Lecture Notes on Nondisjunction and Chromosomal Aberrations

Nondisjunction

  • Definition: Failure of chromosomes (homologs or sister chromatids) to separate properly during meiosis or mitosis.

    • Results in an abnormal number of chromosomes in the gametes.

Types of Nondisjunction

  • Turner Syndrome: Characterized by the presence of 45 chromosomes, specifically the karyotype 45,X.

  • Klinefelter Syndrome: Characterized by the presence of 47 chromosomes with the karyotype XXY.

Mechanism of Nondisjunction

When Can Nondisjunction Occur?

  • Mitosis:

    • Occurs during anaphase.

    • Results in gametes with either 2N+1 (one extra chromosome) or 2N-1 (one missing chromosome).

  • Meiosis I:

    • Tetrads (homologous chromosomes) fail to separate.

    • Produces gametes with N+1 (one extra chromosome) and N-1 (one missing chromosome).

  • Meiosis II:

    • Sister chromatids fail to separate.

    • Results in two normal haploid cells and two abnormal ones (N+1 and N-1).

Nondisjunction Results

  • Nondisjunction in Meiosis I leads to:

    • 2 gametes that are N+1 (extra chromosome) and 2 gametes that are N-1 (missing chromosome).

    • Confirms that all abnormal gametes are produced.

Chromosomal Abnormalities

Classification Based on Chromosome Number

  • Individual with 45 chromosomes is classified as:

    • Monosomy: Defined as 2n−1, indicating one missing member of a chromosomal pair.

    • The normal human chromosomal complement is 46; thus, a total of 45 indicates a monosomy.

Causes of Nondisjunction

Spindle-Assembly Checkpoint

  • Mechanism that monitors proper chromosome segregation; failures can lead to nondisjunction.

Age Factor in Female Meiosis

  • Female meiosis observed to be more error-prone, primarily due to:

    • Cohesin Degradation: Cohesin proteins that hold chromosomes together degrade during meiotic arrests in oocytes, increasing the likelihood of nondisjunction with age.

Statistics Related to Down Syndrome

  • Nondisjunction leads to Down syndrome, with varying incidence based on maternal age:

    • For example:

    • At age 25: 1 in 1,205 births

    • At age 30: 1 in 885 births

    • At age 35: 1 in 270 births

    • At age 40: 1 in 100 births

Monosomy

  • Defined as having 2n−1 chromosomes.

  • Monosomy for Autosomes:

    • Generally not tolerated in animals due to 'haploinsufficiency' where a single copy of a gene is insufficient for life-sustaining functions.

    • Gene Dosage: Refers to the amount of gene product produced, impacting phenotype.

Trisomy

  • Defined as having 2n + 1 chromosomes.

  • An extra chromosome can yield a more viable organism than the loss of one (if smaller).

    • Example Plants:

    • Datura:

      • Diploid number is 24; 12 primary trisomic conditions.

    • Oryza sativa (rice):

      • Diploid number is 12; trisomics with longer chromosomes show slower growth and varied morphology.

Chromosomal Aberrations

Types of Aberrations

  • Deletions: Loss of a portion of a chromosome.

  • Duplications: Repetition of a section of a chromosome.

  • Inversions: Reversal of a chromosomal segment.

  • Translocations: Movement of a chromosomal segment to a different location; can be nonreciprocal or reciprocal.

Deletions in Chromosomes

  • Chromosomal Deletion: A missing region occurs due to chromosome breaks.

    • Can be categorized into:

    • Terminal Deletion: Loss occurs at one end of the chromosome.

    • Intercalary Deletion: Loss occurs from the interior of the chromosome.

Formation of Deletion Loops

  • During synapsis, a normal homolog pairs with a chromosome containing an intercalary deletion, forming a deletion loop. This process allows for pairing despite the unpaired region on the homolog that 'buckles out' into the loop.

Cri du chat Syndrome

  • Caused by the deletion of a small variable part of the short arm of chromosome 5, identified as 46, 5p- (segmental deletion).

  • This condition is not inherited but results from sporadic loss of chromosomal material during gamete formation.

  • Severity of the syndrome varies with the length of the deletion, showcasing the significance of genetic dosage in phenotypic expression.