Meiosis

Reduction

Division

Genetics Vocabulary• Gene:

– a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses) at a specific locus (location)

• Allele:– Any of the alternative versions of a gene that

produce distinguishable phenotypic (visible) effects• Genome:

– The genetic material of an organism or a virus; the complete complement of an organism’s or viruses genes along with its noncoding nucleic acid sequences

Meiosis Vocabulary

• Diploid cell: – a cell that contains all (2n) of the genetic info it

should.

• Haploid cell: – a cell that contains half (n) of the genetic info it

should.

• Homologous chromosomes: – “paired”(at the same locus) chromosomes that have

the same traits arranged in the same order but the are not identical

• Meiosis: – The process that creates cells with a haploid number

of genetic info.

Meiosis Vocabulary• Gametes:

– The only cells in the body not produced by mitosis– Made in the gonads– Sex cells

• Sperm: The male sex cell• Egg: The female sex cell• Each one represents 1 in 8.4 million possible

genetic combinations• Zygote:

– The result of sperm fertilizing egg.– Combo of sperm and egg makes a 1 in 70 trillion

possible combinations• Somatic cell:

– Any other cell other than those involved in gamete formation

Miscellaneous Factoids

• Typically, a thousand or more genes are arranged on each chromosome

• Humans have 46 chromosomes• Why meiosis?

•Offspring get information from mom and dad

– Genetic diversity is key!!

•If that info wasn’t split, you would get twice as much information as they would have (reduction division)

Phases

• Interphase• The same as interphase in mitosis• Chromosomes replicate • Each chromosome then has two

identical sister chromatids held together by a centromere

Prophase I• Chromosomes coil• Spindle forms• Tetrads are formed

– Joining of two homologous chromosomes with two sister chromatids each (called synapsis)

– Bind so tightly that information is exchanged • Now called Chiasmata (Chiasma)

– Called Crossing over (remember the purpose of introns!!!)•Forms recombinant chromosomes

Metaphase I

• Tetrads line up at midline• Homologous chromosomes line up

in pairs

Anaphase I

• Homologous chromosomes separate and migrate towards poles

• Centromeres do not split– This ensures that each new cell gets

only one chromosome

Telophase I

• Spindle breaks down• Chromosomes uncoil• Cytoplasm divides (cytokinesis)• Cell has half of original cells

genetic info but still double the amount it should

Interphase II**

• Very short or non-existent• Chromosomes do not replicate

Meiosis II• Prophase II

– Spindle forms• Metaphase II

– Sister chromatids line up at the equator• Anaphase II

– Centromeres split– Sister chromatids split

• Telophase II– Spindle breaks down– New nuclei form– Cytokinesis occurs

Meiosis in Men and Women

• Male version of meiosis• Spermatogenesis• 4 sperm cells for every 1 cell that divides

• Female version of meiosis• Oogenesis• 4 egg (ova) cells for every 1 cell that

divides• 3 die and 1 survives

Meiosis is the same all around the world!

Alternation of Generations

The adult generation produces spores, while the spore generation produces sex cells. Only seen in plants.

Errors• DNA mutations

– Point• Base-pair substitution

– Silent mutations» Harmless» Occur within non-coding areas or don’t affect the coded

polypeptide– Missense mutations

» Amino acid works but not the way it’s supposed to (i.e., “close but no cigar”)

– Nonsense mutations» Functionless amino acid» Example: a premature stop code

• Base-pair insertions/deletions– Almost always lead to frameshift mutations

Errors• Chromosomal

– Nondisjunction• Leads to aneuploidy (abnormal # of chromsomes)

– Trisomic (2n+1)– Monosomic (2n-1)

• Polyploidy– Triploidy (3n)– Tetraploidy (4n) (gray tree frog)

– Chromsomal structure alterations• Deletion• Duplication• Inversion• translocation

Examples of Aneuploidy

• Trisomy 21: Down’s syndrome

Examples of Aneuploidy

Trisomy 13: Patau syndrome (heart and kidney defects among problems)

Examples of Aneuploidy

• Trisomy 18: Edwards syndrome (body wide effects)

Examples of Aneuploidy

• Klinefelter syndrome: XXY

Examples of Aneuploidy• XYY syndrome:

– Not defined as syndrome– taller than average– Violent tendencies? (Chino

prison study)

Examples of Aneuploidy• XXX

syndrome: relatively normal

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Chorionic villus sampling

&Amniocentesis