Lecture Notes: Understanding Chromosomes and Karyotypes

Introduction to Chromosomes

Chromosomes often depicted in diagrams as "X" shapes.
Misconceptions from diagrams:
- Not all chromosomes are "X" shaped.
- People can have 2 X chromosomes or 1 X and 1 Y chromosome.
- Chromosome representations can vary between 'X' shape and single stick forms.
Humans have 46 chromosomes which are made up of chromatin (DNA and protein).

Chromatin: Made of DNA and proteins.
Packaging:
- DNA is wound around proteins called histones forming nucleosomes.
- Further condensing levels lead to the formation of chromosomes.
Importance of packing:
- Fits DNA into the nucleus.
- Facilitates DNA movement during cell division (mitosis).

Human chromosomes can exist as single components or as replicated units.
During interphase, chromosomes duplicate but are still connected at the centromere.
- Considered one chromosome unit despite duplication.
- Consists of 2 sister chromatids.
In humans: 46 chromosomes, but 92 chromatids post-duplication.
During mitosis, chromatids are pulled apart to ensure each new cell has 46 chromosomes.

Specific genes on chromosomes can be activated or deactivated by various factors.
Karyotype: Image of all chromosomes in a cell.
- Usually taken during mitosis (typically metaphase) due to chromosome condensation.
- Displays chromosomes as homologous pairs.

Homologous Chromosomes:
- Same size; contain the same types of genes.
- One chromosome from each parent.
- 23 pairs; 22 pairs are autosomes; 1 pair are sex chromosomes.
Sex Chromosomes:
- Females: XX, Males: XY.
- Determine biological sex.
Karyotype Importance:
- Useful in detecting genetic disorders, missing or extra chromosomes.
Haploid vs. Diploid:
- Gametes (sperm and egg) are haploid (one set of chromosomes).
- Zygote (fertilized egg) is diploid (two sets of chromosomes).
- Fathers determine biological sex of offspring.