Molecular Embryology I

Question 1

What is an ectopic pregnancy? What is the most common place it occurs? What is the
frequency of ectopic pregnancies?

Answer 1

Abnormal site of implantation. Vast majority of ectopic pregnancies occur in uterine
tube – tubal pregnancy. 1-2% of all pregnancies are ectopic

Question 2

Are studies carried out in mouse relevant to humans? Give an example

Answer 2

Human and mouse: similar genes and similar development. A mutation in kit gene
(required for development and maintenance of pigment cells), leads to similar white
patches on their foreheads.

Question 3

Give two examples of studies carried out in lower eukaryotes, making it relevant to
humans.

Answer 3

Fruit fly gene (Engrailed-I) works fine in mouse. Pax-6 controls eye development and
Pax-6 from other species can grow in Drosophila fruit fly

Question 4

In most cases, ________ are interchangeable between species; _______ are less
interchangeable

Answer 4

genes (coding regions); regulatory regions

Question 5

Without survival signals, cells die. What is a survival signal for muscle? And what are
general cell signals?

Answer 5

ACh. General cell signals – survival, differentiation, division

Question 6

What is the molecular nature of factors responsible for differentiation? How does this
relate to cancer cells?

Answer 6

Protein factors, peptides, fatty acids, thyroid hormone, etc. cause cell differentiation.
Differentiation is specialization and dedifferentiation (back towards embryo). Cancer cells
invariably dedifferentiate and express many embryonic genes.

Question 7

What is the origin of embryonic stem cells? From which tissue can embryonic stem cells
be obtained?

Answer 7

Blastocyst (early embryo)

Question 8

What are the four essential processes by which a multicellular organism is made of?

Answer 8

cell proliferation, cell specialization, cell interaction, cell movement

Question 9

What are exceptions to ‘most human cells have all the DNA intact’?

Answer 9

RBCs, T & B lymphocytes, etc.

Question 10

Give an example of how during development, a different subset of genes is inactivated in
different issues.

Answer 10

Liver has methylation in controlled region of insulin, resulting in inactive insulin.
However, albumin gene is active. Pancrease has methylation in controlled region of albumin, but has an active insulin gene

Question 11

Why do bacteria use methylation strategy?

Answer 11

To protect their DNA from restriction endonucleases

Question 12

Methylation pattern in DNA can be inherited. How does this process work?

Answer 12

Methylated DNA -> DNA replication -> Hemimethylated DNA -> (fully) methylated
DNA

Question 13

During mitosis and meiosis, most _______ are removed.

Answer 13

DNA methylations. Sperm and egg DNA is not methylated and virtually all genes are
active at fertilization. Exception = imprinted genes

Question 14

What is imprinting?

Answer 14

Humans have two copies of each gene (except in males, XY).
In few cases (about 80 genes), only one of the genes is expressed. This is because of GC
methylations in the imprinted genes on one of the chromosome are left alone / or
methylated very early. Result: one allele on autosomes (maternal or paternal gene) is off.

Question 15

What is XIC and XIST? What do they act to do?

Answer 15

XIC – X inactivation center. XIST – X inactivation specific transcript (a noncoding
RNA). One of two X chromosomes in females randomly gets inactivated.

Question 16

How is the inactive X chromosome organized?

Answer 16

Transcriptionally inactive structure: heterochromatin

Question 17

What are two ways of making sister cells different?

Answer 17

Asymmetric division: sister cells born different. Symmetric division: sister cells become
different as result of influences acting on them after birth

Question 18

Ca2+ concentration in a neuronal cell is high in dendrites, low in the cell body and axon,
and intermediate in between. What’s the concept behind this?

Answer 18

Ions (membrane channels) can distribute asymmetrically, as can proteins and mRNAs

Question 19

What is gene expression regulated by?

Answer 19

Binding of transcription factors to regulatory regions in DNA. Regulatory DNA
sequences are usually in the 5’ flanking region; they can be in 3’ flanking or also in the
coding region (usually in introns)

Question 20

Why is DNA bent?

Answer 20

Protein binding often alters DNA shape. Remember how DNA is packaged in
nucleosomes.

Question 21

A protein heterodimer binds to DNA regulatory region. What class of proteins is this?

Answer 21

Helix-loop-helix (HLH) proteins

Question 22

What happens with transcription factors with no DNA binding domain as a result of
alternative splicing?

Answer 22

They form inactive heterodimers; no transcription.

Question 23

Random inactivation of one of the X chromosomes in females is initiated by

Answer 23

X-inactivating non-coding RNA(s)

Question 24

What are main principles behind triplet repeat diseases?

Answer 24

Autosomal dominant. Triplet repeat diseases happen in the germ line so they are in
inherited. Get worse through successive generations. Longer repeat, more severe disease.
Repeat can be in protein coding region or in 5’ or 3’ UTR

Question 25

How do cadherins act in embryological development?

Answer 25

Cadherins form homodimers and form tissues from cells expressing same cadherin.
Note E-cadherin in ectoderm, N-cadherin neural tube. Neural crest does not E- or Ncadherin.

Question 26

How are different cadherin mRNA expressed in different neural tissues

Answer 26

Alternative splicing of cadherin mRNA

Question 27

What happens when you mix cells expressing different cadherins or different levels of
the same cadherin?

Answer 27

Cell sorting to group similar cadherins