Lecture Notes: DNA and Chromosomes

Introduction

• The focus of this lecture is on DNA and chromosomes.
• Comparison between DNA in eukaryotic and prokaryotic cells.
• Lecture includes definitions and description questions to reinforce learning.

Key Definitions

Gene

• A gene is a short section of DNA.
• Codes for a polypeptide and a functional RNA.
• More precise than GCSE definition, which is too vague.

Polypeptide Chains

• Primary structure of a protein.
• Processed in the Golgi apparatus to form functional proteins.

Chromosome

• Made up of tightly coiled DNA.
• Includes a gene as a small section of DNA.

Locus

• Refers to the location of a gene on a chromosome.
• Identical location for the same species.

Allele

• Different form or version of the same gene.
• Results in variation in proteins produced.

Chromosomes in Detail

• DNA storage form in eukaryotic cells.
• Located in the nucleus.
• Humans have 23 pairs of chromosomes (46 total).

Chromosome Appearance

• Stick-like structure when not dividing.
• X-structure after DNA replication during cell division (mitosis/meiosis).

Homologous Pairs

• Chromosome pairs with identical genes, possibly different alleles.
• Formed during fertilization; sperm and egg contribute to 23 pairs.
• Illustrated in a human karyotype.

Sex Chromosomes

• 23rd pair determines biological sex (e.g., XY for males).

DNA Storage

Eukaryotic Cells

• DNA stored as linear chromosomes in the nucleus.
• Wrapped around histone proteins to form nucleosomes.

Prokaryotic Cells

• DNA is circular and much shorter.
• Not associated with histone proteins.
• Free in the cytoplasm, no nucleus.

Organelles with DNA

Mitochondria and Chloroplasts

• Contain their own DNA for essential reactions.
• DNA similar to prokaryotic DNA: short, circular, not histone-bound.

Math Skills Practice

• Calculating DNA length per chromosome.
• Conversion between meters and millimeters.
• Calculating base pairs per chromosome in standard form.

Summary

• Genes code for polypeptides and functional RNA.
• Alleles are variations of genes.
• Humans have homologous pairs of chromosomes.
• Eukaryotic DNA is linear and histone-bound; prokaryotic DNA is circular and not histone-bound.
• Mitochondrial and chloroplast DNA resembles prokaryotic DNA.

Additional Resources

• Practice questions and further resources available online.

Note

• Regular videos and practice questions available for further learning.