1
Biology – Kevin Dees
Chapter 13Meiosis and Sexual Life Cycles
Biology – Kevin Dees
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
2
Biology – Kevin Dees
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
Biology – Kevin Dees
Genetics
• Study may occur at three levels:– Molecular level – DNA
– Cell – within a single nucleus
– Organism – hereditary information passed
3
Biology – Kevin Dees
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.
Biology – Kevin Dees
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
4
Biology – Kevin Dees
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
Biology – Kevin Dees
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
5
Biology – Kevin Dees
• 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)
Biology – Kevin Dees
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 -♀
7
Biology – Kevin Dees
recall chromosome structure
• Replicated homologous chromosomes– Sister
chromatids
– centromere
Biology – Kevin Dees
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
8
Biology – Kevin Dees
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
Biology – Kevin Dees
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
10
Biology – Kevin Dees
Genetic variation• Due to two processes which occur
during meiosis– Crossing over
• during prophase I
– Independent assortment • during metaphase I & II
Biology – Kevin Dees
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
11
Biology – Kevin Dees
Independent assortment
• metaphase I• Tetrads
arrange at metaphase plate
• There is a 50-50 chance that each tetrad will move to each pole
Biology – Kevin Dees
Independent assortment
• Metaphase II
• Sister chromatids line up at metaphase plate
• 50-50 chance for movement to each pole
12
Biology – Kevin Dees
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!!!!
Biology – Kevin Dees
Differences between mitosis and meiosis
• Crossing over
• Tetrads
• Separation of homologous chromosomes
• Meiosis I -reductional division
• Meiosis II –equational division
Top Related