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Meiosis I

Meiosis I

Meiosis is a carefully orchestrated set of cell divisions, the goal of which—in humans—is to produce haploid sperm or eggs, each containing half the number of chromosomes present in somatic cells elsewhere in the body. Meiosis I is the first such division, and involves several key steps, among them: condensation of replicated chromosomes in diploid cells; the pairing of homologous chromosomes and their exchange of information; and finally, the separation of homologous chromosomes by...

Meiosis I

Meiosis I

Meiosis is the division of a diploid cell into haploid cells forming sperm and eggs in animals through differentiation. Meiosis I is the first stage of meiosis, where the genetic recombination of homologous chromosomes and the reduction of the ploidy level by half occurs.
Prophase I is the most extended and complex step of meiosis I characterized by synapsis, chromosome pairing, and recombination of the homologous chromosomes. This process is facilitated by a proteinaceous structure called the...

Meiosis I

Meiosis I

Karyotyping

Karyotyping

Karyotyping

Karyotyping

Nondisjunction

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate correctly and move to the opposite poles of the cells. This produces daughter cells with abnormal chromosome numbers. Nondisjunction is common during anaphase I or anaphase II of meiosis. Mutations in synaptonemal complex proteins that attach homologous chromosomes increase the chances of nondisjunction in anaphase I of meiosis I. In contrast, mutations in topoisomerases and condensins that hold...

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Related Experiment Video

Updated: Mar 12, 2026

Pre-Implantation Genetic Testing for Aneuploidy on a Semiconductor Based Next-Generation Sequencing Platform

Pre-Implantation Genetic Testing for Aneuploidy on a Semiconductor Based Next-Generation Sequencing Platform

Published on: August 17, 2022

Where have all the trisomies gone?

Glenn E Palomaki¹, Geralyn M Lambert-Messerlian¹, James E Haddow¹

¹Department of Pathology and Laboratory Medicine, Women & Infants Hospital, Providence, RI.

American Journal of Obstetrics and Gynecology

|October 30, 2016

Summary

Reliable prenatal screening estimates are crucial for informed choices. Cell-free DNA (cfDNA) screening detects more common aneuploidies, while sequential screening identifies additional abnormalities and structural defects.

Keywords:

Down syndrome biases cell-free DNA detection rate patient preference sequential screening sex chromosome trisomies

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Area of Science:

Obstetrics and Gynecology
Medical Genetics
Fetal Medicine

Background:

Prenatal screening for aneuploidy is essential for patient counseling and policy development.
Sequential screening (serum and ultrasound) is a common primary test for aneuploidy detection.
Cell-free DNA (cfDNA) screening is a newer, noninvasive secondary option for screen-positive cases.

Discussion:

Both sequential and cfDNA screening are noninvasive and detect common aneuploidies.
cfDNA screening identifies more common chromosomal abnormalities like trisomy 21 and sex trisomies.
Sequential screening can detect other aneuploidies (e.g., triploidy) and abnormalities linked to fetal structural defects.

Key Insights:

A direct comparison of sequential and cfDNA screening performance is critical.
The proportion and severity of identified chromosomal abnormalities are key comparative measures.
Accurate reporting of detection rates must account for known biases to ensure reliability.

Outlook:

cfDNA screening may become a general population alternative to sequential screening with future cost reductions.
Informed patient choices depend on reliable comparative performance estimates for all screening methods.
Further research is needed to refine performance estimates and address potential biases in reporting.