Cell Division and Species Variation: Genetics Fundamentals
Reproductive Strategies
Reproductive Strategies
- Asexual Reproduction: A single parent organism produces offspring by making genetically identical copies of itself.
- Sexual Reproduction: Two parent organisms combine genetic material to produce similar but genetically unique offspring.
Asexual Reproduction
- Requires less energy.
- Needs only one parent.
- All organisms are able to reproduce, passing on 100% of their genetic material.
- One organism may begin an entire population.
- Offspring are exact replicas of the parent (clones).
- Less likely to become extinct in stable environments.
- Rapid population growth.
Sexual Reproduction
- Requires much energy (locating mates, exchanging genetic material, overproduction of sperm and males).
- Needs two parents.
- Only passes on 50% of its genetic material to each offspring.
- Two organisms are needed to begin an entire population.
- Offspring are not exact replicas of parents, allowing for more diversity among species.
- Higher probability of mutation and survival in periods of change or instability.
- In general, slower population growth.
The most important advantage of sexual reproduction is the variation produced by continual recombination of sex cells to create unique individuals.
Species Definition
Species: Definition
- Biological Species Concept: A group of organisms that interbreed exclusively and produce fertile offspring.
Problems with the Biological Species Concept:
- Many unicellular organisms do not reproduce sexually.
- Many species of related plants can hybridize with one another in nature and produce fertile offspring.
The “Biological Species Concept” is an idealized concept with limited application.
Alternate & Better Definition of a Species: Separately “evolving” lineage from a single “unique” gene pool.
Observing Inheritance
• Character: Attribute of members of a species that allows comparison with other members of the same or different species.
• Trait: Observable property or variant of a character.
• Variation: Similarities & differences in characters and their traits that may be inherited or acquired (environmentally determined).
Note: “Character” & “Trait” are often used synonymously.
Nucleus & Cell Cycle
Nucleus & Cell Cycle
- Cell’s control center.
- Usually visible.
- Surrounded by a nuclear envelope with:
- Double membrane.
- Nuclear pores allowing passage of substances between cytoplasm & nucleus.
- Contains DNA in alternating forms: chromatin & chromosomes.
- Contains the Nucleolus.
Nucleolus
- Darker staining, spherical bodies within the nucleus.
- Not membrane-bound.
- Composed of clusters of DNA, RNA, & protein.
- Site of ribosome assembly.
Cell Cycle Stages (Human Cells)
- Interphase: Normal cell growth; not dividing. It has three stages:
- G1 (“Gap” phase): Growth and metabolism based on protein replication.
- S (“Synthesis” phase): DNA replication.
- G2 (“Gap” phase): Growth and metabolism, including proteins for cell division.
Mitosis
- Asexual cell division.
- Occurs in “somatic cells.”
- One division cycle.
- Produces two identical daughter cells.
Meiosis
- Sexual cell division.
- Occurs in “germ cells.”
- Two division cycles.
- Produces four similar but different (unique) daughter cells.
Chromatin
- Uncondensed DNA seen during interphase.
- Composed of DNA & coiling proteins (histones).
- Looks like “beads on a string.”
- DNA is available for interphase cell activity:
- For transcription (DNA copying step for synthesizing proteins).
- For replication of DNA (copying step prior to cell division).
- For condensation of DNA (packaging DNA into chromosomes).
Chromosomes
- Each chromosome is formed from a single DNA molecule and contains many genes.
- DNA molecule in chromosome state is highly condensed.
- Chromosomes ensure each daughter cell receives one complete copy of genetic information.
Cytokinesis
Definition: The cytoplasmic division of a cell at the end of mitosis or meiosis, bringing about the separation into two daughter cells.
Differences between animal/plant cells: The key difference is that in plant cells, cytokinesis occurs through the formation of a cell plate, while in animal cells, cytokinesis occurs through the formation of a cleavage furrow.
Chromosome States
Chromosomes – Condensed (packed); Chromatin – Not condensed (unpacked).
Human Chromosomes
- Full complement = 46 chromosomes.
- Somatic cells have 46 chromosomes (23 pairs of homologues, diploid condition = 2n), i.e., full complement.
- Sex cells have 23 chromosomes (haploid condition = n), i.e., half full complement.
Human Chromosomes Organization
- Organized in 23 pairs (2 x 23 = 46).
- One of each pair originally came from the male parent (♂) while the other originally came from the female parent (♀).
- The two chromosomes of each pair are referred to as homologues (short for homologous chromosomes).
- Homologues are grouped as 22 pairs of autosomes (morphologically similar) and 1 pair of sex chromosomes (morphologically different).
Human Karyotype
- Full complement = 46 chromosomes.
- Somatic cells have 23 pairs of homologues – called diploid condition = 2n.
- Sex cells have 23 chromosomes – called haploid condition = n.
Karyotype Details
- Complete set of chromosomes is called a karyotype; individuals of the same species have the same number of chromosomes.
- Karyotype also refers to the organized arrangement of chromosomes by number (e.g., chromosome 21), type (e.g., autosomes), size (e.g., 30µ), and shape (e.g., metacentric).
Chromosome Staining
- Chromosomes are stained with specific dyes: A-T (G bands) and G-C (R bands).
- When stained, chromosomes have a banded structure that unambiguously identifies each one.
Stained (Banded) Chromosomes
Chromosomes are stained with specific dyes (A-T for G bands and G-C for R bands). When stained, chromosomes have a banded structure that identifies each one.
Gametogenesis
Definition: Gametogenesis is the process by which gametes, or germ cells, are produced in an organism.
- Spermatogenesis (Brief explanation): The process of the production of sperms from the immature male germ cells.
- Oogenesis: Oogenesis is the process by which the female gametes, or ova, are created. The female gamete is called an ovum. Sometimes people will refer to female gametes as eggs, but the term egg can include more than one stage of development, and the definition of an egg also changes depending on the type of organism.