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MEIOSISSperm & Eggs & Variation..OH MY!

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SEXUAL REPRODUCTION REQUIRES CELLS MADE BY MEIOSIS

What if a new individual was formed through mitosis?

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MEIOSIS – KEY TERMS allele amniocentesis asexual reproduction autosome binary fission chorionic villi

sampling crossing over diploid exon fertilization

gamete genome haploid homologues intron karyotype nondisjunction placenta reduction division sexual reproduction zygote

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GENOME Genome: Complete complement of an

organism’s DNA. includes genes (control traits) and non-coding

DNA organized in chromosomes

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GENES Eukaryotic DNA is organized in chromosomes

genes have specific places on chromosomes exon: portion of a gene that is translated into

protein intron: non-coding segment of DNA, often found

within an exon; removed before transcription

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HEREDITY Heredity – way of transferring

genetic information to offspring Chromosome theory of heredity:

chromosomes carry genes Gene – “unit of heredity”

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REPRODUCTION Asexual

splitting budding parthenogenesis (egg develops w/o fertilization)

occurs naturally in some invertebrate animal species (e.g., water fleas, aphids, nematodes, some bees, some scorpion species, and parasitic wasps) and a few vertebrates (e.g., some fish, amphibians, reptiles, and very rarely birds)

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SEXUAL REPRODUCTION Fusion of two gametes to produce a

single zygote. Introduces greater genetic variation,

allows genetic recombination. With exception of self-fertilizing

organisms (e.g. some plants), zygote has gametes from two different parents.

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CHROMOSOMES Karyotype:

ordered display of an individual’s chromosomes

collection of chromosomes from mitotic cells

staining can reveal visible band patterns, gross anomalies

Make a Karyotype

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KARYOTYPING Obtain some cells from the individual Culture them in a test tube with nutrients Treat cells w/chemical that stops them

exactly midway through cell division (chromosomes are coiled thickly and more visible than usual)

Cells are placed on a microscope slide and a stain is added that binds to the chromosomes, making them visible

Chromosomes are arranged by size and shape and displayed on a monitor or in a photograph

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KARYOTYPING

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HOMOLOGOUS CHROMOSOMES Chromosomes exist in homologous pairs

in diploid (2n) cells. One chromosome of each homologous

pair comes from the mother (called a maternal chromosome) and one comes from the father (paternal chromsosome).

Homologous chromosomes are similar but not identical. Each carries the same genes in the same order, but the alleles (alternative form of a gene) for each trait may not be the same. 

Exception: sex chromosomes (X, Y)

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HOMOLOGOUS CHROMOSOMES

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IN HUMANS … 23 chromosomes donated by each

parent (total = 46 or 23 pairs). Gametes (sperm/ova):

contain 22 autosomes and 1 sex chromosome

haploid (haploid number “n” = 23 in humans)

Fertilization results in zygote with 2 sets of chromosomes - now diploid (2n).

Most cells in the body produced by mitosis.

Only gametes are produced by meiosis.

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CHROMOSOME NUMBERSAll are even numbers – diploid (2n) sets of homologous chromosomes.

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MEIOSIS – KEY DIFFERENCES FROM MITOSIS

Meiosis reduces the number of chromosomes by half.

Daughter cells differ from parent, and each other. Meiosis involves two divisions, Mitosis only one. Meiosis I involves:

synapsis homologous chromosomes pair up chiasmata form (crossing over of non-sister

chromatids) metaphase I: homologous pairs line up at

metaphase plate anaphase I: sister chromatids do NOT separate overall, separation of homologous pairs of

chromosomes, rather than sister chromatids of individual chromosome

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ANIMATION

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MEIOSIS I

Prophase 1each chromosome duplicates and

remains closely associated (sister chromatids)

crossing-over can occur during the latter part of this stage

Metaphase 1homologous chromosomes align at the

equatorial plate

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MEIOSIS I Anaphase 1

homologous pairs separate with sister chromatids remaining together

Telophase 1two daughter cells are formed with each

daughter containing only one chromosome of the homologous pair

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MEIOSIS II

Second division of meiosis: Gamete formation

Prophase 2DNA does not replicate

Metaphase 2chromosomes align at the equatorial plate

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MEIOSIS II Anaphase 2

centromeres divide sister chromatids migrate separately to

each pole

Telophase 2cell division is complete4 haploid daughter cells

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MITOSIS VS. MEIOSIS

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MEIOSIS CREATES GENETIC VARIATION

During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)

Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing overno daughter cells formed during meiosis

are genetically identical to either mother or father

during sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring

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INDEPENDENT ASSORTMENT

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INDEPENDENT ASSORTMENT

Number of combinations: 2n

e.g. 2 chromosomes in haploid2n = 4; n = 22n = 22 = 4 possible combinations

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IN HUMANS

e.g. 23 chromosomes in haploid2n = 46; n = 232n = 223 = ~ 8 million possible combinations!

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CROSSING OVERChiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids.

Crossing over produces recombinant chromosomes.

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HOW SEX IS DETERMINED IN HUMANS•Females don’t have a Y chromosome in any of their cells, yet they are able to develop and live normal, healthy lives. •For this reason, we know that nothing on the Y chromosome is absolutely necessary.

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SEX DETERMINATION IN OTHER SPECIES

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RANDOM FERTILIZATIONAt least 8 million combinations from Mom,

and another 8 million from Dad …>64 trillion combinations for a diploid

zygote!!!

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MEIOSIS & SEXUAL LIFE CYCLES Life cycle = sequence of

stages in organisms reproductive history; conception to reproduction

Somatic cells = any cell other than gametes, most of the cells in the body

Gametes produced by meiosis

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MEIOSIS & SEXUAL LIFE CYCLES

Generalized animal life cycle

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NONDISJUNCTIONUnequal distribution of chromosomes during meiosis

Resulting gametes zero or two copies of a chromosome instead of a single copy

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TOO MANY OR TOO FEW CHROMOSOMES

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SEX IS COSTLY! Large amounts of energy required to

find a mate and do the mating: specialized structures and behavior required

Intimate contact provides route for infection by parasites (AIDS, syphilis, etc.)

Genetic costs: in sex, we pass on only half of genes to offspring.

Males are an expensive luxury - in most species they contribute little to rearing offspring.

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BUT … More genetic diversity: more potential for survival of species when environmental conditions change. shuffling of genes in meiosis crossing-over in meiosis fertilization: combines genes from 2 separate individuals

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MITOSIS VS MEIOSIS REVIEW In what cellular processes is mitosis

involved? In what cellular processes is meiosis involved?

In what type of cells does mitosis occur? In what type of cells does meiosis occur?

How many times does DNA replicate in mitosis? How many times does DNA replicate in meiosis?

How many cellular divisions occur in mitosis? How many cellular divisions occur in meiosis?

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MITOSIS VS MEIOSIS REVIEW How many daughter cells are formed by

mitosis? How many daughter cells are formed by meiosis?

What is the chromosome number in daughter cells formed by mitosis from diploid parent cells? What is the chromosome number in daughter cells formed by meiosis from diploid parent cells?

In mitosis, are daughter cells identical to or different from parent cells? In meiosis, are daughter cells identical or different from parent cells?

In mitosis, when do synapsis and crossing over occur? In meiosis, when do synapsis and crossing over occur?

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CREDITS cchs.churchill.k12.nv.us/marshk/

Notes/meiosis.ppt Genetic Science Learning Center,

University of Utah, http://learn.genetics.utah.edu

http://www.phschool.com/science/biology_place/labbench/lab3/concepts2.html