Heredity:
Mainly Genes
1. Define the term locus and linkage using the terms genes and chromosomes.
The location of a gene is literally its location on a chromosome. If the loci of two
genes are on the same chromosome they are said to be linked. One could think of
a chromosome as a series of linked genes, like beads on a necklace. Genes that
are linked tend to be inherited together. If the genes are on different
chromosomes they are likely to be separated during meiosis.
2. What is the difference between a gene and an allele?
A gene is a length of DNA at a particular location (locus) on a chromosome. It has
a role in determining a particular characteristic. Sometimes there are alternative
forms of a gene. These alternatives are alleles and lead to variation in particular
characteristics.
3. What is the basic building block of DNA?
Nucleotide
4. What are the three major components of a DNA nucleotide? Two of these
components are constant- which are they?
- Deoxyribose sugar
- Phosphate (link sugars to form uprights of the DNA ladder)
- Nitrogen base (pair to form rungs of the DNA ladder)
The sugar and phosphate parts are constant in DNA.
5. In a nucleotide, one of the component’s chemical makeup can vary. Name the
four variations.
The nitrogen base is variable. Four bases are found in DNA: Adenine (A),
Thymine (T), Guanine (G) and Cytosine (C).
Difference in bases sequences is the basis of the genetic code.
6. What are complementary base pairs?
Bonds will only form between complementary base pairs. The following are the
complementary base pairs of DNA:
Adenine and Thymine
Guanine and Cytosine
7. What type of chemical bond holds the pairs together? How are these bonds
broken?
Hydrogen bonds hold the bases together. Two hydrogen bonds hold adenine and
thymine together while three hydrogen bonds hold guanine and cytosine
together
8. Describe the DNA replication process. It is a simple and error free process.
Explain.
Replication in DNA:
- Hydrogen bonds holding bases together break allowing complementary strands
to unzip.
- Free nucleotides are present in the nucleus.
- The enzyme DNA polymerase is involved in the synthesis of new DNA strands
on the original exposed strands which act as templates.
- For this reason, DNA replication is described as being semi-conservative. Half of
the new molecule is original and half is newly added.
- The result is two exact copies of the original DNA molecule (chromosome).
Replication is generally error-free. The major reason for this is the simplicity of
the mechanism – reliance on base pairing. If an adenine base is exposed, it is only
a thymine that can attach and so on. This simple pattern allows copying within
an organism and the possibility of transferring an exact message from one
generation to the next.
9. When does replication occur in the life cycle of a cell?
Replication occurs in cells before mitosis or meiosis during interphase. It
provides a double set of chromosomes. At this stage, chromosomes are present
as pairs of chromatids. During mitosis or meiosis the pairs of chromatids
separate to different cells. Replication allows maintenance of the quantity of DNA
per cell during cell reproduction.
10. Compare DNA replication and PCR by listing the similarities and differences
between the two processes.
Similarities
Differences
DNA replication
PCR
Occurs within the cell
Occurs in a test tube
Whole chromosomes
Small pieces of DNA are
DNA is copied
are copied
copied
Original strand is denatured Controlled by enzymes
Sample is heated
– strands separated
(helicase and gyrase)
Complementary bases on
Nucleotides are
Nucleotides are added
free nucleotides pair with
present in the nucleus
to the solutions
the separated strands
DNA polymerase assembles
Taq polymerase is used
nucleotides to the 3’ end of
because it is heat
the assembled bases
insensitive
DNA polymerase cannot
start synthesis – it only
extends DNA if a 3’ –OH is
The primer, usually
The primer is a
available at the end of the
RNA, is synthesised by synthetic sequence of
strand. A primer allows DNA
primase
DNA
polymerase to commence
synthesis by providing a 3’
end.
11. What are triplets, codons and anticodons?
The DNA code is a three letter (or base) code. Three bases code for a particular
amino acid. A sequence of three bases in DNA coding for one amino acid is called
a triplet. In messenger RNA a sequence of three bases coding for one amino acid
is called a codon. In transfer RNA a sequence of three bases coding for one amino
acid is called an anticodon. The codon is the complement of the triplet and the
anticodon is the complement of the codon.
12. In terms of nucleotides and triplets, what is a gene?
A gene is a piece of double-stranded DNA that codes information for a particular
function. Most genes code for the production of a protein. Genes vary in length
from thousands to millions of nucleotides. The order of nucleotides and thus, the
order of bases determine the type of protein formed. Three bases or a triplet
code for a particular amino acid. If a gene is three thousand bases long it can be
described as consisting of one thousand triplets. (Note: not all triplets in
eukaryotic genes code for amino acids. Non-coding sections or introns are edited
out during transcription.)
13. What information is encoded by a gene?
A gene codes for the production of a protein. All proteins consist of building
blocks called amino acids. Individual triplets code for particular amino acids.
Therefore a gene is a plan to construct a sequence of amino acids that form a
protein.
14. How many molecules of DNA do you have in most of your somatic (body)
cells?
One chromosome is a large molecule of DNA. You have 46 chromosomes in most
somatic cells. This represents 46 DNA molecules.
15. Approximately how many genes does a human have in a cell?
It is thought that humans have about 21 000 genes per cell.
16. Gene mutations are alterations to base sequences in genes. Describe the
three major types of base mutation.
Gene mutations alter the base sequence in a piece of DNA and can affect the
protein formed. Three types of base mutation are:
1. Substitution – replacement of one nucleotide by another.
E.g.
before
TTT AAG CCG TTG
after
TTA AAG CCG TTG (3rd T substituted for A)
Result: only 1 amino acid in the chain is affected.
2. Addition
E.g.
before
after
TTT AAG CCG TTG
TTA AAA GCC GTT G (base added to second triplet results in
changes to the following triplets)
Result: all amino acids coded by the bases at or after the addition will be
affected.
3. Deletion – removal of one or more nucleotides from a DNA strand.
E.g.
before
TTT AAG CCG TTG
after
TTA AAC CGT TG
(removal of the 6th base alters all later
triplets)
Result: all amino acids coded by the bases at or after the deletion will be
affected.
17. Explain why mutations are usually harmful. Which of the three types of
mutation are the least harmful? Why?
Mutations are generally harmful as they occur at random. It is unlikely to
improve the plan for a protein through random change. A mutation that causes
alteration in few triplets is likely to be less harmful because the sequence of
amino acids will be little altered. A single substitution will alter only one amino
acid so has a relatively small effect. Additions and deletions cause more
problems because they result in a frame-shift. As the code is read in threes,
removing or adding a base alters all triplets that follow. Note: addition or
deletion of three bases would be less harmful as fewer triplets are affected.
18. List some causes of mutation.
Environmental agents: ultraviolet radiation, X-rays, nuclear radiation and certain
chemical agents. The effects of chemical mutagenic agents may depend on their
concentration and length of exposure.
19. Distinguish between germ-line and somatic mutations.
Somatic mutations occur in somatic cells. As they do not occur in reproductive
tissue they are not passed on to the next generation. Somatic mutation can result
in cancer. Germ-line mutations occur during gamete formation and can be
inherited. Note: mutagenic agents may cause mutation but do not direct it.
Mutation is a random process. That is, they can occur anywhere and do not cause
changes in a specific piece of DNA.
20. Which sort of mutation can be beneficial to a species’ long-term survival?
Explain.
Germ-line mutations are the source of new genetic variation in a species. They
are the way in which new alleles for a gene are formed. The results of mutation
are generally harmful but very occasionally a new allele that is expressed as a
better adapted phenotype does occur. Individuals who are better adapted are the
key to a species’ long-term survival. Germ-line mutations can be passed on to
offspring and if a mutation is beneficial it will result in better adapted future
generations.
21. Introns and exons are parts of eukaryotic gene sequences. Which parts seem
to have the more important function?
Exons are the coding part of genes and seem to have the most importance. The
role of introns is less clear but they may have a role in controlling the decoding
process. During the production of a finished piece of mRNA the introns are
edited out.
22. What are the major differences between RNA and DNA?
The major differences between DNA and RNA are:
- DNA is found in the nucleus whereas RNA is found in the nucleus and cytoplasm
(mitochondrial and chloroplast DNA are exceptions).
- The base Thymine (T) is replaced by Uracil (U) in RNA.
- DNA is double stranded where RNA is single stranded.
- Deoxyribose sugar is found in DNA whereas ribose sugar is in RNA.
23. How many different codons are possible in the genetic code?
There are 4 bases. To forma triplet or codon, they are chosen three at a time. Any
base can be chosen for any of the three positions in the codon.
4
4
4
codon base positions
There are 4x4x4 = 64 possibilities for codons.
24. Are all codons needed? How many amino acids are there?
There are twenty different amino acids used to synthesise proteins. Sixty-four
codons are more than sufficient to specify the amino acids. In a number of cases
several different codons specify the same amino acid.
25. Do all codons code for amino acids?
Not all codons specify amino acids. Four codons act as punctuation – there is one
start and three different stop codons.
26. Compare the process of replication, transcription and translation. Not where
each occurs, what materials are involved and the products of each process.
Where process
Materials
Products
occurs
involved
DNA, free
2 molecules of
Replication
Nucleus
nucleotides and
DNA
DNA polymerase
DNA, mRNA,
nucleotides, &
Transcription
Nucleus
mRNA
mRNA
polymerase
mRNA, tRNA,
Ribosomes in the
Translation
amino acids, and
Protein molecules
cytoplasm
enzymes
27. Describe the production of a finished piece of mRNA. Include the role of
enzymes.
Production of a finished piece of mRNA involves:
- RNA polymerase attaches to DNA at the region where the gene to be
transcribed starts.
- the double-stranded DNA unwinds and unzips to expose the template strand’s
bases.
- complementary RNA molecules are added to exposed bases on the DNA
template under the control of RNA polymerase.
- the chain gradually lengthens to form a single-stranded pre-mRNA molecule.
- pre mRNA detaches from the template DNA.
- pre mRNA’s introns are removed and mRNA is capped and tailed to form the
final mRNA product.
28. List the major steps in protein synthesis.
Major steps in protein synthesis:
- transcription from the DNA template to produce a piece of mRNA.
- export of finished mRNA to ribosomes in the cytoplasm.
- attachment of ribosomes to mRNA.
- tRNA molecules in the cytoplasm become attached to specific amino acids.
- translation of the mRNA starts when a start codon is translated.
- tRNA molecules bring amino acids to the mRNA.
- the mRNA message is translated as a tRNA molecule brings its amino acid into
position. (A tRNA anticodon can only attach to its complementary mRNA codon.)
- bonds form between the amino acids.
- a chain of amino acids will form. The tRNA molecules detach and are ready to
pick up another amino acid from the cytoplasm.
- translation ceases when a stop codon is reached. At this stage, the ribosome
detaches from the mRNA molecule along with the product – a protein molecule.
29. Name two types of functions of genes. Explain whether all genes are
responsible for the production of a protein.
Genes can have one of two major functions. Structural genes are directly
responsible for a protein that is part of an organism’s structure or has a function.
The products of Regulator genes control the action of other genes by
determining whether a gene is active or not. In this way, the rate of production of
particular materials can be controlled.
30. What is the aim of genetic engineering (gene manipulation)?
The aim of genetic engineering is to isolate desirable genes in one cell and move
the gene into another cell. The movement of genes can be within a species or to
another species to form transgenic organisms.
31. What are restriction enzymes and how are they used?
Restriction enzymes are used to cut DNA into pieces. There are a range of
restriction enzymes and they differ in the places they cut DNA. A particular
restriction enzyme will only cut DNA at a specific cutting site or recognition
sequence.
32. What is a gene probe? How are they used? Mention autoradiography in your
answer.
A gene probe is a piece of single-stranded DNA (or RNA) that has a base
sequence complementary to a target piece of DNA. The gene probe is marked
with a radioactive label so that it can be detected using autoradiography – a
technique where film sensitive to radioactive emissions is used. Before the probe
can be used the target DNA has to be cut into fragments and then sorted using
gel electrophoresis. The DNA fragments are transferred to a membrane. They are
then denatured to separate the strands. A solution of the labeled probe is then
added to the DNA and the mixture allowed to cool. The probes may stick to the
target sequences of DNA. Their position can be identified using appropriate film,
which then identifies the position of the target DNA.
33. Given that geneticists can cut and rejoin pieces of DNA, how do they
transport DNA from source to target cells?
A method of transporting a piece of DNA from one cell to another is by bacterial
plasmids. Bacterial plasmids are circular pieces of DNA. Geneticists are able to
cut the plasmids and insert the DNA to be transported. Plasmids can pass across
cell membranes and so carry DNA from one cell to a target cell. Once inside the
target cell the plasmid and the inserted gene may be copied.
34. Explain polymerase chain reaction (PCR) and give examples of its use.
The polymerase chain reaction (PCR) is used to make multiple copies of a DNA
sample. This is useful for forensic tests when small tissue samples may remain. It
is also used in prenatal diagnosis to produce enough DNA to analyse for genetic
abnormalities.