Meiosis Flashcards
Heredity
transmission of traits from one generation to the next
Variation
offspring differ somewhat in appearance from parents and siblings
Genetics
study of heredity and hereditary variation
Where do offspring acquire genes?
parents
Genes
coded information, organized into
units that parents pass on to offspring; Genes program specific traits that emerge as we develop; Genes are segments of DNA
allele
alternative copy of gene
coding in genes
Information is coded in DNA in
sequences of 4 bases
Cells translate these DNA sequences in genes,
programming the cell to make
proteins (including enzymes)
All the proteins working together produces
an organism’s inherited traits
Genes are transmitted from one generation to the next by
sperm + ovum
chromosome numbers
DNA is divided into chromosomes, each species has
a specific number
chromosome numbers humans
o Humans- 46 chromosomes (haploid=23)
Locus
gene’s specific
location along the
chromosome
Asexual Reproduction
one parent is involved and
passes copies of its genes to offspring. clones. Offspring have
same genome as the parents except for mutations
clone
group of
genetically identical individuals produced asexually
- Sexual Reproduction
Two parents give rise to offspring that have combination of their genes
Results in
greater variation
human–somatic cell
body cell, contains 46 chromosomes, with two of each type
somatic cell
body cell
gamete
reproductive/sex cell
Homologous chromosomes
chromosomes that make up a
pair, due to
same length, banding pattern, and centromere placement. These two carry genes of the same type. exception: sex chromosomes
sex chromosomes
females–xx, males–xy
autosomes
non sex chromosomes (22)
46 chromosomes are therefore 2 sets of 23- because?
1 set inherited from each parent
Each sperm cell or ovum cell has (chromosomes?)
22 autosomes and 1 sex chromosome
single set of 23 chromosomes
haploid cells
Gamete cells with one set of chromosomes
Gametes
sperm + ovum
syngamy
Fertilization. haploid sperm fuses with a haploid egg
zygote + cell division in zygote
fertilized egg. two haploid sets of chromosomes with genes from the mother and the father. Mitosis in zygote produces more diploid somatic cells. Gametes, which develop in gonads, contain half the
number of chromosomes. Meiosis is the process that reduces the chromosome number in half.
diploid cells
Cells with 2 sets of chromosomes are called diploid cells
2n=46
What keeps a species’ chromosome count constant?
Fertilization and meiosis alternate in sexual life cycles
Meiosis
Results in 4 daughter cells, each w 1/2 number of chromosomes as the parent— haploid
Contains two consecutive divisions called Meiosis I and Meiosis II
Interphase I
Each chromosome replicates, forming
two sister chromatids
prophase I
Lasts longer than in mitosis
o Synapsis occurs, forming tetrad. Chromatids of homologous chromosomes cross over along their
length, forming chiasmata.
o Centrosomes move apart, spindle fibers start to form and attach to kinetochores
o Nuclear membrane and nucleoli disappear
o Can last for days- 90% of meiosis
synapsis
as chromosomes condense,
homologous chromosomes, each made of two sister chromatids, come together as pairs
tetrad
Pairs in synapsis are called tetrad
bc 4 sister chromatids
chiasmata
- can
exchange segments of DNA with one another
Metaphase I
Tetrads arrange on metaphase plate
o Microtubules are
attached
Anaphase I
o Spindle apparatus pulls apart homologous chromosomes, but
sister chromatids stay attached
o Homologous chromosomes move toward opposite pole
Telophase I and Cytokinesis
Each pole has haploid chromosome set, but each chromosome
is still
double stranded
o Cytokinesis occurs simultaneously, forming
two daughter cells
o Some species have second interphase, others head straight into
next meiotic division
o No duplication of chromosomes!
Random Fertilization
each human ovum (from 8 million
possibilities) is fertilized by single sperm (another 8 million
possibilities=
64 trillion possible zygotes
Origins of Genetic Variation
Sexual life styles produce. Independent Assortment of Chromosomes, Crossing over, Random Fertilization
Crossing Over
produces individual chromosomes that combine genes in heritage from both parents
o In humans- 2-3 crossing overs occur per chromosome pair
Independent Assortment of Chromosomes,
during metaphase I, the
chromosomes line up at metaphase plate with
a random orientation of maternal and paternal chromosomes facing the poles
o About 8 million possibilities of combinations (2^23)
Prophase II
Spindle apparatus forms, chromosomes move toward
metaphase plate
metaphase II
Chromosomes are lined up on
metaphase plate
Anaphase II
Centromeres of sister chromatids
separate
o Sister chromatids, now individual chromosomes, move
toward opposite poles
Telophase II and Cytokinesis
o Nuclei form at opposite poles and cytoplasm
splits
o Concludes with
four daughter cells each w a haploid number of chromosomes
unique events of meiosis
Synapsis occurs, forming a tetrad of four sister chromatids
(two homologous chromosomes). Chiasmata, or
areas of crossing over, form between the two chromosomes
2- At metaphase I, paired homologous chromosomes line up instead of individual chromosomes
3- At anaphase I, sister chromatids do not separate- instead, homologous chromosomes separate, each still double stranded
Meiosis II
Meiosis II separates the sister chromatids and is identical to mitosis except for resulting in haploid instead of diploid
comparison mitosis meiosis
Chromosomes # is
mitosis–diploid–2n meiosis–haploid–n–reduced in half in meiosis but not mitosis
2- Mitosis produces 2 cells that are genetically identical to
parent while meiosis
produces four that differ genetically from the parents and from
each other
Spermatogenesis-definition + info
process of forming sperm; Takes place in
testes; Begins with spermatogonia
testes + info
male gonads. in testes, seminiferous tubules have thick wall, the sperm go into the lumen of the walls
spermatogonia
undifferentiated diploid cells
Spermatogenesis process
Spermatogonia divide by mitosis throughout his entire lifetime; Some enlarge and become primary spermatocytes, which undergo meiosis, producing haploid gametes. First meiotic division produces two secondary spermatocytes; Second meiotic division produces four spermatids; each spermatid becomes a sperm cell
oogenesis–def + info
process of ovum formation (pl. ova). Takes place on the ovaries, the female gonads.
Begins with oogonia- undifferentiated diploid cells that form during embryonic development.
oogenesis process step one
Oogonia develop into primary oocytes during prenatal development. By time of birth, they are in prophase I of meiosis and then enter a resting phase.
polar body–
small cell that dies
oogenesis process step two
With onset of puberty, primary oocyte completes the first meiotic division–only one oocyte 1x a month gets ovulated–a couple others complete but end up dying–rest are still in prophase. Forms one polar body and one secondary oocyte, which precedes to meiosis II but stops at metaphase II.
oogenesis process step three
Second meiotic division (occurs at fertilization) gives rise to total of 3 polar bodies (sometimes first polar body divides again, andante the polar body from secondary oocyte) one ovum
