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Lecture 2 - Foundations for Evolution: Cells & DNA

Why study genetics?

  • Genetics provides an understanding of how traits and biological information are inherited and expressed in organisms.

  • The study connects cellular processes to organismal life and evolution.

Cells: basic units of life

  • Somatic cells: form parts of the body (e.g., organs, tissues, etc.).

  • Gametes: Sperm and ova. Only genetic changes in gametes are inherited!

  • Cells contain organelles:

    • Nucleus

    • Mitochondria

    • Other organelles involved in cellular processes, e.g., protein synthesis:

    • Endoplasmic reticulum

    • Ribosomes

  • Deoxyribonucleic acid (DNA):

    • The molecule that transmits genetic information

    • Two types: Nuclear DNA (nDNA) and M mitochondrial DNA (mtDNA)

  • Properties of DNA that make it suitable for inheritance:

    1. Replication

    2. Biological information: can code for proteins (structural genes) and can regulate development (regulatory genes)

    3. Mutation: important for variation!

The DNA Molecule

  • Molecular structure discovered in the 1950s by:

    • James Watson

    • Francis Crick

    • Maurice Wilkins

    • Rosalind Franklin

  • DNA structure explains how it is capable of self-replication.

DNA Structure

  • Two strands of nucleotides in the shape of a double helix.

  • Each nucleotide has three components:

    • A nitrogenous base (A, T, G, C)

    • A sugar molecule

    • A phosphate group

  • The DNA strands are complementary:

    • Adenine -- Thymine

    • Guanine -- Cytosine

  • Complementarity allows DNA to make exact copies during replication.

DNA Replication

  1. Enzymes unwind the DNA molecule.

  2. The two nucleotide chains serve as templates for the formation of new strands.

  3. Unattached nucleotides pair with the appropriate complementary nucleotide.

What are Chromosomes?

  • The physical structures where genes are located.

  • Humans have 46 (23 pairs).

  • Autosomes: non-sex chromosomes (homologous 1-22).

  • Sex chromosomes: X & Y.

  • XY = typical male configuration; XX = typical female configuration.

  • Gametes contain only 23 chromosomes (one copy of each pair).

Chromosome Structure

  • Chromosomes form during cell division.

  • DNA coils tightly (two strands joined at the centromere).

  • One strand of a chromosome is an exact copy of the other.

Cell Division: Mitosis vs. Meiosis

  • Somatic cells:

    • Purpose: growth, tissue repair, etc.

    • Goal: two identical (diploid) daughter cells.

  • Sex cells:

    • Purpose: gamete formation.

    • Goal: four haploid daughter cells.

Mitosis

  • A single somatic cell divides to produce two identical (diploid) daughter cells.

Meiosis

  • Two stages of cell division:

    • Meiosis I: reduction division

    • Meiosis II: similar to mitosis

  • Result: four daughter cells (gametes) with one copy of each chromosome.

  • Maintains constant chromosome number.

DNA Replicates, Chromosome Pairs, and Separation in Meiosis

  • In meiosis:

    • DNA replicates prior to division.

    • Chromosome pairs align and separate.

    • Chromosome strands separate.

  • Germ cells form gametes: 23 chromosomes per gamete.

  • Zygote formation: Egg and sperm unite to form a zygote with 46 chromosomes.

Evolutionary Significance of Meiosis

  • Allows half the genes to come from mom and half from dad.

  • Crossing over and recombination change the genetic composition of the chromosomes.

Evolutionary Significance of Meiosis (continued)

  • Result:

    • Genetic material from each parent is reshuffled in new ways.

    • Daughter cells are different from the parent cell and from each other.

    • Important source of individual variation!

Errors in meiosis

  • Non-disjunction: failure of chromosomes to separate properly.

  • Recombination errors: can cause chromosome structural rearrangements.

  • Result: Gametes with chromosome anomalies; if involved in fertilization, will produce a zygote with chromosome anomaly.

What else does DNA do?

  • DNA directs cells to make proteins.

  • Proteins have many functions:

    • Structural molecules (e.g., collagen)

    • Enzymes (e.g., lactase)

    • Hormones (e.g., insulin)

    • Transportation

    • Antibodies

    • Hemoglobin

    • Regulatory proteins

Proteins

  • Composed of amino acids (20).

  • Different according to the number & sequence of amino acids.

  • Normal functioning of a protein depends on the correct sequence.

  • Example: Hemoglobin

Protein Synthesis

  • Occurs in the cytoplasm at the ribosomes.

  • Ribosomes are where genetic code is “read”.

Transcription (DNA code is copied)

  • A portion of DNA unwinds and serves as a template for the formation of a mRNA (messenger RNA) strand.

  • Differences from DNA:

    • Single stranded

    • Uracil (U) replaces Thymine (T)

    • Uses ribose

  • ext{mRNA} carries the genetic message from the nucleus to a ribosome.

  • The message is translated by transfer RNA (tRNA).

  • tRNA assembles a protein using mRNA as a template.

  • Each codon specifies an amino acid.

Translation (genetic instructions decoded)

  • The process by which mRNA codons are read by tRNA to build a protein.

So what exactly is a gene?

  • The entire sequence of DNA bases responsible for the synthesis of a protein.