10: Changes in Chromosome Number and Structure
Previous chapters described chromosomes as simple linear DNA molecules on which genes are located. For example, your largest chromosome, chromosome 1, has about 3536 genes. To ensure that each of your cells possesses these genes the chromosome has features that allow it to be passed on during cell division. Origins of replication found along its length provide places for DNA replication to start, telomeres protect each end of the chromosome, and a single centromere near the middle provides a place for microtubules to attach and move the chromosome during mitosis and meiosis. This chapter examines: (1) changes in the number of whole chromosomes and how they affect the phenotype of an organism and (2) changes in the structure of individual chromosomes and how they affect meiotic pairing. Human examples will be used to show the phenotypic consequences and methods for detection.
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- 10.1: Changes in Chromosome Number
- This page explains chromosome abnormalities due to nondisjunction in mitosis and meiosis, where improper separation leads to aneuploidy. This condition causes daughter cells to have abnormal chromosome counts, which generally reduces cell viability as they may lack or have excess gene copies, impairing functionality. While some unbalanced gametes might function, they frequently result in non-viable embryos.
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- 10.2: Changes in Chromosome Structure
- If the chromosome is altered, but still retains the three critical features of a chromosome (centromeres, telomeres, and origin of replication), it will continue to be inherited during subsequent cell divisions, however the daughter cell may not retain all the genes. For example, if a segment of the chromosome has been lost, the cell may be missing some genes. The causes of chromosome structural abnormalites, which involves breaks in the DNA that makes up the chromosome.
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- 10.3: Chromosome Abnormalities in Humans
- To better understand the consequences let's consider those that affect people. As you will recall humans are 2n=46. The convention when describing a person's karyotype (chromosome composition) is to list the total number of chromosomes, then the sex chromosomes, and then anything out of the ordinary. Most of us are 46,XX or 46,XY. What follows are some examples of chromosome number and chromosome structure abnormalities.
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- 10.4: Diagnosing Human Chromosome Abnormalities
- This page discusses the use of bright field microscopy and fluorescence in situ hybridization (FISH) for diagnosing chromosomal abnormalities such as Down syndrome and Cri-du-chat syndrome. While bright field microscopy has visibility limitations, FISH offers clearer identification but is resource-intensive. Emerging techniques like PCR and DNA chips show promise for accurate DNA quantification.
Contributors
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Dr. Todd Nickle and Isabelle Barrette-Ng (Mount Royal University) The content on this page is licensed under CC SA 3.0 licensing guidelines.