meiosis Flashcards
p arm
short arm
Q arm
Long arm
centromere
location establishes appearance of cromosome
Types of chromosomes
Metacentric
Submetacentric
acrocentric
Telocentric
Metacentric
Centromere is at middle
Submetacentric
Centromere is between middle and end
Acrocentric
Centromere is close to end
Telocentric
Centromere is at end
general features of chromosomes
-Chromosomes appear most condensed during mitosis.
-Diploid number is 2n
-Chromosomes exist in pairs
-For each chromosome with specific length and centromere placement, there is another with identical features.
-Bacteria and viruses have just one chromosome
karyotype
-Shows human mitotic chromosomes at metaphase
-Chromosomes are photographed, cut out and matched
-All chromosomes are homologous pairs except X and Y
Haploid set
Half of number of chromosomes of diploid number
genetic informatiomin in haploid set constitutes the genome of the species
Different organisms have different haploid number
Homologous chromosomes
Includes copies of all genes and non-coding DNA
One haploid set- one paternal, the other paternal
Biparental inheritance
Inheritance from two parents
Meiosis
-Meiosis produces two daughter cells with full diploid complements
-Meiosis produces gametes/spores- one haploid set of chromosomes
-During sexual reproduction- haploid gametes fuse to constitute a diploid set
-NB: Maintains genetic continuity from generation to generation
Meiotic variation
-First meiosis- Produces gametes with unique combinations due to crossing over
-Reshuffles genetic material to produce diverse offspring
Basis of Mendel’s principles
Crossing over
Genetic exchange between members of each homologous pair of chromosomes
-Creates chromosome-mosaics of maternal and paternal homologs
Prophase 1
-Meiosis begins with diploid cell duplicating its DNA during interphase- like mitosis
-Homologous chromosomes pair up (undergo synapsis) to form bivalents
-Bivalents give rise to tetrads
-Tetrads remain intertwined in some areas ( chiasmata)
-Nonsister chromatids undergo exchange
-Tetrads attach to spindle fibres
Bivalent
A pair of homologous chromosomes that are physically held together by at least one DNA crossover
Tetrad
two pairs of sister chromatids joined at centromere.
Metaphase I
-Chromosomes shortened and thickened
-Terminal chiasmata visible
-Tetrads interact with spindle fibres
-alignment of tetrads are random prior to anaphase I (increases variation)
-One dyad (half of tetrad) pulled by spindle fibres to one or other pole
-single centromeric region holds sister chromatids together by cohesin protein
Anaphase I
-Cohesin degraded between sister chromatids, except at centromere
-Dyad pulled to each polle
-Reductional division- no. of centromeres halved
-seperation (disjunction)
-At completion- series of dyads at each pole equals the haploid number
-Crossing over results in mosaic chromatids of mixed paternal and maternal origin
Non-disjunction
Error where total seperation is not achieved
Telophase I
-some organisms, nuclear membrane forms around dyads and enter short interphase period but chromosomes do not replicate
-Other organisms- cells go directly from anaphase I to meiosis II
-Meiotic telophase- much shorter than mitosis
Meiosis II
Essential for gametes to have a single chromatid from original tetrad
Prophase II
Each dyad has a pair of sister chromatids attached by centromere
Metaphase II
Centromeres positioned on equatorial plate and shugoshin complex is degraded
Anaphase II
Iniated and sister chromatids are pulled to opposite poles
Telophase II
One member of each pair of homologous chromosomes present at each pole
-Each chromosome is now a monad
-Number of centromeres in resulting daughter cells is not reduced during this division (equatorial division)
Following cytokinesis in Meiosis II
Four haploid gametes are formed, crossing over has occured, monads are a combination of maternal and paternal genetic information, genetic variation increased