Chapter 10 - Meiosis and Sexual Life Cycles

Inheritance (Heredity)

transmission of genes from one generation o the next

• results in both similarity and variation between progeny

genetics

study of heredity and the inheritance of variation

what is special about inheritance in prokaryotes

they are able to move genes horizontally

• meaning genes can go from individual to individual in the same generation or species

genes

encodes information for a product (usually a protein) which results in a specific characteristic

alleles

alternate forms of a gene resulting in a different trait expressed

locus

specific location of a gene on a chromosome

human genome

contains 46 chromosomes in somatic cells

• diploid compliment - 22 pairs of homologous chromosomes and 2 sex influencing chromosomes

• sex chromosomes are X and Y (X is very large, Y is very small

homologous chromosomes

same genes in the same places, but not the same alleles

karyotype and why is it helpful

• complete set of an individual’s chromosomes organized by shape and size

• can help us discover an excess or lack of chromosomes or missing a section of chromosomes

• does NOT help us with finding smaller mutations like in nucleotides

why are these homologous but no homozygous

same genes, but different alleles

gametes

specialized cells produced for asexual reproduction

• produced through meiosis (usually)

• contain haploid compliment of DNA

how many autosomes and sex chromosomes are in gametes

22 autosomes and 1 sex chromosome

which sex cell determines sex of offspring

sperm because egg cells only have X chromosomes while sperm cells have X and Y chromosomes

how does sexual reproduction differ sexual in organisms

differs between types of life cycles but always involves both mitosis and meiosis

which between haploid and diploid cells undergo mitosis or meiosis

haploid and diploid cells both go through mitosis, but only diploid cells go through meiosis

why is sexual reproduction costly for an organism

locating a mate and competition for a mate

main benefit of sexual reproduction

genetic variability in progeny

how does variability affect stability in a population

lower variability = lower stability since if all individuals are the same and are exposed to a change (like a virus), all of them will probably be affected

types of life cycles

animals - no haploid multicellular organisms

plants and some algae - alteration of generations

• multicellular diploid and haploid organisms

• cycle between sporophyte (diploid) and gametophyte (haploid)

most fungi and some protists - no diploid multicellular organisms

• gametes are produced through mitosis

how does inherited DNA direct development

nuclei in egg cell and sperm cell contains DNA, so when they fuse in fertilization, DNA is received from both parents

• embryo cells multiply along with copies of that inherited DNA

• leads to offspring receiving traits inherited from both parents

meiosis summary

• double division cycle

• diploid to haploid intermediates (meiosis 1)

• haploid intermediates increase in number (meiosis 2)

Prophase 1

• homologous chromosomes pair up to form tetrads

  • homologous genes on either chromosome are aligned

• crossing over occurs

synapsis

homologous chromosomes paired up together and are held together by synapsis

tetrad

4 sister chromatids in 2 homologous chromosomes held together by synapsis

crossing over and its steps

exchange of genetic info between nonsister chromatids

1. each pair of homologs line up along their length, allele to allele

2. DNA of two nonsister chromatids is broken at corresponding lcations

3. as the DNA condenses, the synaptonemal complex forms and attaches one homolog to the other through synapsis

4. as chromosomes reach the metaphase plate, the synaptonemal complex disassambles

• cross overs between homologs are called chiasmata

chiasmata

cross over between homologs

recombinant chromosomes

end result of crossing-over and synapsis

• two sister chromatids of a single homologous chromosome are no longer identical in alleles

• contain a new combination of alleles compared to the chromosomes from each parent

metaphase 1

• instead of duplicated chromosomes, tetrads are arranged on the metaphase plate

• each homologous chromosomes is attached to only one pole, not both

• independent assortment occurs

independent assortment

happens during metaphase 1

• orientation of maternal vs paternal chromosomes towards a pole is random

• each homologous pair is sorted independently of the other pairs

• this means there are 2^n different possibilities for each mitotic event

• for humans 2^23 =8,388,608 8.4 million

Anaphase 1

• proteins which hold nonsister chromatids together as a tetrad are broken down

• sister chromatids of the same chromosome remain attached together

• homologous chromosomes are moved to opposite poles

telophase 1 and cytokinesis

• haploid set of chromosomes is contained in each half of the cell by a newly formed nuclear membrane

• each duplicated chromosomes now consists of two genetically unique sister chromatids

• haploid cells result from cytokinesis

interkinesis

occurs between meiosis 1 and 2

• similar to interphase, cell synthesizes proteins and stores energy for another round of division

• chromosomes are not fully condensed during interkinesis

• NO REPLICATION OF DNA

• centrosomes DO replicate again

Meiosis 2

basically mitosis again

• no crossing over

• no independent assortment

result of meiosis 1

2 haploid cells containing duplicated chromosomes from the parent cell

• meiosis 1 is the reduction phase

do sister chromatids or chromosomes separate in meiosis 2

sister chromatids

result of meiosis

4 haploid cells, all genetically unique compared to the parent cell, with half the genetic complement in each cell

random fertilization and how it affects genetic variabilit

union of haploid gametes to create a diploid zygote

• which gametes actually participate in any fertilization is random

• each parent produces 8.4 million different gametes due to random assortment

• combination of independent assortment and random fertilization = 70 trillion different diploid combinations possible in the zygote

• crossing over adds to this genetic variance