Genetics class webpage:

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Genetics class
webpage http//www.biol.vt.edu/faculty/tu
rner/biol2004
Password mendel No caps---all
lower case letters for both URL and password
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• SOME QUESTIONS TO TEST YOUR KNOWLEDGE
• When does DNA replication occur during the
  process of cell division?
• When does it occur during meiosis?
• What is the difference between a chromosome and a
  chromatid?
• When does a chromatid become a chromosome?
• How do mitosis and meiosis differ?
• 6. What is synapsis, where does it occur, and
  what is its significance?
• 7. What properties determine if chromosomes
  synapse?
• 8. What are chiasma, where do they occur and
  what is their significance?

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• 9. What are tetrads, dyads and monads?
• What is a bivalent.
• 11. How does the behavior of chromosomes
• during meiosis explain Mendelian
  inheritance?

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Some terms to think about N the number of
chromosomes in the gametes of a particular
organism. The haploid number. 2N the number
of chromosomes in the somatic cells of a
particular organism. The diploid number. C
the amount of DNA in the gametes of a
particular organism.
Usually reported in picograms (10 -12 gm). 2C
the amount of DNA in the somatic cells of a
particular organism.
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The bookkeeping of mitosis
Stage cells csomes ctids Amt
DNA Cmeres Interphase (ltS) 1
2N 2N 2C 2N
Interphase (gtS) 1 2N 4N
4C 2N Prophase
1 2N 4N 4C
2N Anaphase 1
4N 4N 4C 4N
Telophase 2 2N 2N
2C 2N
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The
bookkeeping of meiosis stage cells
csomes ctids cmeres amt DNA division type
Proph I 1 2N 4N
2N 4C Teloph I
2 N 2N N
2C reductional Proph II 1
N 2N N 2C Teloph
II 2 N N N
C equational Net result 4
N N N
C In females, only one cell per meiotic
division is an active gamete, the other 3
division products are polar bodies.
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A bivalent or tetrad
chiasmata (pl)
chiasma
chromatids
2 homologous chromosomes at synapsis
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Proph I. (synapsis) Tetrads or
Bivalents
(4 strands or ctids, 2 homologs)
Anaph I -gt Proph II Dyads (2
strands or ctids) Anaph II -gt Teloph
II Monads (1 strand)
mature gametes
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How many chromosomes are there in a somatic cell
at mitotic prophase? What is the DNA content
of a somatic cell at mitotic prophase?
How many chromatids are there in a somatic cell
at mitotic prophase? How many centromeres
are there in a somatic cell at mitotic metaphase?
How would these answers change if the cell was
at mitotic anaphase? Telophase?
Early interphase (before S)?
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How many chromosomes are there in a somatic cell
at mitotic prophase? Ans. 2N What is the
DNA content of a somatic cell at mitotic
prophase? Ans. 4C How many chromatids are there
in a somatic cell at mitotic prophase? Ans.
4N How many centromeres are there in a somatic
cell at mitotic metaphase? Ans. 2N. How would
these answers change if the cell was at mitotic
anaphase? Telophase? Early interphase (before
S)?
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The human genome contains 23 pairs of
chromosomes. What is the probability that a
gamete you make contains all 23 chromosomes you
got from your mothers egg (i.e., none from
your father)?
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The human genome contains 23 pairs of
chromosomes. What is the probability that a
gamete you make contains all 23 chromosomes you
got from your mothers egg (i.e., none from
your father)? Ans. 1/223 or 2-23 Meiosis
(and sexual reproduction in general) is a machine
for generating genetic diversity
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For a given pair of homologs, the dyad products
of MI are obviously not genetically identical
(MI separates or disjoins homologs). But MII
separates the monads (ctids) that compose a
dyad. Are these sister ctids really genetically
identical? Or, in other words, MI is clearly a
reductional division, but is MII necessarily
equational?
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For a given pair of homologs, the dyad products
of MI are obviously not genetically identical
(MI separates or disjoins homologs). But MII
separates the monads (ctids) that compose a
dyad. Are these sister ctids really genetically
identical? Or, in other words, MI is clearly a
reductional division, but is MII necessarily
equational? Ans. The strands in a dyad
are not necessarily genetically identical
because of crossing over. Chiasma during proph.
I result in exchanges between non-identical
strands in a tetrad. There is almost always
at least one chiasma, and there are often many
more. Thus, calling MII an equational division
is a (useful) oversimplification.