Genetics For Dummies
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Chromosomes are threadlike strands that are composed of DNA. To pass genetic traits from one generation to the next, the chromosomes must be copied, and then the copies must be divvied up. Most prokaryotes have only one circular chromosome that, when copied, is passed on to the daughter cells (new cells created by cell division) during mitosis. Eukaryotes have more complex problems to solve (like divvying up half of the chromosomes to make sex cells), and their chromosomes behave differently during mitosis and meiosis. Additionally, there are various terms to describe the anatomy, shapes, the number of copies, and situations that eukaryotic chromosomes find themselves in. This article gets into the intricacies of chromosomes in the eukaryotic cells, because they're so complex.

Counting out chromosome numbers

Each eukaryotic organism has a very specific number of chromosomes per cell — ranging from one to many. For example, humans have 46 total chromosomes. These chromosomes come in two varieties:

  • Sex chromosomes: These chromosomes determine gender. Human cells contain two sex chromosomes. If you're female, you have two X chromosomes, and if you're male, you have an X and a Y chromosome.
  • Autosomal chromosomes: Autosomal simply refers to non-sex chromosomes. So, sticking with the human example, do the math, and you can see that humans have 44 autosomal chromosomes.

Ah, but there's more. In humans, chromosomes come in pairs. That means you have 22 pairs of uniquely shaped autosomal chromosomes plus 1 pair of sex chromosomes, for a total of 23 chromosome pairs. Your autosomal chromosomes are identified by numbers — 1 through 22. So, you have two chromosome 1s, two 2s, and so on.

When chromosomes are divided into pairs, the individual chromosomes in each pair are considered homologous, meaning that the paired chromosomes are identical to one another in shape and size. For example, your two single chromosome 2s are paired up because they're identical in shape and size. These homologous chromosomes are sometimes referred to as homologs for short.

Chromosome numbers can get a bit confusing. Humans are diploid, meaning we have two copies of each chromosome. Some organisms (like bees and wasps) have only one set of chromosomes (cells with one set of chromosomes are referred to as haploid); others have three, four, or as many as sixteen copies of each chromosome! The number of chromosome sets held by a particular organism is called the ploidy.

The total number of chromosomes doesn't tell you what the ploidy of an organism is. For that reason, the number of chromosomes an organism has is often listed as some multiple of n. Thus, humans are 2n = 46 (indicating that humans are diploid and the total number of chromosomes is 46). A single set of chromosomes referred to by the n is the haploid number. Human sex cells such as eggs or sperm are haploid.

Examining chromosome anatomy

Chromosomes are often depicted in stick-like forms. Chromosomes don't look like sticks, though. In fact, most of the time they're loose and string-like. Chromosomes only take on this distinctive shape and form when cell division is about to take place (during metaphase either through meiosis or mitosis). They're often drawn in this very distinctive shape and form because the special characteristics of eukaryotic chromosomes are easier to see.

The part of the chromosome that appears pinched together (located in the middle of the chromosome) is called the centromere. The placement of the centromere (whether it's closer to the top, middle, or bottom of the chromosome) is what gives each chromosome its unique shape. The ends of the chromosomes are called telomeres. Telomeres are made of densely packed DNA and serve to protect the DNA message carried by the chromosome.

The differences in shapes and sizes of chromosomes are easy to see, but the most important differences between chromosomes are hidden deep inside the DNA. Chromosomes carry genes. Genes are sections of DNA that make up the building plans for physical traits. The genes tell the body how, when, and where to make all the structures that are necessary for the processes of living. Each pair of homologous chromosomes carries the same — but not necessarily identical — genes. For example, both chromosomes of a particular homologous pair might contain the gene for hair color, but one can be a "brown hair" version of the gene — alternative versions of genes are called alleles — and the other can be a "blond hair" allele.

Any given gene can have one or more alleles. The alleles code for the different physical traits (phenotypes) you see in animals and plants like hair color or flower shape.

Each point along the chromosome is called a locus (Latin for "place"). The plural of locus is loci (pronounced low-sigh). Most of the phenotypes that you see are produced by multiple genes (that is, genes occurring at different loci and often on different chromosomes) acting together. For instance, human eye color is determined by at least three different genes that reside on two different chromosomes.

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