bio 109- CHAPTER 11- sexual reproduction and meiosis

meiosis

creating gametes from germ cells

karyotype

total number of chromosomes in a cell

homologous chromosomes

chromosomes with same genes (may be different alleles)

ploidy

number of homologous chromosomes (how many sets)

synapsis

joining of homologous chromosomes

cross-over

transfer of alleles between homologous chromosomes

how many chromosomes do diploid cells have per set

2 (ex. human somatic cells)

how many chromosomes do haploid cells have

1 (human gametes)

how many homologues do offspring inherit from parents

2

sexual life cycles

alternation of haploid and diploid stages

sometimes diploid phase is longer (most animals)

sometimes haploid phase is longer (mosses, fungi, some invertebrates)

features of meiosis

1 round of S phase (like mitosis)

2 rounds cell division

1st separates chromosomes

2nd separates chromatids

synapsis

homologous chromosomes pair up (not in mitosis)

early prophase I

form tetrads (a.k.a bivalents)

cross over

exchange DNA between homologous chromosomes

DNA from maternal now on parental (and vice versa)

sister chromatids are no longer identical

process of meiosis I and II

prophase I

prometaphase I

metaphase I

anaphase I

telophase I

NO S PHASE BETWEEN

prophase II

prometaphase II

metaphase II

anaphase II

telophase II

what are 3 sources of genetic variation

1) crossover

2) alignment of chromosomes

3) alignment of chromatids

meiosis I vs mitosis

3 key features of meiosis

1) synapsis and cross-over, makes unique chromatids (recombine maternal and paternal genes)

2) sister chromatids remain joined in meiosis I, go to the same pole during anaphase I

3) DNA replication is suppressed between meiosis I and meiosis II, half as much DNA in gametes compared to start.

final result of meiosis

four cells containing different half set of original karyotype

in animals, develop directly into gametes

in plants, fungi and many protista, then divide by mitosis

produces greater number of gametes

nondisjunction

failure to separate

produces aneuploid gametes (missing or extra chromosomes)

most common cause of miscarriage in humans