GB 11- Meiosis

Bio Exam 2

Heredity

transmission of traits from one generation to the next (inheritance)

Variation

differences in traits among individuals in a population, often caused by genetic diversity.

Gametes

reproductive cells that transmit genes from on generation to the next and combine during fertilization to form a new organism.

asexual reproduction

the process of producing offspring without the involvement of gametes, resulting in genetically identical offspring to the parent. Single parent produces offspring. In unicellular organisms this is done by splitting. In multicellular organisms this is done by budding or fragmentation A method of reproduction in which offspring arise from a single parent, resulting in clones.

asexual reproduction in eukaryotes:

mitotic division:

1 diploid (2n) parent —→ 2 diploid offspring

1 haploid (n) parent —→ 2 haploid offspring

produces clones- offspring genetically identical to parent

advantages of asexual reproduction: fast, low E required, safe, lots of offspring

sexual reproduction

Fusion of 2 gametes to form a zygote 

Gamete (n) + gamete (n) —→ fertilization —→ zygote (2n) 

gamete usually from different parents (but not always)

offspring not genetically identical to parents 

disadvantages of sexual reproduction

• slow

• high E requirement

• dangerous→ predation

• disease

• often fewer offspring 

advantages of self reproduction

• offspring novel combinations of parents’ genes

• better able to respond to change or stress 

Why variation is good? (sexual reproduction) 

the more variation the less get eaten by predators and better adaptation to changing environments.

Karyotype 

orderly display of chromosomes

mitotic chromosomes are stained  

Humans Karyotype

somatic cells: 46 chromsomes

22 pairs of autosomes

sex chromosomes: X and Y determine sex

Female: XX

Male: XY

Homologous Chromosomes 

same length, centromere position, staining pattern, same genes 

life cycle

sequences of stages from generation to generation

Fertilization and Meiosis

occur in all sexual life cycles

alternate

timing varies between generations and species.

Variety in sexual life cycles 

• Haploid multicellular organism: gameophyte

• haploid unicellular or multicellular organism 

• only 2n cells can undergo meiosis, producing haploid gametes during sexual reproduction.

Steps of meiosis

• reduction division

• 4 stages, involves 2 cell divisions

• interphase, meiosis I and cytokinesis, interkinesis, meiosis II

what is the first step of meiosis

interphase: chromosomes and now centrioles duplicate. Each chromosome now 2 sister chromatids (still chromatin). Humans 2n=46 so 92 chromatids enter meiosis

meiosis I (and cytokinesis)

• first meiotic division: homologous chromosomes seperate, ploidy reduced 

• the four phases of meiosis I: prophase I, metaphase I, anaphase I, telophase I.

first step of meiosis I

prophase I: includes crossing over, synapsis: homologous chromosomes pair up, genes in chromosomes align, results in tetrad, 2 homologous chromosomes (4 chromatids) held together. chromatin condenses, centromeres and kinetochores of homologous chromosomes seperate, sister chromatids are still attached, nuclear envelope breaks down, spindle forms

at the end of prophase I in humans: 2n= 46

# of chromosomes: 46

# of tetrads: 23

# of chromatids: 92

whats the point of synapsis?

• crossing over (homologous recombination) increases genetic diversity by exchanging DNA segments between homologous chromosomes.

• enzyme break and rejoin DNA

• exchanges between non-sister chromatids

• results in new combinations in genes

• important source of genetic diversity

second step of meiosis I

Metaphase I: tetrads align at metaphase plate, homologous chromosomes orient towards opposite poles, both sister kinetochores of one chromosome spindle for same pole, kinetochores of homologous chromosomes —→ spindle for opposite poles

third step of meiosis I 

Anaphase I: homologous chromosomes separate and move to opposite poles, while sister chromatids remain attached. chromosomes act independently. Direction depends on orientation of tetrad.  

Nondisjunction

is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis. This can result in gametes with an abnormal number of chromosomes, potentially leading to genetic disorders. The 2 homologous chromosomes go to the same pole. It is a common error during meiosis

what is the last phase of meiosis I 

telophase I: chromosomes arrive at opposite poles, the nuclear membrane may reform, and the cell divides through cytokinesis. This results in two haploid cells with duplicated chromosomes (sister chromatids are still together)

at the end of telophase I in humans:

2n=46

# chromosomes in each nucleus: 23

# chromatids: 46

# tetrads: 0

what is the third phase in meiosis?

Interkinesis: time between 1st and 2nd meiotic divisions, usually short, similar to interphase, no S phase, no DNA synthesis occurs

what is the last step of meiosis? 

Meiosis II: 2nd meiotic division, chromatids separate into daughter cells. (Prophase II, metaphase II, anaphase II, telophase II). 

• start: 2 cells, n, duplicated chromosomes 

• End: 4 cells , n, unduplicated chromosomes 

• The final step of meiosis, also known as the second meiotic division, results in the separation of sister chromatids into four haploid cells, each with unduplicated chromosomes.

• Each daughter cell is genetically unique 

• no crossing over in meiosis II 

• amount of DNA per cell reduced, ploidy does not change