Module 5

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

Lesson Summary

 

During this lesson you were to concentrate on the following focusing questions:

• How does meiosis contribute to genetic variation?
  
• What differences exist between fraternal and identical twins?

Meiosis is the orderly separation of homologous chromosomes into haploid gametes. Through crossover events in prophase I, and through independent assortment in metaphase I, meiosis ensures the creation of unique gametes. When fertilization later occurs, the new offspring will have a genetic combination never before seen.

 

Fraternal twins result from the fertilization of two individual egg cells by two individual sperm cells. Since meiosis ensures that each gamete produced by a male or female is unique, they will each be genetically different from the other. Identical twins result from the fertilization of a single egg cell by a single sperm cell. Early in development, the cell mass splits into two separate cell masses. Each mass grows by mitosis into a person. Since mitosis does not create variation, these twins are genetically the same, or identical.

 

Lesson Glossary

 

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

 

2n: the symbol referring to a diploid cell

 

alleles: different versions (base sequences) of a gene or trait that will code for slightly different proteins (e.g., sickle cell hemoglobin versus normal hemoglobin)

 

Increased types of alleles in the gene pool increases variation and diversity and protects the species from extinction.

 

clone: to create of an exact replica

 

A cell is a clone if it is the product of asexual reproduction (mitosis or binary fission) that produces two genetically identical cells. An organism can be a clone if it is genetically identical to another organism (e.g., animals can be cloned by taking a nucleus from one animal and inserting it into an empty egg of another, producing an offspring identical to the one that donated the nucleus). Cloning is used in agriculture and pharmaceutical industries to create uniform, consistent products.

 

crossing over: an occurance during meiosis I when homologous pairs and their attached sister-chromatids form tetrads, entwine in synapsis, and may be chopped by enzymes into pieces

 

When chromosomes are reassembled, sections of the mother’s homologue may be exchanged with the father’s, forming chromosomes with new combinations of mother’s and father’s alleles. This increases variation in gametes and offspring, improving the species' chances of survival if the environment changes.

 

diploid: the chromosome number of a somatic (body) cell; both chromosomes of each homologous pair are present; two sets of chromosomes are present, one from each parent

 

fraternal twins: two siblings born at the same time, resulting from the accidental ovulation of two eggs, which are fertilized by two sperm

 

Fraternal twins are as different as any two siblings.

 

genetic variation: the permutations and combinations of genes and alleles possible; refers to different combinations of mother’s and father’s alleles in gametes; increases variation in the offspring and translates into better odds of offspring survival in changing environments

 

haploid: chromosome number of a gamete (sex cell—egg or sperm), resulting from meiosis; only one chromosome of each homologous pair is present; one set of chromosomes present

 

identical twins: result of one egg fertilized by one sperm; occurs when the morula splits into two masses that develop independently in the uterus; offspring are genetically identical

 

independent assortment: one of Mendel’s Laws of heredity

 

There are two uses of this term1) In meiosis I, tetrads line up randomly on the metaphase plate so that either the mother’s or the father’s sister chromatids are on one side. When all the tetrads are pulled apart in anaphase I, the chromosomes that collect at each pole are a unique combination/permutation of mother’s and father’s chromosomes/alleles. The orientation of one tetrad is independent of the others. (2) in reference to whether or not two genes are on the same chromosome or on different ones: If on different chromosomes, one gene (allele) is independent of the other gene in the way their tetrad lines up on the metaphase plate of meiosis I, and much gamete variation can result. If genes are linked on the same chromosome, the two genes cannot line up independently of each other—they are tied together, limiting the variety of permutations/combinations and variation in gametes that can be formed.

 

meiosis I: the first division of meiosis; preceded by DNA replication in interphase; results in one secondary oocyte and first polar body in females and two secondary spermatocytes in males

 

Because homologous pairs are separated from each other in anaphase, cell products are already considered haploid.

 

meiosis II: the second division of meiosis; no DNA replication in interphase; results in one haploid ootid and second polar body in females and four haploid spermatids in males

 

n: symbol referring to a haploid cell

 

ploidy: refers to the chromosome number of a cell or how many sets of chromosomes are present; haploid cells have one set, diploid two, tetraploid four, octoploid eight

 

synapsis: the entwining of the homologous pair and attached sister chromatids in prophase I of meiosis; crossing-over between non-sister chromatids may occur

 

tetrad: formed in prophase of meiosis I when homologous pairs and their attached sister chromatids find each other and entwine in synapsis; may undergo crossing-over with non-sister chromatids, increasing variation in the gametes that result

 

totipotent: cells that have not specialized or differentiated (e.g., zygote, morula); all genes in the cell have the potential to be expressed (turned on)

 

Totipotent cells have the ability to form a complete organism.

 

Biology 30 Learn EveryWare © 2009 Alberta Education