Module 5

Module 5—Cell Division: The Processes of Mitosis and Meiosis

Lesson Summary

 

During this lesson you explored the following focusing questions:

• What kinds of cell division exist and when do they occur?
  
  
• What are the structures that pass genetic information on to the next generation, and how are they observed?

Mitosis and meiosis are the main types of cellular division. Most of what we can see or observe around us are examples of mitotic growth or repair. Perhaps that is because it is fast and accurate. However, in later lessons, as you consider the survival of a species and the changing environmental pressures all life must face, you will soon see the value of meiotic reproduction.

 

In either type, DNA organized into chromosomes is at the heart of proper cell division. In humans, there are 22 pairs of autosomal chromosomes and a pair of sex chromosomes that determines gender. Together, all 46 chromosomes must be duplicated and then passed on to each new daughter cell for those cells to carry out their roles. Chromosomes can be stained and arranged in a karyotpe so scientists can determine the gender of a young developing fetus and whether or not the fetus will be born with genetic disorders.

 

Lesson Glossary

 

Consult the glossary in the textbook for other definitions that you may need to complete your work.

 

allele: a different form of the same gene occurring on homologous chromosomes

 

asexual reproduction: creation of a new organism without the input of cells from two separate organisms of opposite sexes; examples are binary fission, yeast and Hydra budding, and vegetative propagation of plants

 

autosomes: the 22 homologous pairs seen in a karyotype; have nothing to do with gender

 

binary fission: cell division in prokaryotes (bacteria); simple because there is only one circular chromosome so no spindle is needed

 

cell cycle: the period of time between one cell division and the next; consists of interphase, mitosis, and cytokinesis

 

cell division: the period of the cell cycle where the cell is actively dividing; composed of mitosis and cytokinesis stages

 

centromere: a ‘button’ that keeps the two identical sister chromatids together after the S phase of interphase and through mitosis until anaphase

 

chromatid: one-half or one of two threadlike strands into which a chromosome divides during cell division

 

chromatin: long fibres containing DNA, small amounts of RNA, and proteins

 

These fibres form chromosomes when they coil around histones.

 

chromosome: a thick, rod-shaped body in the nucleus that forms when chromatin (long, stringy DNA) supercoils around balls of histone proteins in prophase of mitosis and meiosis

 

cutting: type of vegetative propagation when a stem of a plant is cut off and produces roots, stems, leaves, and flowers; an asexual form of reproduction

 

daughter cell: a cell that is the product of cell division

 

In mitosis, daughter cells are identical to the mother cell; in meiosis, they are not identical to the parent cell.

 

diploid: the term describing a cell that contains two pairs of every chromosome    

 

DNA: the genetic material found contained in the nucleus in eukaryotes (also in mitochondria and chloroplasts) and loose in the cytoplasm in prokaryotes, such as bacteria

 

Down syndrome: typically characterized by some impairment of physical growth, unique physical features, and below average cognitive ability

 

If an n + 1 gamete that results from nondisjunction of a twenty-first chromosome is fertilized by a normal sperm, a Trisomy 21 (2n + 1) offspring is produced with Down syndrome.

 

fertilization: fusion of an egg and sperm (gametes) to produce a zygote; occurs in sexual reproduction only

 

gametes: sex cells (sperm and egg); have half the normal chromosome number so they can participate in fertilization

 

gene: the unit of hereditary information that can be passed on to offspring; includes the specific DNA sequence encoding or regulating the sequence of a protein, tRNA, or rRNA molecule; determines the expression of a trait

 

genetic material: DNA; contains the genes that direct the synthesis of proteins needed by the cell; exists as chromatin or chromosomes

 

haploid: the term describing a cell containing half the chromosomes that a diploid parent cell contains

 

This condition occurs in gametes, either in the egg or the sperm.

 

histones: proteins found in chromosomes that provide scaffolding for DNA to twine around so that the DNA can fit within the confined space of the nucleus

 

karyotype: a pictorial representation of all the chromosomes of a cell arranged in homologous pairs according to size, centromere position, and banding pattern; used to diagnose abnormalities in chromosome number (non-disjunction) and to determine sex chromosomes

 

Klinefelter syndrome: born with primary male sex characteristics but develops female secondary sex characteristics

 

When an XX egg due to nondisjunction is fertilized by a Y sperm, the offspring (XXY) has Klinefelter syndrome.

 

locus: a specific location on a chromosome

 

meiosis: cell division that results in cells that have half the normal chromosome number (haploid gametes); also called reduction division

 

mitosis: cell division that results in identical cells; used for growth and repair of organisms

 

mutation: a permanent change in a cell's genetic structure, often resulting in the expression of a new trait or feature in the affected organism; usually due to random errors occurring during DNA replication or protein synthesis, but can also be caused by chemical or physical mutagens

 

nondisjunction: an error in meiosis that results in non-separation of chromosomes; results in two chromosomes entering one gamete; produces gametes with an extra chromosome (n + 1), or gametes that are missing a chromosome (n – 1)

 

parent cell: a diploid somatic cell about to enter cell division

 

polyploid: the term describing a cell that contains more than two homologous chromosomes

 

resistance: occurs when a drug removes susceptible bacteria or viruses from a population and leaves those variants (mutants) that are resistant to the drug

 

Rapid cell division ensures that the whole population becomes resistant quickly.

 

sex chromosomes: the last (twenty-third) pair of chromosomes seen in a karyotype that determines the gender of an organism

 

X and Y sex chromosomes are not homologous to each other in terms of shape, size, or genetic information.

 

sexual reproduction: creation of offspring through input of genetic material from two different organisms of opposite sexes (sperm from male and egg from female); increases variation

 

somatic cell: the name given to any of the cells of a multicellular organism, including humans

 

The exception is those cells that form gametes, which are not somatic cells.

 

staining: a technique used in slide preparation to make the chromosomes of a dividing cell visible and dark

 

super bugs: bacteria that are immune to many antibiotics

 

Super bugs develop because of an overuse of antibiotics and antibacterials that have destroyed susceptible bacteria, leaving only those bacteria that are resistant to these drugs.

 

variation: the existence of many combinations of genes/traits in a population; improves the probability that some members will survive if environmental conditions change; is high in sexual reproduction

 

X chromosome: the longer sex chromosome

 

Females are XX.

 

Y chromosome: the shorter sex chromosome; determines maleness; has much fewer genes on it than the X

 

Males are XY.

 

Biology 30 Learn EveryWare © 2009 Alberta Education