sister chromatids, but they are now short and compact rather than long and stringy. They are most tightly connected at the centromere region, which is the inward-pinching "waist" of the chromosome.
  4. The chromatids are pulled apart. Each is now considered its own chromosome.
  • Chromosomes, DNA, genes, and genomes are not all the same thing. Sometimes these words are used interchangeably, but they are not the same. DNA is the building block of the genetic material found in all living things. A very long chain of DNA can form a chromosome. In these chromosomes are areas called genes that code for specific proteins. All the different chromosomes of an organism make up that organism's genome. In other words: DNA makes up genes, genes make up chromosomes, and chromosomes make up a genome.

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