Human Genetics 20

Chromosome Structure and Karyotyping

  • Karyotypes

    • Karyotyping involves examining the banding patterns of chromosomes stained with specific dyes.

    • The most commonly used dye is Giemsa stain (G stain), which reveals specific banding patterns on human chromosomes.

  • Key Chromosome Parts

    • Centromere

    • The centromere is the constricted region of a chromosome where the spindle fibers attach during mitosis and meiosis.

    • Composed of heterochromatin which is challenging to sequence due to repeated DNA sequences.

    • Heterochromatin is replicated late during S phase of the cell cycle.

    • Telomeres

    • Telomeres are located at the ends of chromosomes, and they protect the chromosome from degradation.

    • Discussion of subtelomeres, the regions between the centromere and telomeres, which may contain between 8,000 to 300,000 nucleotides.

    • Origin of Replication Sites

    • There are thousands of origin of replication sites per chromosome where replication begins during the S phase of the cell cycle.

  • Chromatin Structure

    • Chromosomes are primarily composed of chromatin, which consists of DNA and proteins, with small amounts of associated RNA.

Heterochromatin and Sequencing

  • Challenges in Sequencing

    • Heterochromatin presents difficulties in DNA sequencing due to its repetitive nature, which was a factor in the incomplete sequencing of the human genome project completed in 2003.

    • Regions such as the ends of chromosomes and centromeres remain hard to sequence because of their repetitive DNA sequences.

  • DNA Repeats

    • The base sequence repeated in centromeric DNA consists of 171 nucleotides, which can be exemplified by sequences like GATC, GATC, etc.

Telomeres and Subtelomeres

  • Structure and Importance

    • Telomeres consist of repetitive sequences to maintain chromosome integrity, preventing degradation and ensuring proper replication.

    • Telomeres are sensitive to the aging process and having functional sequences is vital for longevity of cells.

Gene Families and Pseudogenes

  • Gene Families

    • A gene family arises through duplication and divergence of genes. For example, if one gene is duplicated, the second copy can evolve variations leading to new functions (e.g., wing development in flies).

    • Examples include Hox genes which are critical for body patterning and development in multicellular organisms.

  • Pseudogenes

    • Pseudogenes are nonfunctional genes that have mutations preventing them from producing functional proteins. They are remnants from evolutionary processes and can still hold historical genetic information.

    • Humans have 900 functional and 800 nonfunctional genes within the odor receptor gene family.

Karyotype Procedure

  • Karyotype Analysis

    • Karyotypes are typically prepared from white blood cells, as red blood cells are enucleated and lack DNA.

    • Laboratory technicians isolate cells at mitotic metaphase where chromosomes are most compact and visible.

    • The chromosomes are stained, photographed, and arranged using computer guidance based on size for easier analysis.

  • Cytogenetic Notation

    • Karyotypes are reported using cytogenetic notation (e.g., 46,XX; 47,XX,+21 for an extra chromosome 21 which indicates Down syndrome).

Implications of Karyotyping

  • Karyotyping can provide insight into genetic disorders, reveal effects of environmental toxins, and clarify evolutionary relationships across species.

    • Cytogenetic Notation can reveal chromosomal abnormalities and is essential for prenatal genetic testing and cancer diagnostics.

Chromosome Structure Classification

  • Chromosome classification based on centromere position:

    • Telocentric: Centromere at one end.

    • Acrocentric: Centromere near one end (but not at the tip).

    • Submetacentric: Centromere slightly off-center.

    • Metacentric: Centromere in the center.

Genetic Testing Methods

  • Amniocentesis and Chorionic Villus Sampling (CVS) are traditional methods used for prenatal genetic testing.

    • Amniocentesis: Performed around 15-16 weeks of pregnancy by extracting amniotic fluid containing fetal cells. Results typically take 1-2 weeks due to cell culture requirements.

    • Chorionic Villus Sampling: Conducted earlier (around 10-13 weeks) using a catheter to obtain a sample of placental tissue, which carries genetic information. Higher risk of complications associated with this method (e.g., miscarriage).