SCN3230-ABED_1: Module 5 Lesson 1 - 2

Lesson 1 — Cell Division and Chromosomes

Genetic Material

Read pages 550-553

The essence of a cell, all of what it is and can do, is determined by the genetic code found on DNA (deoxyribonuclieic acid) inside the nucleus of that cell. DNA is organized into segments called chromosomes. For most of a cell's life, these segments of DNA are diffused and cannot be observed under a microscope. During this time, the DNA strands are wrapped around proteins and bundled into structures called chromatins. However, when the cell undergoes division, the chromatins condense into chromosomes and become visible.

For more detail on the organization of chromosomes, read pages 551 and 552 in the textbook.

Organization of Genetic Material. Darryl Leja, National Human Genome Research Institute. Public Domain

Each somatic cell in a human has 23 pairs of chromosomes for a total of 46 chromosomes. Every organism has a specific number of chromosomes in its cells. Each cell (except for gametes or sex cells) contains a complete set of chromosomes. Human body cells, except for cells that produce eggs and sperm cells, are called somatic cells. Each somatic cell has 22 pairs of homologous chromosomes, known as autosomes, as well as one pair of sex chromosomes within the nucleus.

Chromosome pairs 1 to 22 are autosomes, and two copies of each chromosome are in a somatic cell.
The 23rd pair is the sex chromosomes that determine the sex of the organism and can be either an X or a Y chromosome.
A human with two X chromosomes is genetically female, and one with one X and one smaller Y chromosome is genetically male.

Read pages 551 and 552 in the text book to learn more about chromosomes and how they are arranged in cells.

Various Types of Human Chromosomes. Hubbard, et al. (2002). "The Ensembl genome database project". Nucleic Acid Res. 30 (1): 38–41.

Did You Know?

The last chromosome, labelled MT on the table, is the mitochondrial chromosome located in mitochondria. Mitochondria are organelles in the cytoplasm outside the cell's nucleus and they serve as the source of energy for the cell.

The mitochondrial DNA (mtDNA) evolves slowly and can help scientists determine the origin of the human species. Another interesting characteristic of mtDNA is that it can be inherited only from the mother! Therefore, maternal lineage can be traced using the mtDNA.

Chromosome Number

Read page 552

Homologous chromosomes

Chromosomes can be organized further into homologous chromosomes. Two copies of each chromosome are in human cells, resulting in 23 pairs of chromosomes. The chromosomes of each pair are referred to as homologous chromosomes. One copy of the homologous chromosomes comes from the mother, and the other copy comes from the father. Homologous chromosomes are roughly the same size and shape. They contain the same type of genetic information on the same location of the chromosome. Although homologous chromosomes contain the same genes, these genes may not code for the same trait.

For example, chromosome 19 contains the gene for eye colour. Because there are two copies of chromosome 19, the maternal chromosome may code for the blue eye colour and the paternal chromosome may code for the green eye colour. These different traits (blue or green colour) for the genes (eye colour) are called alleles.

Ploidy

Ploidy refers to the number of sets of chromosomes in a cell. A full set of chromosomes includes a single copy of each chromosome and is denoted by 'n'. A single set of chromosomes is called haploid (n) whereas two complete sets of chromosomes are called diploid (2n).

In humans, n = 23 chromosomes. Gametes or sex cells are haploids (n) and contain one set of chromosomes. Somatic or body cells are diploids (2n) and contain two sets of chromosomes.

Each species has a specific number of chromosomes in a set and has a specific number of sets of chromosomes. For example, human cells have 46 chromosomes but dog cells have 78 chromosomes. Humans are diploid (2n), bananas are triploid (3n), and potatoes are tetraploid (4n).

Karyotype

Read pages 548-553

As you have just read, the number and arrangement of chromosomes is very important to regular function. Scientists can make pictures of chromosomes in a cell by staining cells that are about to undergo cellular division. At this point, the chromosomes are most dense and can be sorted into homologous pairs by their length, position of the centromere (a region that appears pinched), and banding pattern.

An individual's chromosome set is known as his or her karyotype. The karyotype image shows that the banding patterns on the homologous chromosomes are almost identical. The banding pattern indicates that the same genes are located on the same spot in the chromosomes. The only exception to this is the XY sex chromosome pair in males. On the karyotype, notice that the X and Y chromosomes are different shape and size. This shows that this individual is genetically a male.

When the chromosomes are sorted, scientists can determine the sex of an organism and whether an abnormal number of chromosomes is present. Certain major syndromes are a result of too many or too few chromosomes because the chromosomes did not separate equally during cell division. This is termed nondisjunction. In humans, Down syndrome is a result of an extra chromosome 21, and Klinefelter's syndrome is a result of an extra sex chromosome (XXY). Lesson 4 investigates this further.

Karyotype of a Normal Male. Darryl Leja, NHGRI (Public Domain).