lecture 11: sexual reproduction Flashcards
What is asexual reproduction?
Produce clones: genetically identical individuals
- Several mechanisms for asexual reproduction exist
- Those mechanisms involve mitotic cell division in both uni-multicellular eukaryotes & binary fission in prokaryotes
- Produce offspring form unfertilized eggs
What is selfing (self-fertilization)?
Sexual reproduction where sperm and egg are produced by same individual
What is sex, where is it observed, and what does it require?
- Sexual reproduction: fusion of two cells and genetic information of each to create a new organism
- Observed in eukaryotes (not in prokaryotes) that evolved 1 BYA
- Requires meiosis in the lifecycle of the organism
Sexual reproduction requires what type of cell division?
Meiosis
Asexual reproduction requires what type of cell division?
Mitosis
What is a life cycle?
Steps of development for one generation —> period where an organisms is formed, develops, and reproduces an offspring of its own
Mitosis produces…
genetically identical cells (same ploidy)
Meiosis produces…
genetically unique cells with half o f the number of chromosomes
What is a homologous pair of chromosomes?
1 from mother and 1 from father: each carries genes for the same traits
What is a gene?
Segment of DNA located in a specific site on a specific chromosome that contain information for producing a particular polypeptide
What are alleles?
Different versions of a gene —> pair of alleles = 1 form mother and other from father
What are the 5 main steps of a meiotic cell division?
- Interphase: chromosomes are replicated (46 —> 92 in one cell)
- Meiosis I
- Cytokinesis I (2 cells with 46 chromosomes each)
- Meiosis II (without replication of chromosomes)
- Cytokinesis II (4 cells with 23 chromosomes each)
—> Meiosis = TWO cell divisions
—> Produce 4 haploid cells
Meiosis steps following ploidy
INTERPHASE
1 diploid cell (single chromosomes: 46) —> Replication —> 1 diploid cell (double chromosomes: 92)
MEIOSIS I
1 diploid cell (double chromosomes: 92) —> 2 haploid cells (double chromosomes: 46 each)
MEIOSIS II
2 haploid cells (double chromosomes: 46 each) —> 4 haploid cells (single chromosomes: 23 each)
How does meiosis differ from mitosis?
MITOSIS:
- produces 2 genetically identical and diploid daughter cells
- involves one division of nucleus and one division of cytoplasm
- involve one chromosomes replication
MEIOSIS:
- produces 4 genetically different and haploid daughter cells
- involves two divisions of nucleus and two divisions of cytoplasm
- involves one chromosome replication
- homologous chromosomes exchange regions
What is “crossing over”?
Homologous chromosomes exchange regions during prophase 1 —> ensures that each haploid cell will be genetically unique
What happens in meiosis I in early prophase I that does not happen in mitosis?
Replicated homologous chromosomes pairs form tetrads —> 4 chromosomes together
—> Stay in tetrads until end of anaphase (when the 2 homologous pairs are pulled to opposite poles)
What happens in meiosis I in late prophase I that does not happen in mitosis?
Crossing over occurs between chromatids of the homologous chromosomes —> “shuffling” —> infinite combinaisons of alleles
What happens during meiosis II?
Is essentially mitosis but without replicated chromosomes (prophase, metaphase, anaphase, telophase —> 4 haploid daughter cells)
Gamete production requires and what are the two types of production of gametes? (2)
- Meiosis
- Differentiation (maturation)
- Spermatogenesis in testes
- Oogenesis in ovaries
What is the outcome of meiosis and sexual reproduction? (3)
- Meiosis produces genetically unique sex cells
- Fusion of these sex cells produces genetically unique zygotes giving rise to genetically unique offspring
- Sexual reproduction produces genetic variation which in turn produces genetic variation in population (only reason why there is meiosis in sexual reproduction)
How does sexual reproduction produce genetic variability? (3)
- Maternal and paternal chromosomes are randomly assorted —> variation in gametes
- Crossing over creates recombinant chromosomes —> variation in gametes
- Fertilization is random —> variation in zygotes
How does meiosis produce genetic variation in gametes? (2)
- Random distribution of homologues during meiosis in gametes —> variation in the combination of maternal and paternal chromosomes
- Crossing over varies combinations of maternal and paternal alleles within each chromosome type
How does distribution of homologues produce genetic variation?
- Orientation of a pair (which side is the maternal/paternal chromosome is on?) is random
- So homologues will line up in ONE of millions of possible ways before they are package into different cells
- So millions of combinations of maternal and paternal chromosomes to be packaged into gametes by meiosis —> millions of combinations of alleles for the genes on our chromosomes
- 2^23 = 8.4 millions of combination possible
How does crossing over produce genetic variation?
- When homologues pair and lineup in meiosis I, corresponding portionsof these chromosomes are exchanged making recombinant chromosomes
- So alleles are shuffled into unique combinations
- So allows even more unique combinations of alleles to be packaged into gametes by meiosis
- Much more than 8.4 million combos can be created for humans
