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the main successes of an organism are
reproduction and survival
reproduction
production of an offspring from an existing organism
asexual (non-sexual) reproduction
doesn’t involve sex, and creates an offspring that’s genetically identical to the parent
unicellular organisms such as yeasts, fungi, and protists use mitosis for
asexual (non-sexual) reproduction
budding is a type of asexual (non-sexual) reproduction that involves
an organism making a tiny copy of itself that stays attached until both are equal in size, then it separates
fission is a type of asexual (non-sexual) reproduction that involves
an organism splitting into 2 halves that develop, grow, and mature into 2 organisms
sexual reproduction involves
gamete fusion to form a zygote
gametes are
sex cells (sperm and egg)
a zygote is
a fusion of sex cells (sperm + egg), 46 chromosomes in total
hermaphroditism is a type of sexual reproduction that involves
an organism (hermaphrodites) producing both sperm and egg cells
hermaphroditism is (1) but (2)
less advantageous for diversity, better than asexual reproduction
gametes are formed via
meiosis
gametes have (1) and are considered to be (2)
23 chromosomes (1 single set), haploid cells (1n)
meiosis is important for
chromosome reduction from 46 to 23 each, so that when gametes fuse they have 46 in total instead of 92
mitosis produces (1), which have (2) in total
diploid cells, 46 chromosomes
an example of cells produced by mitosis is
somatic cells (e.g., body cells)
meiosis produces (1), which have (2) in total
haploid cells, 23 chromosomes
meiosis 1 involves the
reduction division of homologous chromosomes
meiosis 1 produces
2 daughter cells with 1 chromosome that has 2 chromatids
meiosis 2 involves the
separation of sister chromatids
meiosis 2 produces
4 haploid cells that each have 1 chromosomes with 1 chromatid from 2 daughter cells from meiosis 1
during prophase 1
chromosomes condense, nuclear envelope fragments, tetrad forms, crossing over occurs
tetrad
homologous chromosomes brought together by synaptonemal complex
synaptonemal complex
web of proteins that attach homologous chromosomes
crossing over involves
the exchange of chromosomal segments between homologous non sister chromatids (one from each parent)
crossing over occurs between all chromosomal pairs except (1) because (2)
sex chromosomes, they’re not identical
recombinant chromosomes
chromosomes with crossed over segments
non-recombinant chromosomes
chromosomes without crossed over segments
during prometaphase 1
nuclear envelop breaks, spindle fibers attach to the kinetochore, homologous chromosomes are still attached.
prometaphase 1 is different from mitosis prometaphase and metaphase 2 because
the chromosomes are in pairs (2 chromosomes, 4 chromatids total)
during metaphase 1
homologous chromosomes orient in pairs (2 lines) at the cell center randomly, independent assortment occurs
independent assortment
chromosomes can migrate to the poles in various order (e.g., mother chromosomes can go in the same direction, or opposite directions)
during anaphase 1
microtubules pull tetrads apart
during telophase 1
DNA will either condense and a nuclear envelope forms, or cytokinesis occurs without nuclear envelop reformation. Two new non-identical daughter cells are formed
meiosis 2, which is nearly identical to meiosis 1, precedes a unique interphase that’s shorter because it doesn't have an S phase. this is called
interkinesis
meiosis 2 occurs (1) because meiosis 1 produces (2)
twice, 2 daughter cells
during prophase 2
Non-homologous chromosomes reform if they condensed after meiosis 1
during prometaphase 2
microtubules attach to kinetochores present in each non homologous sister chromatid
during metaphase 2
Non-homologous chromosomes align at the cell center in a single line (not in pairs)
during anaphase 2
sister chromatids are pulled apart by microtubules
during telophase 2
chromosomes arrive at opposite ends of the cell and start condensing, nuclear envelopes form. 4 non-identical daughter cells are formed
two meiotic mechanisms that produce haploid genetic variation are
independent assortment and crossing over