Reproductive Strategies and Cell Division
Reproductive Strategies
Asexual Reproduction 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 1 parent
- All organisms are able to reproduce passing on 100% of their genetic material.
- One organism may begin an entire population.
- An exact replica of 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 2 parents
- Only 50 percent of its genetic material to each offspring.
- Two organisms are needed to begin an entire population.
- Offspring are not exact replica of parents and allows for more diversity among species.
- Higher probability of mutation and survival in periods of change or instability.
- In general, slower population growth
Most important advantage of sexual reproduction – variation produced by continual recombination of sex cells to create unique individuals.
Species: Definition
Biological Species Concept – Group of organisms that interbreed exclusively and produce fertile offspring. – Problem with concept: – Many unicellular organisms do not reproduce sexually – Many species of related plants can hybridize with one another in nature and produce fertile offspring. • Biological species concept” is an idealized concept with limited application.
Alternate & Better Definition of a Species Separately “evolving” lineage from single “unique” gene pool. / Observing Inheritance • Character attribute of members of a species that allows comparison with other members of 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) **“Character” & “Trait” often used synonymously.
NUCLEUS & CELL CYCLE
• Cell’s control center • Usually visible • Surrounded by nuclear envelope with – double membrane – nuclear pores allow passage of substances between cytoplasm & nucleus • Contains – DNA in alternating forms- chromatin & chromosomes – Nucleolus
NUCLEOLUS • Darker staining, spherical bodies within nucleus • Not membrane-bound • Composed of clusters of DNA, RNA, & protein • Site of ribosome assembly
Cell Cycle Stages (Human Cells)
1. Interphase – Normal cell growth / Not dividing – 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” • 1 division cycle • 2 identical daughter cells.
Meosis • sexual cell division • occurs in “germ cells” • 2 division cycles • 4 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 of 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 formed from single DNA molecule & contains many genes • DNA molecule in chromosome state is highly condensed • Chromosomes ensure each daughter cell receives 1 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 between plant and animal cytokinesis 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.
Chromosomes– Condensed (packed), Chromatin– Not condensed (unpacked)
Human Chromosomes • Full complement = 46 chromosomes – somatic cells have 46 chromosomes (23 pairs of homologues, diploit condition= 2n) , i.e. full complement. – sex cells have 23 chromosomes (haploit condition = n), i.e. half full complement.
Human Chromosomes•Organized in 23 pairs (2 x 23 = 46) – one of each pair originally came from ♂ parent while other originally came from ♀ parent – the two chromosomes of each pair referred to as homologues (short for homologous chromosomes) – homologues grouped as as 22 pairs of autosomes (morphologically similar) 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 • Complete set of chromosomes is called karyotype – individuals of same species have same number of chromosomes • Karyotype also refers to organized arrangement of chromosomes by number: e.g. chromosome 21 / type: e.g. autosomes/ size: e.g. 30µ 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 (G bands) and G-C (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.
Brief explanation of spermatogenesis: the process of the production of sperms from the immature male germ cells.
Oogrnesis: 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.