Lecture Notes: Understanding Chromosomes

Introduction

• Misconceptions about chromosomes arise from diagrams.
• Chromosomes often depicted in X shape in diagrams.
  • Misconception that all chromosomes are X-shaped.
  • Confusion over chromosomes resembling alphabet letters.

Chromosome Basics

• Chromosomes can be found in both bacteria (circular) and eukaryotes.
• Focus on human chromosomes.

Structure of Chromosomes

• Chromosomes are made of chromatin (DNA + protein).
• DNA is wound around histones to form nucleosomes ("beads").
• Further packaging condenses into chromosomes.
• Importance of packing:
  • DNA must fit inside nucleus.
  • Easier DNA movement during cell division (mitosis).
• DNA is not always tightly packed (e.g., during DNA replication).

Human Chromosomes

• Can appear as single components or replicated.
• During interphase, chromosomes duplicate but are bound at the centromere.
  • Still counted as one chromosome but consists of two sister chromatids.

Counting Chromosomes

• Humans have 46 chromosomes.
• Post-duplication in interphase: 46 chromosomes = 92 chromatids.
• During mitosis, chromatids are pulled apart.

Gene Expression

• Certain genes on chromosomes can be turned on/off.
• Different cells express different genes (e.g., eye cells vs. stomach cells).

Karyotypes

• Karyotype: image of all chromosomes.
• Used in medical screenings.
• Karyotypes typically taken during metaphase when chromosomes are condensed.
• 46 chromosomes in a karyotype can be confusing due to chromatid visibility.
• Chromosomes in karyotypes arranged in homologous pairs.
  • 22 pairs of autosomes (not related to sex).
  • 1 pair of sex chromosomes (X and Y).

Haploid and Diploid Cells

• 23 chromosomes from each parent.
• Gametes (sperm and egg) are haploid (1 set of chromosomes).
• Fertilized egg (zygote) is diploid (2 sets of chromosomes).
• Fathers determine biological sex (X or Y chromosome).

Importance of Chromosome Study

• Understanding genetic disorders.
• Karyotypes reveal chromosome abnormalities.
• Expanding field of genetics and potential careers, e.g., genetic counseling.

Stay curious about your studies in genetics and chromosomes!