Biology Chapter 13.ppt - Wharton County Junior...

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Biology – Kevin Dees

Chapter 13Meiosis and Sexual Life Cycles


Reproduction

• Characteristic of all living things

• Reproduction also involves the transmission of traits from one generation to the next; inheritance– Heredity – Latin = heres, heir

• With inherited similarity there is also some variation– Offspring differ from parents and siblings

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Variation

• Essential to life on earth

• Produces biodiversity; formation of new species by natural selection

• Human ‘artificial selection’– Breeds of dogs; varieties of crops

• The study of heredity and hereditary information - Genetics


Genetics

• Study may occur at three levels:– Molecular level – DNA

– Cell – within a single nucleus

– Organism – hereditary information passed

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It’s in your genes!!!!Inheritance of genes

• Unit of heredity – Gene– Genes are segments of DNA– Sequences of bases along the sugar-phosphate backbone for a

genetic language– Most genes code for the synthesis of proteins

• The DNA of eukaryotic organisms is subdivided into chromosomes– Each species has a characteristic number of chromosomes;

humans have 46

• A gene’s location on a chromosome is termed the locus.


Where did you get those genes???

• Inheritance from parents – reproduction– Two basic forms of reproduction

• Asexual - no fusion of gametes

• Sexual – involves fusion of gametes

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Asexual reproduction• Single celled organisms reproduce

by mitotic division

• Multicellular organisms can also reproduce asexually– Budding by Hydra

• Organisms produced asexually are clones – genetically identical to parent

• How does change occur ? Mutations – changes ion DNA sequences


Sexual reproduction• Sex - fusion of gametes

– from the Latin – sexus (to divide; as in different sexes male and female)

– Male gamete – sperm– Female gamete – ova or egg

• Parents give rise to offspring that have unique combinations of genes; not clones

– Two essential questions have to be asked -

• How does this genetic variation arise?• How are offspring produced with the correct number of

chromosomes?– Recall that each species has a predetermined number of

chromosomes

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• In each human somatic (normal body) cell there are 46 chromosomes

• The 46 chromosomes occur as homologous chromosome pairs– One maternal chromosome– One paternal chromosome– Each homologous chromosome carry genes controlling the

same inherited trait

• Cells that have pairs of homologous chromosomes are said to be diploid (2n)– In humans, 23 pair is the diploid number

• Cells that have half of a homologous chromosome pair are haploid (n)– In humans – gametes (sperm and egg)


Karyotypes – used to observe cells ploidy level

– Karyotype• Ordered display of condensed chromosomes

• Cells arrested in metaphase

• Karyotypes are often used to screen for genetic anomalies; Down syndrome

• Karyotypes can also be used to determine sex –male or female

Sex chromosomes – in humans, 23rd pair

(Other 22 pair are known as autosomes)

XY -♂ XX -♀

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recall chromosome structure

• Replicated homologous chromosomes– Sister

chromatids

– centromere


What does this have to do with sex????

• Sex involves the fusion of gametes during fertilization forming a zygote (2n)

• Zygote – single cell

• Zygote grows and develops by mitosis

• In order to reproduce sexually the gametes must be haploid

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Need for cell division process to produce haploid cells for reproduction

• Meiosis– Cell division with single

replication of the genetic material followed by two consecutive cytoplasmic divisions

• Meiosis I

• Meiosis II

– End result :• Four genetically unique haploid

cells


Process of meiosis• Very similar to mitosis

• Be sure to note the differences!!!!

• These subtle differences allow for genetic variation in cells produced

• Begin with cell which has completed Interphase – DNA replication has occurred

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Genetic variation• Due to two processes which occur

during meiosis– Crossing over

• during prophase I

– Independent assortment • during metaphase I & II


Crossing over• Synapsis

– Replicated homologous chromosome pairs line up and are physically connected

– Form tetrads– Chiasma

• Point where non-sister chromatids overlap and genetic rearrangement occurs

• This produces recombinant chromosomes – carry genes from two

different parents

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Independent assortment

• metaphase I• Tetrads

arrange at metaphase plate

• There is a 50-50 chance that each tetrad will move to each pole


Independent assortment

• Metaphase II

• Sister chromatids line up at metaphase plate

• 50-50 chance for movement to each pole

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The possible number of combinations when chromosomes sort independently is

• 2n

– Where n = haploid number of the organism

– In humans• 223 = 8,388,608

• Couple this with probabilities which factor in crossing over and then random fertilization (one from male and one from female)

– 223 x 223 = 70,368,744,177,664 (>70 trillion)

– WOW – unique we each truly are!!!!


Differences between mitosis and meiosis

• Crossing over

• Tetrads

• Separation of homologous chromosomes

• Meiosis I -reductional division

• Meiosis II –equational division

Biology Chapter 13.ppt - Wharton County Junior...

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