setrpal.blogg.se - Prophase meiosis

The sequence of these bases determines the genetic information encoded in DNA. The four nitrogenous bases are adenine (A), thymine (T), guanine (G), and cytosine (C). Each nucleotide consists of a sugar molecule, a phosphate group, and a nitrogenous base. It does not store any personal data.DNA structure and biochemical composition: DNA is a double-stranded helix made up of nucleotides. The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. The cookie is used to store the user consent for the cookies in the category "Performance". This cookie is set by GDPR Cookie Consent plugin.

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Trisomy - three copies of a chromosome, e.g.

Monosomy - one copy of a chromosome, e.g.

Clinical Relevance - Chromosomal abnormalities Figure 1 - An overview of meiosis Meiosis I Reciprocal translocations take place when the chromosomes break within the arms of the chromosome and Robertsonian translocations take place when whole chromosomes join end to end. There are two types of chromosomal translocation. If there is no gain or loss of genetic material, this is a balanced translocation, however, if the exchange of chromosomal material results in extra or missing genes in a daughter cell, it is known as unbalanced and can have clinical effects. Edward’s syndrome (trisomy 18)Ībnormalities in chromosome structure are often due to translocations, where there is an exchange of material between two chromosomes, resulting in an abnormal rearrangement.

Trisomy – three copies of a chromosome, e.g.

Monosomy – one copy of a chromosome, e.g.

This leaves one gamete short of some genetic information, and the other with additional genetic information. This is often due to nondisjunction where there is failed separation of chromosomes during anaphase, so either whole chromosomes (error occurring in meiosis I) or chromatids (error occurring in meiosis II) move to the same pole of the cell. Errors in the process of meiosis can lead to abnormalities in either chromosome number or structure.Ībnormalities in chromosome number include aneuploidy, where there is loss or gain of a whole chromosome. While this is happening, the chromosomes continue to condense.Ĭlinical Relevance – Chromosomal abnormalitiesĬhromosome abnormalities occur in approximately 0.6% of live births, however, they are common in pregnancy losses.

Spindle fibres attach to the chromosomes at points along the chromosomes called centromeres. This process is known as crossing over, and the points at which this occurs on a chromosome are referred to as chiasmata. To further increase genetic diversity, homologous chromosomes exchange small parts of themselves, such that one chromosome contains both maternal and paternal DNA. Spindle fibres appear which are important for the successful division of the chromosomes. The nuclear envelope disintegrates and the chromosomes begin to condense. There are initially four chromatids (c) and two chromosomes (n) for each of the 23 chromosome pairs (4c, 2n). Prior to prophase, chromosomes replicate to form sister chromatids. In meiosis I, homologous chromosomes are separated into two cells such that there is one chromosome (consisting of two chromatids) per chromosome pair in each daughter cell, i.e.