Meiosis || Gen Bio

lessons 4.1 to 4.6

meiosis

• cellular division that produces sex cells

• involves two divisions of nucleus (meiosis 1 & 2)

• starts with homologus chromosomes

• also called reduction division

gametes

reproductive cells; sperm and egg

reduction division

• first stage of meiosis

• diploid (2n) divides into two haploids (n)

diploid (2n)

• the full set of 46 chromosomes

• occurs in body cells

haploid (n)

• a set of 23 chromosomes; half of diploid

• occurs in sex cells

homologus chromosomes

• separate chromosomes; one set from father, the other from the mother

• similar in size, shape, length, and genetic content

• do not contain exact copies of each other

1. prophase 1

2. metaphase 1

3. anaphase 1

4. telophase 1

what are the stages of meiosis 1?

prophase 1

• chromosomes condense and become visible

• homologus chromosomes pair up

• tetrads and spindle fibers form

• crossing over happens (DNA is exchanged)

• nuclear membrane breaks down

metaphase 1

• homologus chromosomes line up at middle

• each pair is called tetrad

• spindle fibers attach to each chromosome

• orientation is random

anaphase 1

• homologus chromosomes separate

• chromosomes move to opposite pole

• sister chromatids stay together
  

telophase 1

• chromosomes reach opposite poles

• nuclear membranes may reform

• cell divides through cytokinesis

• two haploid cells form

1. prophase 2

2. metaphase 2

3. anaphase 2

4. telophase 2

what are the stages of meiosis 2?

prophase 2

• chromosomes condense again

• spindle fibers form

• nuclear membrane breaks down

metaphase 2

• chromosomes line up at center

• spindle fibers attach to sister chromatids

anaphase 2

• centromere divides; sister chromatids separate

• chromatids are pulled to opposite ends

telophase 2

• chromatids reach poles

• nuclear membranes reform

• cells divide through cytokinesis

• four haploid cells form

gametogenesis

• process involving production of gametes

• two types: spermatogenesis (male), oogenesis (female)

spermatogenesis

process of sperm production

oogenesis

process of egg production

independent assortment

random distribution of homologus chromosomes

random distribution

• creates many random combinations of chromosomes in gametes

• maternal and paternal chromosomes separate independently

genetic variation

• unique mix of maternal and paternal chromosomes

• important for evolution and diversity

random fertilization

the chance of fusion of any sperm with any egg during fertilization

chromosonal abnormalities

incorrect number of chromosomes

nondisjunction

chromosomes don’t separate properly during meiosis

• homologus chromosomes fail to separate in meiosis 1

• sister chromatids fail to separate in meiosis 2

can result to trisomy or monosomy in babies

1. deletion

2. duplication

3. inversion

4. translocation

reasons for breakage/changes in chromosomes

trisomy

cells have 3 copies of 1 chromosome

monosomy

cells have only 1 copy of a chromosome

1. down syndrome

2. jacobsen syndrome

human chromosome disorders

down syndrome

• trisomy 21 (3 copies of chromosome 21)

• frequency correlates to mother’s age (older mom = bigger chance)

jacobsen syndrome

several genes on chromosome 11 are missing

1. klinefelter’s syndrome (male)

2. jacob’s syndrome (male)

3. trisomy x (female)

4. turner syndrome (female)

sex chromosomes abnormalities

klinefelter’s syndrome

• XXY

• sterile male sex organs

• feminine characteristics

• tall w/ normal intelligence

jacob’s syndrome

• XYY

• extra y chromosome making them slightly taller and more active

• normal intelligence and sexual development

• delayed emotional maturity and slight learning disabilities

trisomy x

• XXX

• produces healthy females

• presence of barr body because of one inactive x chromosome

turner syndrome

• X or X0

• webbed neck with short stature

• sterile

• degree of effects vary

deletion

loss of a chromosonal segment

duplication

repeat a segment

inversion

reverses a segment

translocation

move segment from one chromosome to another