Developmental biology

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Surname 1

BIOL 316
Take Home Quiz 1 36 pts total
Dupressoir et al., 2009

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Read questions carefully: many require more than one answer!
Do not copy and paste answers from the paper- write in your own words!
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on Ecampus
1. Function and origin of syncytins (8 pts):
a. Where did the syncytin genes come from?
Syncytin emerged from a virus. Syncytin is an envelope gene meaning
that it is a viral gene. It, therefore, originated as a result of a retroviral
infection. The basic expression of envelope gene allows a retroviral
element to target and integrate with particular cell types easily. The
retroviral infection entry into the primate descent can be traced back
to 25-40 million years ago. The syncytin gene is specifically expressed
in the placenta. Accordingly, syncytin 1 is viewed as an example of
contemporary attainment of a HERV gene (endogenous retrovirus).
b. How did they end up in the mammalian genome
Syncytins originated from a virus but ended up in the mammalian
genome. It is therefore clear that that specific virus must have infected
various mammals, and the resultant concern is how it did so. The
rationale for this is that, when the virus is incorporated into embryo
germ cells, the same can be transferred to the next generations.
Therefore, they ended up in the mammalian genome when virus
invaded either sperm or eggs of mammals and consequently changed
the genetic code. The virus targeted the DNA within the egg or sperm.
It then integrated its viral-structured DNA within that of the host, and
when a viable offspring was produced, then the viral gene that resulted
into syncytins was transferred to the next generations of mammals.
c. What do cells in vitro (in a culture dish), do when expressing
syncytins?
When cells in vitro are expressing syncytins, there is usually a
stimulation of cell to cell fusion. This almost certainly occurs through
interactions with receptors.
d. What was the ancestral (original) function of syncytin proteins?
The ancestral function of syncytin proteins involved the integration of
particular bacterial elements within animal cells. This was a significant
function during the evolution process as it also led to the development
of elements such as mitochondria. Soon after, the primary function of
syncytin proteins was encoding a protein within placental cells. It is

Surname 2
influential in the development of a cellular layer allowing placenta and
uterus fusion.
2. The authors generated syncytin-A knockout mice using homologous
recombination, which introduces a piece of DNA into embryonic stem cells
that lead to the removal of the gene of interest from the genome. The resulting
cells are typically heterozygous and are implanted into a female uterus so they
can develop into SynA +/- mice (8 pts).
a. What was the phenotype of heterozygous knockout mice (SynA +/)?
The heterozygous knockout mice had syncytin A (+/-) which proved to
be phenotypically unclear. However, the heterozygous mice were
fertile, viable and illuminated no apparent phenotypic flaws.
b. When two SynA +/- mice were crossed with each other,
heterozygotes (+/-) and homozygous wild-type (wt +/+) young
were obtained at “the usual Mendelian ratio.” What is this ratio for
these two genotypes?
After the intercross of two SynA +/- mice, no feasible homozygous null
offspring was established. On the other hand, wild-type and
heterozygous offspring exhibited (after computation of Mendelian
ratio) a reality that syncytin is crucial for the embryo and lack of it is
lethal. The ratio was 29(14) on E13.5 and 13(13) on E14.5.
c. What predicted the percentage of offspring of this cross were expected
to be homozygous knockouts (Syn -/-)?
d. What was the actual percentage of Syn -/- offspring? Explain your
answer.
The actual percentage of Syn -/- offspring was 0. T...


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