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Master how genetic information is passed to offspring through reproduction, cell division, DNA synthesis, and inheritance patterns.
Reproduction is the biological process by which organisms produce new individuals. There are two main types: sexual reproduction (involves fusion of gametes) and asexual reproduction (single parent, no gamete fusion).
| Type | Sexual Reproduction | Asexual Reproduction |
|---|---|---|
| Parents | Two parents | One parent |
| Gametes | Fusion of gametes (fertilization) | No gametes |
| Genetic Variation | High - offspring genetically unique | None - offspring are clones |
| Speed | Slower | Faster |
| Adaptation | Better for changing environments | Better for stable environments |
Mammalian reproduction is controlled by a complex system of hormones that regulate gamete production, ovulation, pregnancy, and birth. Understanding these hormonal pathways is critical for Band 6.
Pregnancy is maintained by hormones that prevent menstruation and support fetal development. Birth is initiated by a positive feedback loop involving multiple hormones.
Explain why a woman who takes a pregnancy test 4 days after fertilization might get a negative result, even though she is pregnant.
Answer:
Pregnancy tests detect HCG hormone in blood or urine. HCG is produced by the embryo after implantation.
Timeline issue:
Conclusion: Testing too early (before implantation) gives false negative. Most tests are accurate from approximately 1 week after fertilization, or around the time of missed period (2 weeks post-fertilization).
Humans have developed technologies to manipulate reproduction in both animals and plants for agricultural productivity and to overcome infertility. Understanding both the benefits and ethical concerns is essential.
Mitosis and meiosis are two types of cell division with fundamentally different purposes and outcomes. Understanding their differences is critical for Module 5.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth and repair of body tissues | Production of gametes (sex cells) |
| Where it occurs | Somatic (body) cells | Germ cells (testes/ovaries) |
| Number of divisions | One division | Two divisions (Meiosis I and II) |
| Number of daughter cells | 2 daughter cells | 4 daughter cells |
| Chromosome number | Diploid (2n) → Diploid (2n) - Maintains chromosome number | Diploid (2n) → Haploid (n) - Halves chromosome number |
| Genetic variation | Daughter cells genetically identical to parent | Daughter cells genetically different from parent and each other |
| Crossing over | No crossing over | Crossing over occurs in Prophase I |
| Pairing of homologous chromosomes | No pairing (synapsis) | Homologous pairs form bivalents |
| Example in humans | 46 chromosomes → 46, 46 (skin, muscle, liver cells) | 46 chromosomes → 23, 23, 23, 23 (sperm and egg cells) |
DNA replication is the process by which a cell copies its DNA before cell division. This ensures each daughter cell receives a complete set of genetic information. The process is semi-conservative: each new DNA molecule consists of one original strand and one newly synthesized strand.
| Feature | Leading Strand | Lagging Strand |
|---|---|---|
| Synthesis direction | Continuous (toward fork) | Discontinuous (away from fork) |
| Template orientation | 3' to 5' | 5' to 3' |
| Number of primers | One | Many (one per Okazaki fragment) |
| Okazaki fragments | None | Yes (1000-2000 nucleotides each) |
| DNA Ligase needed? | No | Yes (to join fragments) |
Given a DNA template strand: 3'-TACGGATCG-5', what will be the sequence of the newly synthesized complementary strand? Show the direction.
Step 1: Identify template direction
Template: 3'-TACGGATCG-5'
Step 2: Remember base pairing rules
Step 3: Build complementary strand
Genetic variation is essential for evolution and adaptation. Meiosis creates genetic diversity through several mechanisms, ensuring that no two gametes (or offspring) are genetically identical.
| Source of Variation | When | What Happens | Impact |
|---|---|---|---|
| Crossing Over | Prophase I | DNA exchanged between non-sister chromatids | New allele combinations |
| Independent Assortment | Metaphase I | Random orientation of bivalents | 2^n combinations |
| Random Segregation | Anaphase I | Homologs separate randomly | Unpredictable distribution |
| Random Fertilization | Conception | Any sperm + any egg | 2^2n combinations |
The cell cycle is the series of events from one cell division to the next. Proper regulation ensures cells divide only when appropriate and prevents uncontrolled growth (cancer).
The organization and structure of DNA differs significantly between prokaryotes (bacteria) and eukaryotes (animals, plants, fungi, protists). These differences affect how genes are expressed and regulated.
| Feature | Prokaryotes (Bacteria) | Eukaryotes (Animals, Plants, Fungi) |
|---|---|---|
| DNA Shape | Circular (single chromosome) | Linear (multiple chromosomes) |
| Location | Nucleoid region (cytoplasm, not membrane-bound) | Nucleus (membrane-bound organelle) |
| Histones | No histones (DNA not wrapped around proteins) | Histones present (DNA wrapped around histone octamers → nucleosomes → chromatin) |
| Size | Small (approximately 4 million base pairs in E. coli) | Large (approximately 3 billion base pairs in humans) |
| Introns/Exons | No introns - genes are continuous coding sequences | Introns (non-coding) and Exons (coding) - genes are interrupted |
| mRNA Processing | None - mRNA used directly | Extensive: 5' cap, 3' poly-A tail, splicing (introns removed) |
| Plasmids | Yes - small circular DNA molecules with extra genes (e.g., antibiotic resistance) | Rare (yeast have plasmids, most eukaryotes don't) |
| Gene Density | High (approximately 90% of DNA codes for proteins) | Low (approximately 2% in humans codes for proteins, rest is non-coding) |
| Transcription & Translation | Coupled (happen simultaneously in cytoplasm) | Separated (transcription in nucleus, translation in cytoplasm) |
Transcription is the process of copying a gene's DNA sequence into RNA. In eukaryotes, this produces pre-mRNA, which is then processed into mature mRNA that can be translated into protein.
Given a DNA template strand: 3'-TACGGCATG-5', write the mRNA sequence that would be transcribed. Include directionality.
Step 1: Identify the template strand
Template DNA: 3'-TACGGCATG-5'
Step 2: Apply RNA base pairing rules
Step 3: Write mRNA sequence
Translation is the process by which ribosomes read mRNA and synthesize proteins. The sequence of nucleotides in mRNA determines the sequence of amino acids in the protein.
Given the mRNA sequence: 5'-AUGCCGUACUGA-3', determine: (a) the amino acid sequence of the polypeptide, and (b) how many amino acids are in the final protein.
Step 1: Identify codons
mRNA: 5'-AUG CCG UAC UGA-3'
Divide into 3-nucleotide codons starting from 5' end
Step 2: Translate each codon
AUG = Methionine (Met) - Start codon
CCG = Proline (Pro)
UAC = Tyrosine (Tyr)
UGA = STOP - Stop codon (no amino acid)
Mutations are changes in DNA sequence. They can occur spontaneously or be induced by environmental factors. Understanding mutation types and their effects is crucial for genetics and evolution.
| Mutation Type | Change in DNA | Effect on Protein | Severity |
|---|---|---|---|
| Silent | Nucleotide substitution | Same amino acid | None |
| Missense | Nucleotide substitution | Different amino acid | Variable |
| Nonsense | Creates stop codon | Premature termination | Usually severe |
| Frameshift (Insertion/Deletion) | Nucleotides added or removed | All downstream amino acids changed | Usually severe |
Autosomal inheritance follows Mendel's laws and involves genes located on autosomes (non-sex chromosomes). Understanding dominance relationships and using Punnett squares are fundamental skills for Module 6.
In pea plants, purple flower (P) is dominant to white flower (p). A purple-flowered plant is crossed with a white-flowered plant, and the offspring are 50% purple and 50% white. What is the genotype of the purple-flowered parent?
Step 1: Identify what we know
Step 2: Test both possibilities
If purple parent is PP:
PP × pp → all Pp (100% purple) ✗ Does not match
If purple parent is Pp:
Pp × pp → 50% Pp (purple), 50% pp (white) ✓ Matches!
Sex-linked traits are controlled by genes located on sex chromosomes (X or Y). Most sex-linked traits in humans are X-linked because the Y chromosome carries few genes.
Not all traits follow simple Mendelian patterns. Several non-Mendelian inheritance patterns produce different phenotypic ratios and outcomes.
| Pattern | Heterozygote Phenotype | F2 Ratio (if applicable) | Example |
|---|---|---|---|
| Complete Dominance | Same as dominant homozygote | 3:1 | Pea plant height |
| Incomplete Dominance | Intermediate (blend) | 1:2:1 | Snapdragon color |
| Codominance | Both expressed | 1:2:1 | ABO blood type |
| Multiple Alleles | Varies | Varies | ABO blood type |
| Polygenic | Continuous variation | Bell curve | Human height |
Pedigrees are family trees that show inheritance patterns of traits across generations. Analyzing pedigrees helps determine whether traits are dominant, recessive, autosomal, or sex-linked.
| Pattern | Skip Generations? | Sex Distribution | Key Clue |
|---|---|---|---|
| Autosomal Recessive | Yes | Equal M/F | Unaffected parents → affected child |
| Autosomal Dominant | No | Equal M/F | Appears every generation |
| X-Linked Recessive | Yes | Mostly males | Carrier mother → affected sons |