ChromoSocks Lesson 1

What is a chromosome? A chromosome is a piece of DNA that is maintained inside the cell. It replicates or copies itself and is segregated with each cell division. Different chromsomes contain different numbers of genes. DNA is tightly coiled in the nucleus in long strands called chromotin.

What are homologous chromosomes? Humans typically have 46 chromosomes, organized into 23 pairs. Out of each pair, one is inherited from the mother, and the other is inherited from the father. The two copies of each pair are called homologs..

What are homologous chromosomes? Homologs are the same size, shape and have genes located in the same place on the chromosome. Although they are similar, they are NOT identical. For example, a person can inherit a gene for curly hair from one parent and straight hair from the other parent.

What are replicated chromosomes? Before cells divide, the DNA is replicated or copied, and condensed even more. The two replicated copies of each chromosome are called sister chromatids. They are held together at a specific location called the centromere.

What is Meiosis? Meiosis is a type of cell division that occurs only in the production of sex cells. The purpose of meiosis is to reduce the chromosome number (46) by half (23) that will be present in the mature egg or sperm. The result of this is that when the egg and sperm unite, the embryo will have the correct number of chromosomes (46).

What is Meiosis? NOTE: 46 is the chromosome number for humans. Other organisms have different chromosome numbers. For humans, 46 is called the diploid number, and 23 is called the haploid number. During meiosis, the chromosome number is reduced, going from 2 copies of each chromosome (diploid) to 1 single copy (haploid).

What is Meiosis? At the end of meiosis, 4 haploid cells are produced, known as gametes. In males, 4 functional sperm are produced while in females, one large egg and 3 polar bodies are produced. (Polar bodies are non- functional and store extra chromsomes)

What are the stages of Meiosis? S Phase: DNA within the cell is replicated. (this is not a stage of meiosis) During this phase, the physical amount of DNA is doubled, but the cell remains diploid.

MEIOSIS 1 There are 4 stages in Meiosis 1. You can remember them in order by remembering the phrase PMAT. 1. Prophase: Chromosomes condense and become visible. Each chromosome forms a pair with its homolog.

Prophase 1 Crossing over may occur: equal portions of DNA from maternal and paternal homologs are exchanged. Crossing over ensures genetic variation!

Metaphase 1 Homologous chromosomes line up side by side in the middle of the cell.

Anaphase 1 The spindle fibers shorten which pulls the homologous chromosomes to opposite poles of the cells. Each pair segregates randomly and this contributes to genetic diversity.

Telophase 1 The centrioles and spindle fibers break down and the nuclear membrane separately reforms around two distinct sets of chromosomes.

Cytokinesis The remainder of the cell divides around the two new nuclei. This results in 2 haploid cells, each with 23 chromosomes (in humans)

Meiosis 2 The second cell division of meiosis is called Meiosis 2. It begins with Prophase 2

Prophase 2 Prophase 2: centrioles and spindle fibers form and attach to the centromere of each chromosome. The centromeres of sister chromatids act independently with a spindle fiber from one pole attaching to the centromere of 1 sister chromatid and a spindle from the other pole attaching to the other sister chromatid.

Prophase 2

Metaphase 2 Chromosomes line up along the spindle of the cell.

Anaphase 2 The sister chromatids are pulled to opposite ends of the cell due to the shortening of the spindle fibers.

Telophase 2 The centrioles and spindle fibers break down and the nuclear membrane reforms.

Cytokinesis In cytokinesis, the cytoplasm divides and the cell membrane pinches the cell in two. This occurs for both of the cells produced at the end of Meiosis 1 and results in 4 genetically diverse, haploid cells.

Errors in Meiosis The most frequent cause for meiotic errors is a process called nondisjunction. In nondisjunction, something doesn’t separate like it should. If a pair of homologous chromosomes fails to separate in Meiosis 1, this would result in two sex cells with an extra copy of that chromosome and two sex cells that lack any copy of it.

Nondisjunction If sister chromatids fail to separate in Meiosis 2, one sex cell would have an extra copy of that chromosome and one cell would lack it. The other two would have the correct number of chromsomes. Trisomy: having an extra chromosome Monosomy: Missing a chromosome

Monosomy

Polyploidy If the cell completely fails to divide, it would result in a sex cell with an extra copy of all chromosomes. This is called polyploidy. In females, nondisjunction occurs in about 20% (1 in 5) female meiotic events. The rate is lover in males.

Trisomy 21 (Down syndrome) Individuals have 3 copies of chromosome 21. Causes a range of physical and developmental differences such as facial features, low muscle tone, heart and intestinal abnormalities, and y some level of intellectual and developmental delays.

Down Syndrome

These are more severe than Trisomy 13. Individuals with Edwards syndrome have three copies of chromosome 18, while those with Patau syndrome have three copies of chromosome 13. Characterized by severe delays in growth and development and multiple skeletal and organ abnormalities. Most do not survive birth and if they do, they usually pass in the 1 st year. Trisomy 18 (Edward Syndrome) and Trisomy 13 (Patau syndrome)

Edward Syndrome

Patau Syndrome

Monosomy X (Turner Syndrome) Individuals with Turner Syndrome are females with one X chromosome instead of the typical two X chromosomes. They are usually shorter than average, have characteristic facial features, and often have heart and kidney abnormalities. These females also do not have ovarian function and are infertile.

Turner Syndrome

XXY (Klinefelter syndrome) Individuals with Klinefelter syndrome are males with an extra X chromosome. They are tall, with long arms and legs, have low muscle tone, and less than typical hair on the body. They also have an increased chance for learning disabilities and are infertile.

Klinefelter Syndrome

XYY Individuals with XYY are males with an extra Y chromosome. They are taller than average, have an increased risk of learning disabilities and have minor behavior problems.

XYY