Chapter 3 Flashcards
Chromosome
- an arrangement of DNA with genes at specific regions (loci)
some determine sex (sex chromosomes)
some don’t (autosomes)
Ploidy
the number of copies of each chromosome in any given cell
diploid (2n) cells have two copies of each chromosome
(ex. somatic cells)
- two copies of genetic material
haploid (n) cells have one copy of each chromosome
(ex. germ cells)
Homologous chromosomes
contain the same genes in the same order
Diploid vs haploid lifecycles
humans mostly diploid
- except meiosis to fertilization
fungi and unicellular eukaryotes mostly haploid
- except nuclear fusion to meiosis
- mature, ecologically form is haploid
Mitosis vs meiosis and ploidy
mitosis
- produces diploid cells
- genetically identical chromosomes
- how somatic cells reproduce
meiosis
- produces haploid cells
- not genetically identical
- how gametes are produced (sperm and eggs)
Cell division issues
too much
= cancer, morphological abnormalities, and death
too little
= inappropriate development and growth
Three stages of mitosis
M-phase:
- cell division
Interphase:
- 3 parts
G0:
- division arrested
- specialized cells eventually die
3 parts of interphase
G1
- high gene expression to prepare for replication
S phase
- DNA replication occurs
G2
- preparation for cell division
chromosomes duplicated
Parts of M phase
Prophase
- chromo condense
Prometaphase
- nuclear envelope breaks down
- microtubules attach
Metaphase
- chromo align at plate
Anaphase
- sister chromatids separate
Telophase
- new nuclear membrane forms
Cytokinesis
- daughter cells separate
Nuclear contents of a cell during mitosis
End of…
G1
- 46 somes
- 46 tids
S
- 46 somes
- 92 tids
Mitosis
- 46 somes
- 46 tids
Cell cycle checkpoints
- failure = death
- mutations affecting cycle controls = growth abnormalities
- loss of control = cancer
checkpoints at the end of the phase
Metaphase checkpoint
- chromos attached to mitotic spindle
G1 checkpoint
- cell size, nutrients available, growth factor signals
S phase checkpoint
- DNA replication complete
G2 checkpoint
- cell size and complete chromo replication
Meiosis
- produces gametes for sexual reproduction
- produces haploid cells
- two stages = meiosis I and meiosis II
- no DNA replication between phases
Steps of meiosis
diploid precursor cell
heterozygous (ex. Gg)
chromatids duplicated into homologous chromosomes during interphase
first meiosis separates homologous chromos into separate cells
second meiosis separates each sister chromatid into separate cells
= gametes
Meiosis 1 (3 main events)
- homologous chromosomes pair
- crossing over / recombination
- duplicated homologous chromosomes separated into two daughter cells
Steps of meiosis 1
Prophase 1
- homologous chromosomes pair and recombine in tetrads
Metaphase 1
- homologous pairs align
Anaphase 1
- chromosomes separate
- sisters still attached
Telophase 1
- nuc membrane forms and cleavage furrow
Cytokinesis
- cells separate
Meiosis 2 (1 main event)
separation of sister chromatids into separate daughter cells
results in 4 distinct haploid daughter cells
Recombination
occurs during prophase 1 in meiosis 1
before homologs separate
mediated by the synaptonemal complex that forms between homologous chromosomes
Nuclear contents of a cell during meiosis
G1 and S same as mitosis
G1
- 46 somes
- 46 tids
S
- 46 somes
- 92 tids
Meiosis 1
- 23 somes
- 46 tids
Meiosis 2
- 23 somes
- 23 tids
Allele segregation
Idk bout this
during recombination (prophase 1) and when homologs separate (metaphase 1)
metaphase 1 arrangement dictates independent assortment of multiple genes
Oogenesis and spermatogenesis
produced through meiosis
Oogenesis
- produces 1 egg cell per division and 1-3 polar bodies
Spermatogenesis
- produces 4 sperms per cell division
Primary oocytes
established in the mother 20 weeks after conception - during embryonic development
7 million present
1-2 million by birth
60 000 - 80 000 by puberty
500 mature oocytes produced during a woman’s life
Oogenesis
in utero:
2n oogonium
2n primary oocyte
after puberty:
n secondary oocyte (ovulated) + polar body
n meiosis to completion + polar body
To determine total DNA content of daughter cells
compare number of chromatids
Drosophila experiment results
genes are carried on chromosomes
- Thomas Hunt Morgan rediscovered Mendel’s laws
- also contributed to the understanding of the chromosomal basis of sex determination
looked at wild type (usually normal flies) compared to mutated flies
X-linked inheritance discovery
- discovered by Nettie Stevens
- looked at chromosomal differences in male and female beetles
2n female cells: 20 large chromos
2n male cells: 19 large, 1 small
small = y chromo
White-eyed mutant flies
discovered by morgan
recessive to red eyes
sex linked on the X chromosome
X-linked recessive inheritance
traits will be more common in males
inherit the condition directly from carrier mothers
3:1
dom:rec
but the 1 will be the male
ex. colourblindness, hemophilia A
Hemizygous
having only one copy of a gene or locus in a diploid organism
ex. males are hemizygous for the X chromosome
Sex determination due to chromosomes
different animals
Humans/mammals
- Y determines male
Platypus
- 5 pairs of sex chromosomes
5XX = female, 5XY = male
Birds/reptiles/butterflies/moths
- ZW system
ZW = female, ZZ = male
Drosophila sex determination
- Y no effect
- X:A ratio system
- ratio of X to autosomes determines sex
X expresses sisterless
autosomes express deadpan
when sisterless dimerize (high concentrations), female flies produced
when deadpan dimerizes with sisterless, males produced
XX = females
X = male
Y chromosome
contains gene called sex determining region of Y (SRY)
males
- XY
- XXY, XYY, XYYY
females
- XX
- XO, XXX, XXXX
Z/W sex determination
opposite of humans
females - ZW
males - ZZ
X-linked dominant inheritance
- hetero females transmit dom to half their offspring
- daughters of dominant males will pas the trait directly to their daughters
Congenital hypertrichosis (CGH)
rare x-linked dominant condition
large increase in number of hair follicles on the body
Dosage of X-linked genes too high
females express double what males do
undergo dosage compensation
in mammals, one X randomly inactivated in each somatic cell
X-inactivation in female placental mammals
occurs in early embryonic development
inactivates either the maternal or paternal X in females random
cells are therefore mosaics; one expresses paternal while the other expresses maternal
Mosaicism - humans and cats
- visible in women with x-linked skin conditions
“Lines of Blaschko”
calico cats
one allele black, one for orange
each has unique patch patterns
