Lecture 4 - Cellular Mechanisms and Genetic Transmission
Evolutionary mechanics and moving from history to mechanisms
The instructor frames today as a continuation of thoughts on evolution, shifting from the historical trajectory (Darwin and natural selection) to the mechanics of how evolution works at the cellular/genetic level.
There is an assignment component: occasionally students respond to readings; next time (Friday) there is an assignment on Canvas tied to the reading on genetic and environmental influence on human phenotypes.
Outside reading for Friday: Matt Ridley’s Geno, a non-academic science writer’s take on how much is genetic vs environmental, and a prompt will be posted on Canvas for a response before Friday’s class.
Ridley’s Geno is described as an older but interesting piece that questions the assumption that traits are purely genetic and beyond environmental influence.
The instructor emphasizes cross-connection: we’ll move from cell/DNA details to how those processes produce physical traits and variation.
Reading folders are on Canvas under Files > Supplemental Readings (PDFs).
The topic for today includes Darwin’s natural selection and the details Darwin didn’t know, notably the cellular and genetic basis for variation and inheritance.
The course aims to bring anthropology and biology together: you don’t need to be a geneticist to understand the basics of evolution and variation; the goal is a common baseline.
Contextual note: the instructor grew up in the Midwest and spent time in Arizona; personal anecdotes about climate and moving are used to illustrate experience, not core content.
The class will touch on how cells and DNA work, the transmission of information, and how variation arises and is inherited, with a focus on gametes (sex cells) vs somatic (body) cells.
Cells: somatic vs. gametes; basic cell organization
There are two broad categories of cells in the body:
Somatic cells: structural components like skin, hair, heart, liver cells; these contribute to tissues and organs.
Gametes (sex cells): sperm and egg, produced to form a new individual.
The genetic material within cells is organized into organelles with specific roles; some key organelles discussed:
Nucleus: contains nuclear DNA (nDNA).
Mitochondria: contain mitochondrial DNA (mtDNA) and are the site of energy production; mitochondria have their own DNA and are inherited maternally.
Endoplasmic reticulum (ER) with ribosomes: site of protein synthesis.
Important concepts:
Nuclear DNA is unique to you: half from your mother, half from your father; mtDNA is inherited from the mother.
Changes in gametes (not somatic cells) are the ones that can be passed to the next generation.
Lamarck vs Darwin note: Lamarck argued for inheritance of acquired characteristics; Darwin proposed natural selection acting on heritable variation—modern understanding ties variation to genetic changes in gametes.
Quick recap of terminology:
Nucleus contains nuclear DNA; mitochondria contain their own DNA (mtDNA).
mtDNA is inherited maternally and can be used to trace maternal lineages; mtDNA changes over time but is often inherited more or less intact across generations.
DNA codes for proteins and regulatory elements; it can replicate itself with high fidelity, enabling growth, development, and repair.
DNA: structure, bases, and replication
DNA as the genetic blueprint:
DNA codes for biological information; some genes code for proteins, others regulate gene expression (switching on/off) and development.
DNA can replicate itself accurately, enabling cell division and inheritance.
Mutations introduce new genetic variation, which can be beneficial, neutral, or deleterious depending on context.
Key structural features:
DNA forms a double helix with rungs made of bases and a sugar-phosphate backbone.
The four DNA bases are: A (adenine), T (thymine), G (guanine), C (cytosine).
Base pairing is highly specific: A-T and G-C, enabling exact copies during replication.
Each unit along the chain is a nucleotide: base + deoxyribose sugar + phosphate group.
The name DNA comes from DeoxyriboNucleic Acid.
Why replication matters:
When cells divide, DNA must be copied to produce identical daughter cells for growth, development, and tissue repair.
Enzymes unwind DNA, breaking hydrogen bonds between bases and creating templates for new strands.
New nucleotides pair with exposed bases (A with T, G with C) to form two complete double helices.
Chromosomes arise when DNA condenses for cell division:
In humans, there are 46 chromosomes, arranged in 23 pairs.
The first 22 pairs are autosomes; the 23rd pair are sex chromosomes (XX in females, XY in males).
Chromosomes form from DNA when a cell is about to divide; otherwise, DNA exists as loose chromatin.
A karyotype visualizes the 46 chromosomes to determine sex and chromosomal composition.
Mitochondrial DNA vs nuclear DNA; inheritance patterns
Nuclear DNA (nDNA): located in the nucleus; inherited from both parents (half from each): maternal and paternal contributions.
Mitochondrial DNA (mtDNA): located in mitochondria; inherited maternally; used to trace maternal lineages; mtDNA is present in many copies per cell, which can make ancient DNA easier to recover.
Important implications:
mtDNA provides a trace of maternal ancestry and can be extracted from old samples more readily due to high copy number.
mtDNA only tells part of the story of genetic history since it reflects the maternal line alone.
The discovery and structure of DNA (historical context):
The structure was elucidated by Watson and Crick, with critical contributions from Maurice Wilkins and Rosalind Franklin (X-ray crystallography data).
Franklin’s data and the teamwork environment led to the double-helix model, though she did not receive the Nobel Prize due to her death before the prize.
Chromosomes, karyotypes, and inheritance patterns
Chromosome organization:
Humans have 46 chromosomes organized into 23 pairs: 22 autosomes + 1 sex chromosome pair.
Sex chromosomes: XX in typical females; XY in typical males.
Gametes and haploidy:
Gametes (sperm and egg) are haploid, containing one copy of each chromosome (22 autosomes plus one sex chromosome, totaling 23 chromosomes).
Fertilization restores the diploid number in the zygote (two copies of each chromosome).
Chromosome structure and condensation:
DNA in non-dividing cells is chromatin; prior to division, DNA condenses into chromosomes with a centromere and sister chromatids.
The concept of a genome size:
The human genome comprises roughly 3 imes 10^9 base pairs.
Mitosis vs meiosis: how cells divide and produce variation
Mitosis (somatic cell division):
Goal: produce two identical daughter cells (diploid) to support growth, tissue maintenance, and repair.
Start with a diploid cell containing 46 chromosomes; DNA replicates to form sister chromatids; the cell divides to yield two daughter cells with 46 chromosomes each (single-stranded after dividing).
Meiosis (gamete formation):
Goal: produce gametes with half the genetic content (haploid) to enable random combination of parental genes.
Process results in four haploid gametes, each with 23 chromosomes (one copy of each autosome and a sex chromosome).
Meiosis includes two rounds of division: meiosis I and meiosis II.
Key difference:
Mitosis yields two genetically identical diploid cells; meiosis yields four genetically distinct haploid gametes.
Genetic variation: crossing over, recombination, and sources of variation
Crossing over and recombination (Meiosis I):
During the first division, chromosome pairs come together and exchange genetic material (recombination).
This reshuffling creates new chromosome variants, increasing genetic diversity in offspring.
Consequences for variation:
Offspring are not simply copies of the parents; recombination and assortment generate trillions of possible genetic combinations.
This variation underpins evolutionary change and the range of traits (e.g., beak size in Darwin’s finches).
Nondisjunction and aneuploidy:
Errors can occur when chromosomes fail to separate properly during meiosis, leading to aneuploid gametes (too many or too few chromosomes).
If such a gamete participates in fertilization, the offspring may have an abnormal chromosome number.
Example: Down syndrome is typically trisomy of chromosome 21 (trisomy 21).
Aneuploid conditions and recombination errors:
Structural rearrangements can occur during recombination, potentially creating abnormal chromosome configurations.
Protein synthesis: transcription and translation
Proteins as functional molecules:
Structural proteins (e.g., collagen) contribute to tissue strength and elasticity; regulatory proteins control processes like enzyme activity and hormone signaling; enzymes themselves are proteins.
Examples: lactase (enzyme that digests lactose), insulin (hormone that regulates glucose), hemoglobin (protein that transports oxygen).
The role of DNA in protein synthesis:
DNA provides the coding sequence for proteins; the gene sequence determines the amino acid sequence of the protein, influencing structure and function.
The central dogma steps:
Transcription: DNA is transcribed into messenger RNA (mRNA). Key features:
mRNA is single-stranded; uracil (U) substitutes for thymine (T).
The information is copied from a DNA template to an mRNA molecule that can travel out of the nucleus.
Translation: mRNA is read by the ribosome, with transfer RNA (tRNA) bringing amino acids to form a polypeptide chain.
Codons are read in groups of three nucleotides (three-base sequences) on the mRNA.
Example codons: the start codon is typically AUG; amino acids are specified by various codon sequences.
The genetic code is degenerate (redundant): more codons than amino acids, so some amino acids are encoded by multiple codons.
The ribosome and the endoplasmic reticulum:
The ribosome is the molecular machine that translates mRNA into a protein chain.
The endoplasmic reticulum (ER) provides a surface for protein synthesis (rough ER with ribosomes).
The journey of genetic information:
DNA → transcription → mRNA → translation → protein.
Genes, noncoding regions, and regulatory elements
A gene is more than just a sequence coding for a protein:
Regions exist that are noncoding and do not become part of the final protein.
Some noncoding regions regulate when and where genes are turned on or off, or influence cell type development.
Introns are noncoding segments that may be edited out during processing; exons code for the final protein.
Regulatory genes and development:
Some regulatory genes govern body plan and development (e.g., HOX genes determine segmental identity along the spine, such as the number and arrangement of vertebrae: 7 cervical, 12 thoracic, 5 lumbar typically).
Genes and environment:
Gene expression can be influenced by environmental factors; the relationship between genes and environment shapes phenotypes.
Protein function and illustrative examples
Structural proteins:
Collagen is the most abundant structural protein in the body and contributes to bone and connective tissue strength with a degree of elasticity.
Enzymes and regulation:
Enzymes are proteins that catalyze biochemical reactions; lactase is one example that digests lactose (the sugar in milk).
Lactase production typically declines with age in mammals; some human populations retain lactase production into adulthood, an adaptive variation.
Transport and regulation:
Hemoglobin is a protein composed of four amino acid chains that transports oxygen in the blood.
Protein composition and sequence:
Proteins are polymers of amino acids whose function depends on the length and sequence of the amino acid chain.
The discovery of DNA structure and implications for replication
DNA structure and replication underlie inheritance:
The double helix format explains how DNA can replicate with high fidelity, enabling exact copies for cell division.
The complementary base-pairing rules (A-T, G-C) ensure accurate copying.
Key historical figures:
James Watson, Francis Crick; Maurice Wilkins; Rosalind Franklin (X-ray crystallography) contributed data that aided the discovery.
Franklin did not receive the Nobel Prize due to her death before the prize was awarded.
Ancient DNA, Neanderthals, and modern human origins (Svante Pääbo emphasis)
Ancient DNA (aDNA) work:
Scientists obtain DNA from ancient remains (e.g., Neanderthals) to reconstruct genomes and understand human evolution.
Svante Pääbo (Max Planck Institute) pioneered reconstructing the Neanderthal genome, enabling comparisons with modern humans.
Neanderthal genome and introgression:
The work enabled comparisons between Neanderthal DNA and modern human genomes.
The video notes that modern humans outside Africa carry Neanderthal DNA, with the speaker mentioning a figure like 2.5\% Neanderthal ancestry in some populations.
The effort has progressed from partial drafts (e.g., ~55% of the genome discussed in earlier work) to more complete sequences.
Implications for human origins and migration:
The data support an origin of anatomically modern humans in Africa around 100{,}000 years ago, followed by migrations that met other hominin groups (e.g., Neanderthals) in parts of Eurasia.
Outside Africa, genetic variation is distributed differently, with Africa showing greater genetic diversity than populations elsewhere (the variation outside Africa is described as having fewer variants in some segments of the talk).
The interplay between modern humans and Neanderthals is used to understand admixture events and population history.
Practical notes on aDNA work:
Extracting DNA from ancient remains requires careful methods to avoid contamination; the process involves sequencing many DNA molecules to reconstruct genomes.
Emphasis of the talk:
The video presents DNA at a basic level and then demonstrates how the genetic processes underpin broader evolutionary questions and population relationships.
Connections to readings, assignments, and class context
Connections to outside readings: the Matt Ridley Geno piece (outside reading) discussing gene-environment interactions in traits such as intelligence; the instructor invites reflection on how much of trait variation is genetic vs environmental.
Course logistics mentioned:
Readings are available on Canvas (Files > Supplemental Readings).
Friday’s class includes discussion of genetic/environmental influence on human phenotypes and a reaction to the Ridley reading.
Ethical and philosophical implications (implicit in the discussion):
Understanding the genetic basis of traits and the role of environment has implications for how we view autonomy, responsibility, and the interpretation of “nature vs nurture.”
The use of ancient DNA raises ethical questions about how genetic information from extinct populations is interpreted and applied.
Quick reference: key terms, numbers, and concepts to memorize
Key numeric anchors:
Humans have 46 chromosomes in most somatic cells; 23 pairs total; 22 autosomes + sex chromosomes.
Sex chromosome configurations: XX (female) vs XY (male).
Gametes are haploid with 23 chromosomes; zygote restores the diploid number (46).
Cervical, thoracic, lumbar vertebrae typical counts: 7 cervical, 12 thoracic, 5 lumbar (regulated by HOX genes).
Start codon: AUG; codons read in triplets; genetic code shows redundancy (more codons than amino acids).
Genome size: roughly 3\times 10^9 base pairs.
Mitochondrial DNA: maternally inherited.
Neanderthal ancestry in some modern humans outside Africa: about 2.5\% in some populations (as described in the talk).
Modern humans originated in Africa around 100{,}000 years ago.
The human genome contains roughly billions of nucleotides; comparisons show variation within Africa is greater than that outside Africa.
Core processes to know:
DNA replication relies on complementary base-pairing (A-T, G-C) and the unwinding of the double helix by enzymes.
Mitosis yields two identical diploid cells; Meiosis yields four haploid gametes with half the genetic content.
Crossing over during meiosis I increases genetic variation via recombination.
Non-disjunction can produce aneuploidies (e.g., trisomy 21 in Down syndrome).
Protein synthesis steps: transcription (DNA to mRNA) and translation (mRNA to protein) with ribosomes and tRNA; the role of the codon structure and the redundancy of the genetic code.
Real-world relevance:
Understanding cellular and molecular mechanisms clarifies how genetic variation arises and is transmitted, which underpins anthropology’s interest in human evolution and population history.
Ancient DNA research (e.g., Neanderthals) reshapes views on human origins and interactions with other hominins.