homologous chromosomes

(homologs)

two chromosomes of each type

-same size and same shape

-carry the same genes

gene

section of DNA that influences some heriditary trait of and individual

allele

different versions of same gene

(homologs carry the same genes, but each may contain different alleles)

karyotype

the # and types of chromosomes present

Diploid

"double form"

two versions of each type of chromosome

-2 alleles for each gene

haploid number

the number of distinct types of chromosomes in a giving cell

(humans-23)

Ploidy

combination of the # of sets and n(haploid #)

2n diploid organisms-human 46

polyploid

instead of 2 homologs these have many forms

-have 3 or more of each type of chrom in each cell

chromosome

structure made up of DNA and proteins

carries the cells hereditary infor (genes)

autosomes

non sex chromosomes

unreplicated chromosome

chrom that consists of a single copy

replicated chromosome

chrom has been coplied; 2 linear structures joind at the centromere

sister chromatids

-chrom copies in a relicated chrom

non-sister chromatids

chromatids belonging to homologous chromosomes

tetrad

homologous replicated chroms taht are joined together

haploid #

the number of different types of chroms

diploid #

the number of chroms present

during Meiosis I

homologs in each chrom pair separate from each other; one homolog goes to one daughter cell and one goes to the other

-the diploid parent cell produces 2 haploid daughter cells; each chrom still contsists of 2 sister chromatids (chroms are still replicated)

During M II 

-sister chromatids from each chromosome separate

-cell produced also have one of each type of chromosome  but now chroms are unreplicated

reduction division

the outcome of meiosis is a reduction in chrom#

Gameotogenesis

homologous chroms form egg cell or sperm cell

Interphase (Meiosis I)

-chromes replicate in parent cell

-in uncondensed state

forming sister chromatids

Early Prophase I

 -chromes condense

-nuclear envelope breaks up

-spindle apparatus forms

-synapsis of homologous chromosomes (tetrads)

-kinetochore microtubules attach to the kineotoches at the centromeres of chroms

Late Prophase I

crossing over of nonsister chromatids

(mixing from maternal and paternal chroms)

chiasma

when nonsister chromatids cross over each crossover forms X shaped called______

Metaphase I

migration of tetrads to metaphase plate is complete

Anaphase I

homologs separate and begin moving to opposite sides of cell

Telophase I and Cytokinesis

chromes move to opp side of cell

-cell divides

(nuclear envelope may reform)

Prophase II

spindle apparatus forms

(if nuclear envelop formed at end M1 breaks apart)

Metaphase II

chromes line up at metaphase plate

(consisting 2 sister chromatids)

Anaphase II

sister chromatids separate (become unreplicated chromes)

-->begin moving in the opposite side of cell

Telophase II and Cytokinesis

chromes move to opposite side of cell

cell divides

results in 4 haploid cells

Keye difference btwn Meiosis and Mitosis

homolog pair early in meiosis but not during mitosis

-bc homologs pair in prophase of meiosis I they can migrate to the metaphase plate together and then separate during anaphase of meiosis I resulting in reductive devision

Prophase I (step 1)

step(1): after chrom replication complete-sister chromatids stay tightly joined along entire lenght

Prophase I (step 2)

when homologs synapse, 2 pairs of nonsister chromatids are brought close togher and held there by a net work of proteins (synaptonemal complexes)

Prophase I (step 3)

crossing over occurs when a complex of proteins cut the chroms and then reattches the pieces so that segments are swapped btwn adjacent homologs

Replication

-during interphase sister chromatids are held together by proteins along the chrom "arm" and at the centromere

Synapsis

during prophase 1

homologous chroms are held together by proteins in the synaptonemal complex

Crossing Over

during prophase 1

-complex of proteins form where crossing over will occur

(chrom segments are swapped btwn non-sister chromatids)

2 aspects of meiosis that create variation among chromosomes

(1) separation and distribution of homologous chromosomes

(2) crossing over

genetic recombination

-crossing over produces new combinations of alleles w/in a chrom; combinations that did not exist in either parent

ways to drastically INC genetic variablity

crossing over

and 

recombination

outcrossing

gametes from different individuals combine to form offspring;

INC genetic diversity of offspring bc it combines chroms from diff individuals which contain different alleles

deleterious alleles

alleles that function poorly and lowers fitness of an individual

(sexual indivuals are likely to have offspring that lack these)

purifying selection

natural selection against deleterioius alleles 

(over time it should steadily reduce to numerical advantage of asexual reproduction)

Nondisjunction

if both homologs or both sister chromatids move to the same pole of the parent cell

Aneuploid

cells that have too many or too few chroms

for gamete to get one complete set of chroms 2 steps need to occur perfectly

(1) the chroms in each homologous pair must separate from each other during the first meiotic division, so that only one homolog ends up in each daughter cell

(2) sister chromatids must separate from each other and move to opp poles of the dividing cell during meiosis II