Download Topic 4 Genetics

Topic 4 Genetics
4.1.1 State that eukaryotic chromosomes
are made of DNA and proteins.
[The names of the proteins ( histones)
are not required, nor is the structural
relationship between DNA and the
proteins.]
DNA is very long, in order to manage
it within a cell, it is wound around
histones to consolidate it. The term supercoiled, refers to the DNA tightly coiled up
prior to mitosis or meiosis. This is when
chromosomes are visible.
4.1.2 Define gene, allele, and genome
[Gene: a heritable factor that controls
a specific characteristic. ( The difference
between structural genes, regulator genes,
and genes coding for tRNA and rRNA are
not expected at SL]
Humans have around 25,000 different
genes. Note that another reason different
organisms have similar DNA sequences is
because all organisms need to make tRNA
and rRNA… just like all organisms have
the same enzymes and proteins to respire
with. If you have the same chemical
pathways, you have the same genes.
[Allele: one specific form of a gene
differing from other alleles by one or a
few bases only and occupying the same
gene locus as other alleles of the gene.]
You get one set of alleles from your
mom, and one from your dad. Which
allele that gets expressed depends upon
which allele dominates the other….
Sometimes both alleles are expressed (
codominance) Humans have 3 possible
alleles for blood type: type A, Type B, and
Type O.
[Genome: the whole of the genetic
information of an organism]
Remember the human genome
project. The original human genome was
determined from a mix of 18 peoples’
DNA. We have currently completely
mapped only two individuals DNA: James
Watson, and Craig Ventner. It costs
about $1 million to map an individuals’
genome.
4.1.3 Define Gene mutation
[ The terms point mutation or
frameshift mutation will not be used]
A mutation is any change in the DNA
sequence. I will expect you to know what
a point mutation or frameshift mutation
is.
Mutations can be at the chromosome
level. This includes polyploidy, which is
the cause of Down’s syndrome, Turner’s
syndrome and Kleinfelter’s syndrome.
During meiosis, a chromosome fails to
separate from its copy. This leaves a
gamete with either an extra chromosome,
or a missing chromosome. Usually, this
results in spontaneous abortion.
Other chromosome level mutations
are translocations and crossing over.
Pieces of the chromosome break off and
re-attach elsewhere, or exchange with
homologous alleles.
Mutations can occur at the base-pair
level. Point mutations involve a single
change in a nucleotide base. This may or
may not result in a different amino acid…
which may or may not result in the
protein having a different shape. Sicklecell anemia is a point mutation ( see
4.1.4), as is Cystic Fibrosis.
Frame shifts involve deletions or
additions of base pairs. This throws off
the triplet codon code, and can result in
dramatically different amino acid
sequences. The worst frame shift occurs
at the beginning of a gene, since every
codon after it is potentially different.
Ultra-violet light can cause frame shifts.
The light causes adjacent thymines to
break their bonds with adenine, and form
bonds with each other. Correcting
enzymes cut out the adjacent thymines,
and form a frame-shift.
4.1.4 Explain the consequence of a base
substitution mutation in relation to the
processes of transcription and translation,
using the example of sickle-cell anemia.
[GAG has mutated to GTC causing
glutamic acid to be replaced by valine,
and hence sickle-cell anemia.
There has been natural selection
occurring in humans in favor of sickle-cell
anemia in malarial areas. Only the
homozygous condition gives sickle cell
anemia. The heterotrophic individuals are
more resistant to malaria. The
plasmodium protist does not do well in
the slightly misshapen red blood cells of
the heterotrophic individuals.]
Transcription is DNA to RNA,
translation is RNA to protein. The base
substitution occurring in DNA can affect
the amino acid sequence of the protein,
and therefore its shape. Shape is
everything for a protein.