Chapter 11 and 51 Flashcards
sexual life cycle
- made up of meiosis and fertilization
- diploid cells
- somatic cells of adults have 2 sets of chromosomes
- haploid cells
- gametes have only 1 set of chromosomes
features of meiosis
meiosis includes 2 rounds of division
1. meiosis I and meiosis II
2. each has prophase, metaphase, anaphase, and telophase
synapsis
- during early prophase I
- homologous chromosomes become closely associated
- includes formation of synaptonemal complexes
crossing over
- genetic recombination between nonsister chromatids
- Chiasmata: site of crossing over
- contact maintained until anaphase I
- humans typically have 2 or 3 crossing over per chromosome
unique features of meiosis
- 1st meiotic divsion is termed the “reduction division”
- NO DNA replication between meiotic divisions
- 2nd meiotic division does not further reduce the number of chromosomes
the process of meiosis
- meiotic cells have an interphase period that is similar to mitosis with G1, S, and G2 phases
- after interphase, germ-line cells enter meiosis I
meiosis prophase I
- chromosomes coil tighter and become visible, nuclear envelope breaks down, spindle forms
- each chromosome composed of 2 sister chromatids
- Synapsis:
- homologues become closely associated
- crossing over occurs between nonsister chromatids
- remain attached at chiasmata (chiasmata move to the end of the chromosome arm before metaphase I)
meiosis metaphase I
- terminal chiasmata hold homologues together after crossing over
- microtubules attach to each homologue
- homologues are aligned at the metaphase plate side-by-side
- orientation of each pair of homologues on the spindle is random-independent assortment
meiosis anaphase I
- microtubules of the spindle shorten (chiasmata break)
- homologues are separated from each other and move to opposite poles
- each pole has a complete haploid set of chromosomes consisting of one memeber of each homologous pair
meiosis telophase I
- nuclear envelope re-forms around daugther nucleus
- sister chromatids are no longer identical because crossing over (prophase I)
- cytokinesis may/may not occur after telophase I
meiosis II
**Resembles a mitotic division
**
* Prophase II: nuclear envelopes dissolve and new spindle apparatus forms
* Metaphase II: chromosomes align on metaphase plate
* Anaphase II: sister chromatids are separated from each other
* Telophase II: nuclear envelope re-forms around 4 sets of daughter chromosomes; cytokinesis follows
final results of meiosis
- 4 cells containing haploid sets of chromosomes
- in animals, develop directly inot gametes
- in plants, fungi, and protists, divide mitotically
(produce greater # of gametes, adults with varying # of gametes)
errors in meiosis
nondisjunction: failure of chromosomes to move to opposite poles during either meiotic division
aneuploid gametes: gametes with missing/extra chromosomes
(^^ the most common cause of spontaneous abortion) (75% of embryos have abnormal # of chromosomes = miscarriage)
meiosis vs mitosis
meiosis is characterized by 4 features:
1. synapis and crossing over
2. sister chromatids remain joined at their centromeres throughtout meiosis I
3. kinetochores of sister chromatids attach to the same pole in meiosis I
4. DNA replication is suppressed between meiosis I and II
asexual reproduction
- genetically identical cells are produced from a single parent cell through mitosis
- single-celled organisms utilize fission
- cnidarians reproduce by budding
parthenogenesis
- females produce offspring from unfertilized eggs
- common in arthropods
- known in lizards
Hermaphroditism
- one individual has both testes and ovaries
- may be sequential: change sex
- protogyny: female to male change
- protandry: make to female change
amphibian reproduction
- in most species fertilization is external
- eggs of most species to develop in water
Internal fertilization has led to three strategies for development
1. Oviparity
Fertilized eggs are deposited outside mother’s body to complete their development
2. Ovoviviparity
Fertilized eggs are kept within mother to complete development, young obtain food from egg yolk
3. Viviparity
Young develop within mother and obtain nourishment from her blood
bird reproduction
(oviparity)
- Internal fertilization but males lack a penis
- Amniotic egg
Extraembryonic membranes
Adaptation for life on land
**- All birds practice internal fertilization
**
- Hard calcareous shell on egg
- Birds are endotherms
- Must incubate eggs to keep them warm
reproduction in mammals
monotremes
* lay eggs and incubate eggs in nest
* lack nipples, milk seeps for skin when licked
marsupials
* give birth to incomplete developed fetuses, which mature in the mothers pouch
placentals
* retain young in uterus for long period of development
* nourished by placenta from mother’s blood
male reproductive system
- seminiferous tubules: sites of sperm production
- Leydig cells produce testosterone
converts indifferent external genitalia into penis & scrotum
begins being secreted 9-10 weeks after conception
Gamete production- spermatogonium (germ cells) divides by mitosis to produce two diploid cells
- total of 4 haploid spermatids produced
female reproduction system
at puberty, granulosa cells secrete estrogen
- triggers menarche, 1st menstrual cycle
- stimulates secondary sexual characteristics
at birth, ovaries contain 1 millon follicles
during each menstrual cycle, a group of follicles are used to reinitiate developement
