Meiosis Flashcards
what is meiosis
the process of cell division in eukaryotes consisting of 2 consecutive cell divisions (meiosis 1 and meiosis 2)
when does DNA synthesis occur
- happens only once and after their is 2 nuclear divisions
what does the two nuclear divisions result in
- 1 diploid (2n) parent cell which produces 4 haploid daughter cells
what are haploid cells also known as
gametes
where does meiosis happen
testis and ovary
what does the fusion of 2 haploid gametes produce
a diploid zygote
what does meiosis do
- produces 4 haploid cells from a single diploid cell
- it scrambles information in alleles. creating genetic diversity and is accomplished by crossing over and independent assortment
why do we need meiosis
- it halves the number of chromosomes in sex cells
why does meiosis halve the number of chromosomes in sex cells
the sperm and egg each have 23 chromosomes so that when they come together they have 46 chromosomes
What are the stages of Meiosis
- Meiosis 1
- Meiosis 2
what happens during meiosis 1
- the homologous chromosomes get separated
what happens in meiosis 2
- sister chromatids get separated
what happens during prophase 1
- duplicated chromosomes align with their homologues (synapsis) to form homologous pairs
what is a tetrad
the 4 chromatids of a homologous pair
what is crossing over
the exchange of segments between non sister chromatids during synapsis
what is chiasmata
the sites where chromosomes attach so they exchange genetic material
what are homologous chromosomes
- pair of chromosomes (one maternal and one paternal) that are similar in shape and size
- carry genes controlling the same inherited traits at the same locus
- each locus is the same position on homologues
what are the types of chromosomes that humans have
- autosomes
- sex chromosomes
how many autosomes and chromosomes do humans have
- 22 pairs of autosomes
- 1 pair of sex chromosomes
what happens during metaphase 1
- chromosomes (homologous pairs in tetrads) line up at the equator
- they get attached to spindle fibres at their centromeres
- spindle fibres connect them to the centrioles
what happens during anaphase 1
- the spindle guides the movement of the chromosomes toward the poles
- the homologous pairs of chromosomes separate and move toward the opposite poles as spindle fibres contract
What happens during Telophase 1 and cytokinesis
- cytokinesis occurs simultaneously with telophase 1
- this is the end of the first meiotic cell division
- the cytoplasm divides, forming two new daughter cells
- each newly formed cells has half the number of the parent chromosomes
what is meiosis 1 also called
reduction division
what is cytokinesis
- in plant cells: cells plate forms that grows outward forming 2 daughter cells
- in animal cells: cleavage furrow pinches inward to form 2 daughter cells
When does meiosis 2 begin
-similar to mitosis
- right after telophase 1 and cytokinesis
- no further replication of chromosomes and no interphase 2
what happens during prophase 2
-each daughter cell forms a spindle, usually at right angle to the first spindle
- chromosomes (with 2 chromatids) move toward the equator
- no crossing over happens
what happens during metaphase 2
- the chromosomes are positioned on the metaphase plate in a mitosis like fashion
- no homologous pairs
what happens during anaphase 2
- centromere of sister chromatids separate and move toward opposite poles
- individual chromosomes after anaphase 2 have only one chromatid
what happens during telophase 2 and cytokinesis
- nuclei form at opposite poles of the cell and cytokinesis occurs
- after cytokinesis there are 4 haploid daughter cells
When can individual chromosomes be seen
- when they are in prophase and metaphase cause they have condensed enough to be seen under a light microscope
how are karyotypes storted
- size
- position of centromere
- pattern of dark and light bands
what is a karyotype
- method of organizing the chromosomes of the cell in relation to number, size, and type
- generally organized from largest to smallest chromosomes
what are the different types of chromosomes that are based on the position of the centromere
- telocentric chromosome
- acrocentric chromosome
- submetacentric chromosome
- metacentric chromosome
what is a telecentric chromosome
a chromosome whose centromere is located at one end
what is a acrocentric chromosome
a chromosome whose centromere is located near the end
what is a submetacentric chromosome
a chromosome whose centromere is located near the middle
what is a metacentric chromosome
a chromosome whose centromere is centrally located
what are the key differences between mitosis and meiosis
- synapse, chiasma formation and crossing over in meiosis
why is genetic recombination important
leads to variation or diversity in a population
why is variation important
important raw material for natural selection and evolution helps to adapt to changing environments
how does meiosis create genetic variation
- sexual reproduction - fusion of sperm & egg/ egg & pollen
- crossing over (prophase 1)
- independent assortment (metaphase 1)
what happens during sexual reproduction
fusion of sex cells (egg and sperm) of two individuals to form an offspring that inherits genetic information form both parents
what is spermatogenesis
the process in which four sperms are produced in male testes by the process of meiosis
what is oogenesis
the process in which one ovum and 3 polar bodies are produced in female ovaries by the process of meiosis
what is fertilization
the process by which a sperm fuses with an egg to form a zygote and restore the diploid number of the species
why can spermatogonia and oogonia reproduce
- they have 46 chromosomes which is the number needed to do mitosis
what are the phases in spermatogenesis and oogenesis
- multiplication phase
- growth phase
- differentiation phase
what happens during the multiplication phase in spermatogenesis
one diploid spermatogonium multiplies to become 2 primary diploid spermatocytes
what happens during the growth phase in spermatogenesis
each of the primary spermatocytes split and become 2 secondary haploid spermatocytes
what happens during the differentiation phase in spermatogenesis
each secondary spermatocytes becomes 2 haploid spermatids which then become sperm
what happens during the multiplication phase in oogenesis
one diploid oogonium multiples to become 2 primary diploid oocytes
what happens during the growth phase in oogenesis
each primary oocytes becomes a secondary haploid oocyte and the first polar body
what happens during the differentiation phase in oogenesis
the secondary oocyte splits to become a haploid ootid and first polar body grows a second polar body which then becomes the ovum
what does crossing over do
creates new combinations of genetic material
what is independent assortment
- orientation of each chromosome pair is random with respect to other chromosome pairs in metaphase 1
- daughter cells inherit a mix of maternal and paternal chromosomes
- produces 2^23 gametes
what happens if there are errors in meiosis
- humans get disorders called syndromes
- e.g. down syndrome
how do errors in meiosis happen
- happens in non-disjunction
what is non-disjuction
- when chromosomes fail to separate
when can non disjunction happen during meiosis
- anaphase 1 when homologous pairs fail to separate
- anaphase 2 when sister chromatids fail to separate
What happens during non-disjunction in anaphase 1
- homologous pairs fail to separate
- two gametes get an extra chromosome (trisomy; n+1); two gametes get none (n-1)
- abnormal gametes: 100%
what happens during non-disjunction in anaphase 2
- sister chromatids fail to separate
- two gametes are normal (n), one has an extra chromosome (trisomy: n+1), one gamete has one less chromosome (n-1)
- abnormal gametes: 50%
what are some examples of non-disjunction
- down syndrome: 21 trisomy
- turner syndrome: X chromosome monosomy
- edward’s syndrome: trisomy 18
what are the two types of chromosomal abnormalities
- abnormal chromosome number
- abnormal chromosome structure
what is aneuploidy
- non disjuction occurs leading to a cell which is trisomy (n+1) or monosomy (n-1)
- most situations are lethal in humans
what is polyploidy
- a zygote produced by fertilization of a haploid gamete with a diploid cell producing a cell with more than 2 sets of chromosomes
what can cause abnormal chromosome structure
- deletion
- translocation
- duplication
- inversion
when can abnormal chromosome structure happen
- crossing over when genetic material that is exchanged doesn’t reattach properly
- mistakes during DNA replication
what happens in deletion
- detached fragment of DNA does not get reattached and is lost
- example: Wolf-Hirschhorn syndrome
what happens in translocation
- detached segment of DNA reattaches to a non - homologous chromosome
what happens in duplication
- a segment of DNA is duplicated during DNA replication
what happens in duplication during crossing over
- results in one chromosome becoming longer while the other is shorter
- longer chromosome will have a duplicated section
- shorter chromosome will have a deletion
what happens in inversion
- detached fragment reverses its orientation and reattaches to the original chromosome
