Cell Division

Learning Objectives

By the end of this section, you will be able to:

  • Describe the structure of prokaryotic and eukaryotic genetic make-up
  • Distinguish between chromosomes, genes, and traits
  • Describe the mechanisms of chromosome compaction

The continuity of life from one cell to another has its foundation in the reproduction of cells by way of the cell cycle. The cell cycle is an orderly sequence of events that describes the stages of a cell’s life from the division of a single parent cell to the production of two new daughter cells.

Genomic DNA

The cellular DNA, packaged as a double-stranded DNA molecule, is called its genome. In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in a loop located within the nucleoid(Figure 1).

The illustration shows a prokaryotic cell with a single, circular chromosome floating free in the cytoplasm.

Figure 1. Prokaryotes, including bacteria and archaea, have a single, circular chromosome located in a central region called the nucleoid.

The 23 chromosomes from a human female are each dyed a different color so they can be distinguished. During most of the cell cycle, each chromosome is elongated into a thin strand that folds over on itself, like a piece of spaghetti. The chromosomes fill the entire spherical nucleus, but each one is contained in a different part, resulting in a multi-colored sphere. During mitosis, the chromosomes condense into thick, compact bars, each a different color. These bars can be arranged in numerical order to form a karyotype. There are two copies of each chromosome in the karyotype..

Figure 2. There are 23 pairs of homologous chromosomes in a female human somatic cell. The condensed chromosomes are viewed within the nucleus (top), removed from a cell in mitosis and spread out on a slide (right), and artificially arranged according to length (left); an arrangement like this is called a karyotype. In this image, the chromosomes were exposed to fluorescent stains for differentiation of the different chromosomes. A method of staining called “chromosome painting” employs fluorescent dyes that highlight chromosomes in different colors. (credit: National Human Genome Project/NIH)

In eukaryotes, the genome consists of several double-stranded linear DNA molecules (Figure 2). Each species of eukaryotes has a characteristic number of chromosomes in the nuclei of its cells.  Diploid describes a nucleus containing two copies of genetic material or a complete set of chromosomes.  For humans, diploid would be shown as 2n = 46.  Haploid, found in our sex cells, would have only one copy of the genetic material or a half a set of chromosomes.  So, n = 23 for humans.

Matched pairs of chromosomes in a diploid organism are called homologous (“same knowledge”) chromosomes. Homologous chromosomes are the same length and have genes in exactly the same location, or locus. Genes, the functional units of chromosomes, determine specific characteristics.  Traits are the variations of those characteristics. An example would be hair color with traits that are blonde, brown, or black.

 The variation of individuals within a species is due to the specific combination of the genes inherited from both parents. Even a slightly altered sequence within a gene can result in an alternative trait.  For example, there are three possible gene sequences on the human chromosome coding for blood type: sequence A, sequence B, and sequence O. Because all diploid human cells have two copies of the chromosome for blood type, the blood type(trait) is determined by which two versions of the gene is inherited.

Minor variations of traits, such as blood type and eye color, contribute to the natural variation found within a species. The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosome uniformity.  The X and Y chromosomes do have variations to accurately produce gametes, but other slight variations do exist.

Eukaryotic Chromosomal Structure and Compaction

There are five levels of chromosome organization. From top to bottom: The top panel shows a DNA double helix. The second panel shows the double helix wrapped around proteins called histones. The middle panel shows the entire DNA molecule wrapping around many histones, creating the appearance of beads on a string. The fourth panel shows that the chromatin fiber further condenses into the chromosome shown in the bottom panel.

Figure 3. Double-stranded DNA wraps around histone proteins to form nucleosomes that have the appearance of “beads on a string.” The nucleosomes are coiled into a 30-nm chromatin fiber. When a cell undergoes mitosis, the chromosomes condense even further.

DNA must be tightly packaged to fit in the cell’s nucleus. At the same time, it must also be readily accessible for gene expression.  During some stages of the cell cycle, the long strands of DNA are condensed into compact chromosomes.  Chromosomes are compacted in a number of ways to include:

(1)  short stretches of the DNA double helix wrap around a core of eight histone proteins at regular intervals along the chromosome forming nucleosome(Figure 3);

(2)  nucleosomes coil into chromatin fiber;  further shortens the chromosome; forms chromatin fiber

(3)  proteins are used to pack the chromatin;  ensure each chromosome (in a non-dividing cell) occupies a particular area of the nucleus that does not overlap with any other chromosome (see the top image in Figure 3).

DNA replication occurs in interphase during the S phase of the cell cycle.  After replication, a chromosome is composed of two linked sister chromatids held together at a compacted region called the centromere.

Link to Learning

This animation illustrates the different levels of chromosome packing:

Section Summary

Prokaryotes have a single looped chromosome composed of double-stranded DNA in the nucleoid.  Eukaryotes have double-stranded  linear chromosomes composed of chromatin in the nucleus.  A full set of matched chromosomes is referred to as diploid.  Haploid is having a half a set of chromosomes. Genes are segments of DNA that code for a specific protein. An organism’s traits are determined by the genes inherited from each parent. Duplicated chromosomes are composed of two sister chromatids held together by the centromere.  Chromosomes are compacted using a variety of mechanisms during the cell cycle which is necessary for chromosomal segregation during mitosis.

Additional Self Check Questions

1. Define diploid and haploid.

2.  Eukaryotic chromosomes are thousands of times longer than a typical cell. Explain how chromosomes can fit inside a eukaryotic nucleus.

AnswERs

 

1. Diploid is a full set of matched chromosomes inside a cell nucleus.  Haploid is a half a set of chromosomes.  Diploid is represented as 2n, while haploid is represented as n. 
2. The DNA double helix is wrapped around histone proteins to form structures called nucleosomes. Nucleosomes are coiled into a chromatin fiber. During cell division, chromatin is further condensed.