2.3. Track 2[[Student version, December 8, 2002]] 61
T 2 Track 2
2.3.4′ Since its enunciation in the 1950’s, several amendments to the above simplified picture of
information flow have been found. (Others were known even at the time.) Just a few examples
include:
1 ′.It is an overstatement to claim that all of the cell’s heritable characteristics are determined by
its DNA sequence. A cell’s entire state, including all the proteins and other macromolecules
in its cytoplasm, can potentially affect its descendants. The study of such effects has come
to be called “epigenetics.” One example is cell differentiation: Once a liver cell forms, its
descendants will be liver cells. A cell can also give its daughters misfolded proteins, or
“prions,” transmitting a pathology in this way. Even multiple clones of the same animal are
generally not identical.^8
Moreover, the cell’s DNA can itself be modified, either permanently or temporarily. Examples
of permanent modification include random point mutations (Chapter 3), random duplication,
deletion, and rearrangement of large stretches of the genome from errors in crossing-over
(Chapter 3), and insertion of foreign DNA by retroviruses such as HIV. Temporary, reversible
changes include chemical modification, for example methylation.
2 ′.Other operations, such as “RNA editing,” may intervene between mRNA synthesis and trans-
lation.
3 ′.Apolypeptide can be modified after translation: Additional chemical groups may need to be
added, and so on, before the finished protein is functional.
4 ′.Besides chaperones, eukaryotic cells also have special enzymes to destroy polypeptides that
have improperly folded.Problems....................................................
2.1All Greek to me
Now’s the time to learn the Greek alphabet. Here are the letters most often used by scientists. The
following list gives both lower and upper case (but omits the upper case when it looks just like a
Roman letter):
α, β, γ/Γ,δ/∆, ,ζ,η,θ/Θ,κ,λ/Λ,μ,ν,ξ/Ξ,π/Π,ρ,σ/Σ,τ,υ/Υ,φ/Φ,χ,ψ/Ψ,ω/ΩWhen reading aloud we call them alpha, beta, gamma, delta, epsilon, zeta, eta, theta, kappa,
lambda, mu, nu, xi (pronounced “k’see”), pi, rho, sigma, tau, upsilon, phi, chi (pronounced “ky”),
psi (pronounced “p’sy”), omega. Don’t call them all “squiggle.”
Practice by examining the quote given in Chapter 1 from D’Arcy Thompson, which in its entirety
reads: “Cell and tissue, shell and bone, leaf and flower, are so many portions of matter, and it is
in obedience to the laws of physics that their particles have been moved, moulded, and conformed.
They are no exception to the rule that Θ oς α ` `ιγ ωμ τ ρ ˆι.” From the sounds made by each letter,
can you guess what Thompson was trying to say? [Hint: ςis an alternate form ofσ.]
(^8) Identical twins are more similar, but they share more than DNA—they come from a common egg and thus share
its cytoplasm.