Chapter 13: Meiosis & Sexual Life Cycles
Anaphase I (Meiosis)
HOMOLOGOUS CHROMOSOMES SEPARATE from one another & move to pop ends of the cell. -sister chromatids of a homolog will remain attached
Which of the following events occurs in meiosis but not in mitosis? -Homologous chromosomes synapse. -Chromatin condenses. -Chromosomes are replicated. -Daughter cells are produced. -Chromosomes align at the metaphase plate.
Homologous chromosomes synapse.
Stages of Prophase I: Diplotene (diplonema)
Homologs begin moving away from each other (repulsion) bc the synaptonemal complex breaks down - chiasma becomes apparent - condensation of chromosomes (still)
What is independent assortment? -It is the random way in which each pair of homologous chromosomes lines up at the metaphase II plate. -It is the random way in which sister chromatids line up at the metaphase II plate. -It is the random distribution of the sister chromatids into the two daughter cells during anaphase II. - It is the random way in which each pair of homologous chromosomes lines up at the metaphase I plate. -It is the random nature of fertilization.
It is the random way in which each pair of homologous chromosomes lines up at the metaphase I plate.
False
True or False: If pairs of homologous chromosomes are aligned at the metaphase plate with both homologous chromosomes in the pair attached to one pole, then the resulting daughter cells will contain the correct number of daughter chromosomes at the end of telophase I.
True
True or False: The chiasmata is an X-shaped region that represents where crossing over has taken place.
T/F: False
True or False: The midpiece of a mature sperm contains enzymes that will allow the sperm to penetrate the egg cell upon fertilization.
T/ F: False
True or False: When a primary spermatocyte divides by meiosis I, four secondary spermatocytes form.
chiasmata
X shaped region that REPRESENTS where cross-over occurs -becomes visible in diplotene
first polar body
smaller cell that is produced from M1, it is smaller due to unequal cytoplasmic division -degenerates & never starts M2 *If FERTILIZATION OCCURS= then secondary Oocyte will complete Meiosis II> Fertilized Egg> Embryogenesis, Meiosis II> 2nd Polar Body> Degenerate **IF NO FERTILIZATION= then secondary oocyte never completes meiosis II
Gametes
sperm and egg
Tetrad
structure containing 4 chromatids that forms during meiosis
Genetics
the scientific study of heredity (transmission of traits from one generation to the next) and variation (differences in appearance) -Genes are passed to the next generation by reproductive cells called gametes
Stages of Meiosis II
*before meiosis II, a period of rest can occur & it is called INTERKINESIS *the events are the same as mitosis
Telophase I and Cytokinesis (Meiosis)
-Spindle disassembles - each pole contains a HAPLOID set of chromosomes -Depending on species, nuclear envelope will reform around each set & chromosomes may uncoil - Cleavage Furrow= animal cells >actin & myosin contract & deepens furrow until the cells split in 2 plant cells *plant cells= cell plate forms (vesicles from golgi) RESULT: 2 HAPLOID cells w/ duplicated chromosomes that are GENETICALLY DISSIMILAR due to CROSSING OVER & INDEPENDENT ASSORTMENT
structure of mature sperm
-Tail= movement of sperm (flagella) -Midpiece= contains mutiple mitochondria, produces ATP for tail -Head= contain HAPLOID nucleus & acrosome - Acrosome= cap containing enzymes in a vesicle that will help penetrate the egg
Prophase II (meiosis)
-chromosomes condense -spindle forms -attachment of sister chromatids to spindle -movement of sister chromatids to metaphase plate
Origins of Genetic Variation Among Offspring
-crossing over -independent assortment of chromosomes (8.4 million possible chromosome combos) - random fertilization (70 trillion diploid combos) -mutations (change in an organism's DNA)
Anaphase II (meiosis)
-one chromatid attached to one polar & other chromatid attached to other pole -nuclear envelope reform around each set of unduplicated chromosomes -Spindle disassembles -Chromosomes Uncoil
Gametogenesis
-production of gametes, spermatogenesis & Oogenesis
Spermatogenesis (in detail)
-stem cells are located on outer edge of semiferous tubule & as stem cell undergoes division, the progeny move inward & to the lumen (opening) & into epididymus (sperm storage & maturation) -spermatogonial stem cell= @ puberty, this stem cell replicates by mitosis
haploid
1 set of chromosomes (n)
Animal Sexual Life Cycle
1. begin w/ fertilization (union of haploid egg & sperm) 2. (zygote) 2n= one celled organism w/ 2 sets of chromosomes 3. mitosis= chromosomal # stays the same (2n> 2n); cells identical to parent 4. 2n diploid multicellular organism 5. meiosis= half chromosomal # (2n> n); cells diff than the parent & to each other 6. n gametes 7. Startover
Overview of Meiosis: Meiosis I
1st nuclear & cytoplasmic division reductional division = diploid> haploid
diploid
2 sets of chromosomes (2n) -1 mom and 1 dad
Overview of Meiosis: Meiosis II
2nd Nuclear & cytoplasmic division, equational division
Human Cells:
46 chromosomes (23 pairs)
Which of the following is an example of alternation of generations? -A diploid animal produces gametes by meiosis, and the gametes undergo fertilization to produce a diploid zygote. -A haploid mushroom produces gametes by mitosis, and the gametes undergo fertilization, which is immediately followed by meiosis. -A diploid plant (sporophyte) produces, by meiosis, a spore that gives rise to a multicellular, haploid pollen grain (gametophyte). -A grandparent and grandchild each have dark hair, but the parent has blond hair. -None of the above
A diploid plant (sporophyte) produces, by meiosis, a spore that gives rise to a multicellular, haploid pollen grain (gametophyte).
The following are a pair of homologous chromosomes with genes A, B, C and D. The chromatids are numbered 1, 2, 3 and 4. If a single crossover event occurs between chromatids 2 and 3 at genes B and C (as shown below), what will be the gene sequence in the resulting chromatids? The choices below are given in the order of: chromatid 1, chromatid 2, chromatid 3 and chromatid 4. AA aa BB bb CC cc DD dd 1 2 3 4
ABCD, AbcD, aBCd, abcd
What are the alternate versions of a gene called?
Alleles
During which phase of meiosis, do homologous chromosomes separate and move to opposite ends of the cell?
Anaphase I
During what phase of meiosis, do sister chromatids separate and move to opposite ends of the cell?
Anaphase II
Prophase I (Meiosis)
Chromosomes condense & at end they will become visible under microscope - centrosomes move to opp. ends of cell & Spindle forms (made of microtubules; move chromosomes around the cell) - nuclear envelope disappears - nucleolus disappears -microtubules of the spindle attach to the kinetochores on the homologs - homologs start to move @ metaphase plate ** difference from mitosis: has stages (subdivisions)
Stages of Prophase I: Pachytene (pachytema)
Continued condensation of chromosomes synapsis is complete (pairing is complete & synaptonemal complex holds them together) ***crossing over occurs*** -tetrad: 4 chromatids- bivalent
Telophase II and Cytokinesis (Meiosis)
Cytoplasmic Division RESULT: 4 cells that are haploid w/ unduplicated chromosomes that are genetically different
During what stage of prophase I, does the maximum condensation and maximum repulsion of homologous chromosomes occur?
Diakinesis
Allele
Different forms of a gene
During which stage of prophase I, does the chiasmata first become apparent?
Diplotene
What is the chromosomal makeup of each daughter cell at the end of telophase I? -Each daughter cell contains a haploid set of chromosomes, and the chromosomes are each composed of a single chromatid. -One daughter cell contains a diploid set of chromosomes and the other daughter cell contains a haploid set of chromosomes, and all chromosomes are composed of two chromatids. -Each daughter cell contains a diploid set of chromosomes, and the chromosomes are each composed of a single chromatid. -Each daughter cell contains a haploid set of chromosomes, and the chromosomes are each composed of two chromatids. -Each daughter cell contains a diploid set of chromosomes, and the chromosomes are each composed of two chromatids.
Each daughter cell contains a haploid set of chromosomes, and the chromosomes are each composed of two chromatids.
Which of the following is true of a species that has a chromosome number of 2n = 16?
Each somatic cell will have 8 pairs of homologous chromosomes.
Completion of Meiosis I & onset of Meiosis II (oogenesis cont.)
FSH & LH will cause a primary oocyte to complete meiosis I & start meiosis II. It will get arrested in Metaphase II (follicular phase-maturation of oocyte) -secondary oocyte gets arrested @ metaphase of meiosis II
In a human karyotype, chromosomes are arranged in 23 pairs. If we choose one of these pairs, such as pair 14, which of the following do the two chromosomes of the pair have in common?
Length, centromere position, staining pattern and genes
What is the location of a gene called?
Loci
Stages of Prophase I: Diakinesis
Maximal repulsion of homologous & maximal condensation of chromatin
During what phase of meiosis, do the tetrads align down the metaphase plate?
Metaphase I
Crossing over in Pachytene
Occurs when DNA molecules of non-sister chromatids are broken by proteins & rejoined to each other; exchange of genetic info btwn non-sister chromatids Result: Chromosome that has MATERNAL & PATERNAL info (recombinant chromosome) *** MAJOR source of genetic diversity*** (look @ notes)
Oogenesis (in detail)
Production of oocyte (egg cells); occurs in ovaries. Process is PROLONGED & does NOT occur in a continuous process. It does NOT occur throughout life -At birth: 1-2 million oocytes are arrested in prophase I -only ~500 will mature btwn puberty & menopause *Primary Oocyte: begin meiosis I during embryological development & cells get stopped in prophase I. -FSH & LH will cause a primary oocyte to complete meiosis I & start meiosis II. It will get arrested in Metaphase II (follicular phase-maturation of oocyte) -secondary oocyte gets arrested @ metaphase of meiosis II -smaller cell that is produced from M1, it is smaller due to unequal cytoplasmic division -degenerates & never starts M2 *If FERTILIZATION OCCURS= then secondary Oocyte will complete Meiosis II> Fertilized Egg> Embryogenesis, Meiosis II> 2nd Polar Body> Degenerate **IF NO FERTILIZATION= then secondary oocyte never completes meiosis II
During what phase of meiosis, do homologous chromosomes first condense and attach to the spindle apparatus?
Prophase I
#23 chromosome
SEX Chromosomes: XX= Female XY= Male
What is the zipper-like structure that allows homologous chromosomes to pair up alongside each other during prophase I?
Synaptonemal complex
2n=6
The maternal and paternal set of chromosomes is this equation. (n=3)
#1-22 chromosomes
autosomes-DO NOT CODE FOR SEX
Metaphase II (meiosis)
chromosomes align on metaphase plate
Stages of Prophase I: Lepotene
condensation of chromosomes (chromatin)
Stages of Prophase I: Zygotene (zygote)
continued condensation of chromatin ** homologs begin to pair up gene-by-gene w/ each other using a protein (synaptomal complex) that zippers the homologs together. -pairing of homologs is called synapsis- Synapsis> bivalent: homologous chromosomes are in pairs
Sister chromatids of one duplicated chromosome
identical to each other; result of s phase of interphase
2 non sister chromatids in homologous pair
non identical chromatids, 1 mom, 1 dad
independent assortment (2n)
occurs when a pair of homologous chromosomes orient independently of the other pairs when the maternal & paternal chromosomes align down metaphase plate 2^n > n= haploid # Ex: 2^23= 8.4 million diff egg combos 2^23=8.4 million diff sperm combos = 16.8 million diff combos in total
Karyotype
ordered display of the chromosomes taken from metaphase of mitosis (WBC's)
Metaphase I (Meiosis)
pairs of homologous chromosomes align down the metaphase plate (middle of cell) w/ one homolog facing one side of cell & other facing the other side of cell. -kinetochore of the homolog is attached to the spindle -sister chromatids attached to spindle?? -attachment of homolog within a pair is random
homologous chromosomes
pairs; same exact length, have same centromere position, have same bonding patterns, have same genes @ same loci (location) but the allele can be different. Ex: Hair color (gene) Allele: brown, black, red, blonde
Centromere
pinched region on a chromosome; Region where sister chromatic are more closely associated
Oogenesis
production of mature oocytes (Large & non-motile) occurs in ovaries -not a continuous process -not prolific
Spermatogenesis
production of sperm (smaller & motile) occurs in testes in seminiferous tubules (highly coiled tubes) -continuous process -prolific process -100's of sperm per day
Life Cycles
• Generation-to-generation sequence of stages in the reproductive history of an organism • The timing of meiosis and fertilization can vary from species to species
Meiosis
• Reduces the number of chromosome sets from diploid to haploid and it takes place in two consecutive cell divisions, called meiosis I and meiosis II • The two cell divisions result in four daughter cells and each daughter cell has only half as many chromosomes as the parent cell
Overview of Meiosis: Interphase
•Prepatory phase composed of G1, S, G2 G= gap phase -cell growth, production of proteins & ATP, metabolism, organelles duplicate G2= nucleus is present, nucleolus (ribosomal subunits made) is present, DNA is loosely coiled around proteins (chromatin) S= DNA replication **centrosomes replicate**