Unit 8 - Control of Gene Expression

Question 1

what causes gene mutations?

Answer 1

DNA replicational errors in interphase generally by substitution, addition or deletion of bases from the normal DNA sequence
occur naturally

Question 2

what increases the rate of mutations?

Answer 2

mutagenic agents:
UV light
ionising radiation
chemical carcinogens e.g. tobacco tar & asbestos

Question 3

define mutagenic agent

Answer 3

a factor that increases the rate of gene mutations

Question 4

most mutations have…

Answer 4

a negative/neutral impact (selected against) on the organism but some are selected for (as they create beneficial alleles) by natural selection

Question 5

what are the 3 types of base substitution mutations?

Answer 5

silent mutation
mis-sense mutation
non-sense mutation

Question 6

describe silent mutation

Answer 6

base substitution - one base substituted for another
new codon codes for same amino acid
because DNA code is degenerate
so no effect on 1y structure
so no effect on 2y or 3y structure
so protein function unaffected

Question 7

describe mis-sense mutation

Answer 7

base substitution - one base substituted for another
new codon codes for a different amino acid
so different 1y structure
so different 2y & 3y structure due to different H bonds, ionic bonds or disulfide bonds b/w R groups
so different specific 3d shape & function

Question 8

describe non-sense mutation

Answer 8

base substitution - one base substituted for a stop codon
causes premature translation
shorter 1y structure
so different 2y & 3y structure
different H bonds, ionic bonds & disulfide bonds formed b/w R groups
different specific 3d shape
loss of function - no ESCs formed if about enzymes

Question 9

describe addition & deletion mutations

Answer 9

addition - a base is added to the DNA sequence
deletion - a base is removed from the DNA sequence

causes frameshift, shifting the last base of each codon into the next codon to produce a different sequence downstream of the mutation
addition causes frameshift to the right
deletion causes frameshift to the left
so different 1y structure
different 2y & 3y structure

NB addition/deletion of bases in multiples of 3 does not cause frameshift so is less detrimental to the overall protein function

Question 10

describe base duplication mutation

Answer 10

one or more bases are repeated, causing frameshift to the right
different 1y, 2y, 3y etc.

Question 11

describe inversion of bases mutation

Answer 11

a group of bases becomes separated from the DNA sequence & re-joins at the same position but in the inverse order
different 1y, 2y, 3y etc.

Question 12

describe translocation of bases mutation

Answer 12

bases are separated from the DNA sequence on one chromosome & re-join on a different chromosome
affect gene expression & phenotype
can lead to cancer

Question 13

summary of protein synthesis

Answer 13

all cells (apart from gametes & RBCs) have the same DNA/genes but they express the genes differently = control of gene expression
transcription factors control which genes are transcribed
splicing
siRNA can destroy mRNA molecules
control of how much mRNA is translated
activity of a protein can be altered by other enzymes - secondary messenger model

Question 14

all cells in the body…

Answer 14

have the same DNA/genes but have different structures & functions
they are specialised (differentiated) for a specific function & only transcribe & translate the proteins they need

Question 15

sperm + egg =

Answer 15

zygote

Question 16

describe totipotent stem cells

Answer 16

a fertilised egg/zygote
early cells derived from zygote by mitosis for a limited time in a mammalian embryo
unspecialised
capable of differentiation into any specialised cell
able to divide for long periods = self-renewal

Question 17

how do totipotent stem cells develop,

Answer 17

translate only part of their DNA, leading to cell speicalisation

Question 18

describe pluripotent stem cells

Answer 18

embryonic stem cells in humans
can give rise to most cell types needed for a foetus to develop
cannot form placental cells

Question 19

what is the medical use for pluripotent stem cells?

Answer 19

can divide an unlimited number of times (self-renewal)
& can be used to treat a variety of human disorders e.g. genetic disorders like type 1 diabetes (B cells in pancreas) & paralysis (nerve cells)

Question 20

describe multipotent stem cells

Answer 20

adult stem cells e.g. in bone marrow
can differentiate into a limited range of specialised cells

Question 21

describe unipotent cells & e.g.

Answer 21

cells that can only divide to form one cell type e.g. formation of cardiomyocytes

Question 22

why is specialisation irreversible?

Answer 22

most animal adult cells are specialised & unable to divide
(stem cells replace them by mitosis)
although cells retain all the genes of an organism, many genes are permanently switched off (not expressed)

Question 23

why is it controversial to use pluripotent embryonic stem cells?

Answer 23

right to life
embryo cannot consent
human rights argument

Question 24

describe induced pluripotent stem cells

Answer 24

generated by using appropriate protein transcription factors
similar to embryonic stem cells
capable of self-renewal
could replace embryonic stem cells in medical research & treatment
would not have the same ethical & rejection (iPS made from patient’s adult cells) issues associated with the use of embryonic stem cells

Question 25

how is transcription of target genes controlled?

Answer 25

transcription of target genes can be stimulated or inhibited when specific transcription factors move from the cytoplasm to the nucleus

Question 26

why might embryonic pluripotent stem cells lead to harm?

Answer 26

might differentiate into the wrong types of cells
might divide out of control, leading to a tumour/cancer

Question 27

why are embryonic pluripotent stem cells suitable to treat damage?

Answer 27

they can replace any type of cell