week 4

Question 1

what are the 2 general types of cells in plants and animals

Answer 1

somatic:
- make up vast majority of individual’s cells
- either in G0 or actively going thru cell cycle and dividing via mitosis
- diploid in humans (2n)

germ:
- pockets of specialized cells set aside during embryogenesis
- become incorporated into reproductive organs
- only cells that undergo meiosis to produce haploid gametes (egg/sperm)

Question 2

general overview of meiosis

Answer 2

2 rounds of cell division

chromosomes duplicate only 1x (before 1st round) but nuclei divide 2x

meiosis 1 reduces chromosome # from 2n to n (homologous chrom separate)

in meiosis 2, chromosome # doesn’t change (sister chromatids separate)

Question 3

meiosis i

Answer 3

separation of homologous chromosomes 2n -> n

• homologous chromosomes pair
• “crossing over” occurs between homolog chrom where parts of chrom swap
• homolog chrom separate 2n->n (reductional division)
• sister chromatids remain attached thru mei i

Question 4

meiosis ii

Answer 4

separation of sister chromatids and move to opp poles
- n -> n
- equational division
- very similar to mitosis

Question 5

phases of meiosis 1

Answer 5

1. prophase i:
   - leptotene
   - zygotene
   - pachytene
   - diplotene
   - diakinesis
   ** substages of prophase i
2. metaphase i
3. anaphase i
4. telophase i (telo = end)

Question 6

what occurs during the first 3 substages of prophase i (leptotene, zygotene, pachytene)

Answer 6

dna has already replicated to form sister chromatids prior to start of meiosis

leptotene:
- chromosomes condense, centrosomes move to opp poles and produce spindle fibres (microtubules)

zygotene:
- homolog chrom find each other and pair (synapsis)

pachytene:
- crossing over occurs

Question 7

leptotene

Answer 7

1st substage of prophase i

chromosomes condense, centrosomes move to opp poles and produce spindle fibres (microtubules)

Question 8

zygotene

Answer 8

2nd substage of prophase i

homolog chrom “zipper” together with the help of specific proteins

2 homolog chrom attached = tetrad (grp of 4 chromatids) or bivalent (2 chrom)

tetrad formation: complex of proteins attaching homolog chrom is called synaptonemal complex

Question 9

pachytene

Answer 9

3rd substage of prophase i

crossing over occurs

recombination nodules (protein assemblies) appear along synaptonemal complex

nodules facilitate exchange of DNA at random pts (avg 3-4 per chrom)

intersection pts between homolog chrom are called chiasmata

mixes maternal and paternal homolog chromosomes
- can result in new allelic combos

Question 10

diplotene

Answer 10

4th substage of prophase i

synaptonemal complex dissolves but crossover pts (chiasmata) remain and hold homolog chromosomes together (tetrads remain intact)

meiotic arrest occurs at this time in many species

Question 11

diakinesis

Answer 11

nuclear membrane breaks down and spindle continues to form

chromatids thicken and shorten

tetrad condenses further

Question 12

metaphase i (meiosis)

Answer 12

tetrads line up along metaphase plate

each chrom of a homolog pair attached to a spindle fibre from a diff pole

Question 13

anaphase i (meiosis)

Answer 13

chiasmata dissolve

homolog chrom move to opposite poles

sister chromatids remain connected to each other at their centromeres

Question 14

telophase i and interkinesis

Answer 14

nuclear membranes form around chrom at each pole

sister chromatids remain attached at their centromeres

in some species, chrom decondense during interkinesis, in others they don’t

during interkinesis, cells prepare for meiosis ii, but chromosomes do not replicate

each daughter cell is haploid (1/2 the number of chromosomes as the parent cell)

Question 15

phases of meiosis ii

Answer 15

1. prophase ii
2. metaphase ii
3. anaphase ii
4. telophase ii (telo = end)

Question 16

prophase ii

Answer 16

chromosomes condense

centrioles move towards poles

nuclear envelope breaks down at the end of prophase ii

Question 17

metaphase ii

Answer 17

chromosomes align at the metaphase plate

sister chromatids attach to spindle fibres from opposite poles

Question 18

anaphase ii

Answer 18

sister centromeres detach from each other, allowing sister chromatids to move to opposite poles

Question 19

telophase ii

Answer 19

chromosomes begin to uncoil

nuclear envelopes and nucleoli re-form

Question 20

cytokinesis (meiosis)

Answer 20

cytoplasm divides, forming 4 new haploid cells

Question 21

differences between mitosis and meiosis i in types of cells they occur in and how many rounds

Answer 21

mitosis:
- somatic cells, germ-line precursor cells
- haploid and diploid cells
- one round of division

meiosis:
- germ cells as part of sexual cycle
- only diploid cell
- 2 rounds of division: meiosis i and ii

Question 22

differences between mitosis and meiosis in terms of preparation

Answer 22

mitosis:
- preceded by S phase (chrom duplication)

meiosis:
- chrom duplicate prior to meiosis i but not before meiosis ii

Question 23

differences between mitosis and meiosis in terms of homolog chrom

Answer 23

mitosis:
- homolog chrom do not pair
- genetic exchange between homolog chromosomes is very rare

meiosis:
- during prophase of meiosis i, homolog chrom pair (synapse) along their length
- crossing over occurs between homologous chromosomes during prophase of meiosis i
- attach to spindle fibers from opposite poles during metaphase i

Question 24

differences between mitosis and meiosis in terms of sister chromatids

Answer 24

mitosis:
- attach to spindle fibers from opp poles during metaphase
- centromeres separate at the ebeginning of anaphase

meiosis:
- attach to spindle fibers from opposite poles during metaphase ii
- centromeres separate at the beginning of anaphase ii

Question 25

differences between mitosis and meiosis in terms of final cell products

Answer 25

mitosis: - produces 2 new daughter cells, identical to each other and og cell - genetically conservative meiosis: - produces 4 haploid cells 1 egg/all sperm of which can become gametes - none of the cells are identical to each other/the og cell bc meiosis results in combinatorial change

Question 26

does mitosis and meiosis still work with mendel's laws?

Answer 26

yes! law of segregation: - during gamete formation, the 2 alleles for a trait separate from each other at random into separate gamete - ex. in heterozygous yellow pea plants, the yellow and green alleles separate each into a separate gamete law of independent assortment: - alleles of 2 or more different genes sort into gametes independently from one another - ex. in heterozygous round yellow pea plants, the separation of yellow and green alleles into separate gametes does not influence the separation of round and wrinkled alleles into separate gametes

Question 27

explain oogenesis generally

Answer 27

egg production in humans mitotic divisions of oogonia (germ cells) produces many diploid cells called primary oocytes asymmetrical meiosis - at each div, one large and one small daughter cell is produced primary oocyte goes thru meiosis i and produces a polar body and a secondary oocyte secondary oocyte undergoes meiosis ii and produces a polar body and a mature ovum polar bodies disintegrate meiosis of a single oogonium generates 1 egg, NOT 4

Question 28

oogenesis in the context of a whole human

Answer 28

begins 6 months after conception, when ovaries are fully formed each ovary contains ~500k primary oocytes - remain arrested in diplotene (homolog chrom still attached) until after puberty when primary oocyte is released during ovulation, it proceeds to metaphase ii a primary oocytes only completes meiosis ii if it is fertilized

Question 29

spermatogenesis in humans

Answer 29

sperm production mitotic divisions of spermatogonia (germ cells) produce many diploid cells called primary spermatocytes spermatogenesis is symmetrical - 4 haploid sperm (equal in size) produced by meiosis of each spermatocyte primary spermatocyte undergoes meiosis i to produce 2 secondary spermatocytes secondary spermatocytes undergo meiosis 2 to each produce 2 spermatids spermatid grow flagella (tails) and mature into sperm

Question 30

spermatogenesis in the context of a whole human

Answer 30

doesn't begin until puberty after puberty, spermatogenesis is continuous thruout a male's life - millions of sperm always in production - in a lifetime, individuals can produce 100s of billions of sperm process of spermatogenesis takes ~48-60 days

Question 31

how do we know that traits are carried on chromosomes

Answer 31

1. sec is associated w/ inheritance of particular chrom - suggests specific traits are carried on chromosomes 2. mitosis and meiosis together ensure a constant # of chrom in somatic cells of a species - no other molecule is so conserved in each cell - suggest chromosomes could be the agent of inheritance

Question 32

what did the white eyed drosophila mutant test cross show? (morgan)

Answer 32

chromosomes carry heritable traits "criss-cross" inheritance pattern - females inherit trait from father - males inherit from mother

Question 33

what did the calvin bridges experiments do

Answer 33

confirm traits are determined by genes on chromo9somes nondisjunction

Question 34

theory of chromosomal inheritance

Answer 34

1. genes reside on chrom - dna sequencing - FSH (fluorescent in situ hybridization) 2. genes determine traits - genetic eng - ex. inserting gene for human insultin into e-coli chrom causes ecoli to produce insulin 3. chromosomes are the vehicle of inheritance - nuclear cloning

Question 35

aneuploidy

Answer 35

loss/gain of 1+ chromosome

Question 36

aneuploids

Answer 36

individuals who have an abnormal of chromosomes per cell

Question 37

polyploidy

Answer 37

more than 2 full sets of chromosomes

Question 37

euploids

Answer 37

individuals who have a normal number of chromosomes

Question 38

monosomy

Answer 38

cells have one chromosome fewer than the normal diploid number (2n-1)

Question 39

trisomy

Answer 39

cells have one chromosome more than the normal diploid number (2n+1)

Question 40

tetrasomy

Answer 40

cells have 4 copies of a particular chromosome rather than the normal 2 (2n+2)

Question 41

autosomal aneuploidy

Answer 41

generally harmful to organisms trisomies are sometimes viable in humans monosomies are never viable in humans

Question 42

down syndrome

Answer 42

trisomy 21 developmental delays mild to moderate intellectual disability characteristic physical features chromosome 21 is smallest chromosome - affects least amnt of genes, generally well tolerated

Question 43

what is aneuploidy caused by

Answer 43

non-disjunction during meiosis - failure of chromosomes to segregate normally - can occur during meiosis i or 11

Question 44

why is aneuploidy harmful

Answer 44

gene transcription gets fucked up because your cells expect 2 copies, not 3

Question 45

if aneuploidy is a thing, how do male humans survive with only one X chromosome?

Answer 45

only 1 copy of an autosome is detrimental because gene dosage is insufficient dosage compensation! - offsets what would be double the gene products in the homogametic sex (females in humans) compared to the heterogametic sex (males in hum) in female humans, one X is randomly inactivated, - the inactive X chromosome is condensed into a barr body - "X inactivation" or 'lyonization'

Question 46

X inactivation

Answer 46

occurs about 2 weeks after fertilization in embryos with XX genotype embryo has 500-1000 cells and each activated one X chrom at random - some cells will contain maternally inherited active X and others the paternal all mitotic decedents of a given cell will have the same X inactivated for most traits, there is enough "mixing" of active/inactive X thruout the body that heterozygotes with a mutant recessive X allele show the wild type phenotype for some X linked traits, X inactivation results in a visible mosaic phenotype - mosaic expression: XX ~50/50 expression of each X chromosome thruout body ex. calico pattern in some mammals

Question 47

does x inactivation occur in drosophila?

Answer 47

no instead, males produce double the # of gene products from their single X to compensate ** this is why XXX is viable in humans but not flies

Question 48

how to account for female humans not haveing any Y chromosomes at all if X inactivation explains same gene dosage of X linked genes

Answer 48

Y chromosome includes: - sex determining region of Y (SRY) - 3 genes needed for spermatogenesis - 8 essential genes scattered thru MSY region - ~30 genes in the two PAR regions 8 genes and PAR region genes are present in X too. these parts of the X chromosome are excluded from inactivation in the barr body

Question 49

aneuploid mosaics

Answer 49

occurs due to mitotic nondisjunction or chromosome loss early in. embryo development same idea as X chromosome inactivation resulting in a mosaic of cells in a fully developed human but instead of some cells having one active X and others having the other X active, some cells are trisomic or monosomic and otherss are typical

Question 50

turner syndrome

Answer 50

females have only one X chromosome instead of two, or are partially missing an X chromosome leading to the complete or partial deletion of the pseudoautosomal regions in the affected X chromosome.

Question 51

parthenogenesis

Answer 51

reproduction without fertilizatiojn

Question 51

drosophila bilateral gynandromorph

Answer 51

is an XX fly zygote loses an X chromosome during the first mitotic division, one daughter nucleus will be XX and the other will be XO these individuals grow into gynandromorphs depending on the alleles carried by X, the 2 halves of the fly could appear different and one will have a male specific sex comb

Question 51

asymmetrical gynandromorph

Answer 51

patches of male and female phenotype but not perfectly bilateral ex. butterflies male = ZZ, female = ZW loss of 1 Z chromosome in 2nd/3rd mitotic division in a male embryo

Question 52

haplodiploidy

Answer 52

unfertilized eggs (haploid) -> males fertilized eggs (diploid) -> females sex isn;t linked to inheritance of specific set of chromosomes ants, bees, wasps

Question 53

triploids

Answer 53

almost always sterile extra chrom can't pair at meiosis sexual reprod results in unbalanced gametes the more chromosomes in a set, the less likely to get a functional gamete rare in nature, but we can make them (seedless watermelon) can be beneficial for aquaculture - trip fish live longer and grow larger as they don't need to expend resources on reprod

Question 54

how to make triploid organisms

Answer 54

using pressure to retain polar body cross between diploid and tetraploid -> need to create tetraploid from diploid (treat seedling with chemical that inhibits spindle formation)

Question 55

larger chromosome numbers lead to what in plants

Answer 55

increased size presumably bc increased amnt of gene product present due to increased number of gene copies

Question 56

can individuals of different species mate and create viable offspring?

Answer 56

yes, but usually sterile mules are usually sterile bc the chromosomes can't pair during meiosis i