Download BIO.6

BIO.6
The student will investigate and understand common mechanisms of inheritance and protein synthesis.
Key concepts include
a) cell growth and division;
In prokaryotic cells, cell growth and division is by the process of binary fission. The circular strand
of DNA is first copied and then the cell divides. New membrane forms between the two DNA copies and
the cell pinches and constricts in the middle. A new cell wall then forms around the new membrane and the
cell pinches into two identical daughter cells.
Eukaryotic cells go through a cell cycle, consisting of several stages. 90% of the time is spent in
the interphase of the cell cycle consisting of several substages: G1 normal growth & metabolism; S
synthesis of new copies of DNA called chromatids. Each chromosome now has two chromatids (called
sisters) attached at a centromere. In the second growth phase, G2, the cell prepares to divide;
microtubules form and are arranged for division. Interphase ends as mitosis, the actual division, begins.
The microtubules attach to the centromeres and pull the sister chromatids apart to opposite poles of the
dividing cell; a new nuclear membrane forms around the chromosomes at each end of the cell. The last
phase is called cytokinesis in which the cytoplasm divides and a new cell membrane forms between the
two identical new cells.
Mitosis is divided into several phases: prophase (chromosomes condense & nuclear envelope
dissolves), metaphase (chromosomes line up along equator of cell with spindle fibers, i.e. microtubules
attached), anaphase (microtubules contract pulling sister chromatids apart at the centromere), and
telophase (nuclear envelope reforms and chromosomes uncoil).
b) gamete formation;
In eukaryotic cells, sex cells undergo a complex division, called meiosis, to produce haploid (one
copy of each chromosome) gametes from diploid (two copies of each chromosome) cells. This is an
important process because it allows tremendous variability to be introduced into the genome of a
population when gametes unite to form the next generation. Meiosis consists of two cell divisions. In the
first division, DNA is copied as it is in mitosis. However, in the second division, DNA is not copied, so each
cell gets only one chromosome of a pair.
In a diploid cell, there are two homologous chromosomes, each one coding for the same genes.
This homologous pair of chromosomes is not necessarily identical. For example, one chromosome may
have a version of the gene coding for straight hair and the other may have a version coding for curly hair.
When the DNA is copied prior to the first division, each chromosome of the homologous pair is copied and
joined at the centromere as identical sister chromatids. Now there will be two copies of the straight hair
version and two copies of the curly hair version. In the first division, the homologous chromosomes are
separated. One daughter cell will now have 2 copies of the curly hair version of the gene and the other 2
copies of the straight hair version.
These two cells are still diploid, with two copies of each chromosome in the form of sister
chromatids. In this second division, the sister chromatids will separate, each becoming one copy of a
chromosome in the now haploid daughter cells. There will be four daughter cells (two from each of the first
cell division) called gametes. Thus two gametes will have the curly hair version of the gene and two
gametes will have the straight hair version of the gene.
The incredible variety of genomes in eukaryotic populations arises from several factors. First, most
organisms have more than one chromosome. These chromosomes separate independently of each other
during the first meiotic division. Suppose, for example, that in addition to the chromosome containing the
gene for curly vs. straight hair, our organism had a second chromosome coding for black vs red hair.
During the first meiotic division, independent assortment of chromosomes would indicate that some
divisions would produce daughters with straight red hair versions, curly red hair versions, straight black hair
versions and curly black hair versions. Where n is the haploid number of chromosomes, the potential
variability is 2n or 4 possible combinations. If there were 3 chromosomes, the possible combinations would
be 23 or 8 possible combinations! Think what it would be with humans with 23 pairs of chromosomes!
There is another source of variability in this process. During the first prophase (first division) when
homologous pairs of chromosomes line up, crossing-over can occur. This is an event in which one
chromatid of one homologous chromosome exchanges a segment with a chromatid from the other
homologous chromosome. Now the sister chromatids are not necessarily identical, leading to four entirely
different gametes if there is only one chromosome, and sixteen different possible gametes if there are two
pairs of homologous chromosomes. In humans with 23 homologous pairs of chromosomes and with
random fertilization, independent assortment, and crossing over, the possibilities seem endless.
For illustrations of this process, please view the power point on meiosis.
c) cell specialization
In true multicellular organism, the tasks associated with maintaining life and reproduction are
assigned to different specialized cells. During development, cell specialization occurs. Stem cells are
those cells that have not differentiated into different specialized cells. All cells in an organism contain the
same genetic content. During development, some portions of the genome of some cell lines are inactivated
or become activated for different lengths of time compared to other cell lines. These cells develop into
tissues (a distinct group of cells with similar structure and function). Tissues can be organized into organs
with further differentiation of cells. Organs are then coordinated into organ systems such as the circulatory
system, the skeletal system, the respiratory system, etc.
d) prediction of inheritance of traits based on the Mendelian laws of heredity
Mendel carried out a series of breeding experiments on garden peas. What made his work unique
and significant is that he quantified his results. He was also fortunate in choosing garden peas. Had he
chosen snapdragons, genetics would have been set back by many decades as flower color in snap
dragons is not simple inheritance.
Mendel began by obtaining true breeding (purebred) plants for 7 different characters such as flower
color (purple or white), seed color (yellow or green), pod shape (round or constricted), etc. This means that
if he crossed a purple flowered plant with another purple flowered plant, he would always get purple
flowered plants. This was referred to as the parental generation or P generation.
He then crossed two purebred plants with different traits for the same character. For example, he
crossed a purple flowered plant with a white flowered plant. The offspring of this cross were referred to as
the F1 generation (F for filial). For each character studied, all of the offspring exhibited only one of the
traits. In the case of flower color, they were all purple.
His next cross involved two of the F1 generation. Among their offspring, the missing trait
reappeared in each character (1/4 of the offspring), in the F2 generation. The ratio of traits was always 3:1
for each character.
From this data, Mendel was able to identify dominant traits (exhibited by F 1 generation) and
recessive traits, the trait that reappeared in the F2 generation. We now call his characters “genes” and his
traits are called “alleles.”
His results can be illustrated and the outcome of crosses predicted using Punnett Squares.
R
R
Rr
Rr
r
r
Rr
Rr
The above Punnett Square represents Mendel’s first cross of a plant with a round seed pod (RR genotype)
with a constricted seed pod (rr). Capital letters represent dominant alleles and lower case represent
recessive alleles. The letter is taken from the first letter of the dominant allele. The letters across the top
represent the gametes produced by the purebred (homozygous dominant) round seed pod. The letters
down the side represent the gametes produced by the purebred constricted seed pod (homozygous
recessive). The pairs of letters within the boxes are the possible genotypes produced by this cross. All of
them are identical. From the round seed pod parent, the offspring could only get the round allele. From the
constricted seed pod parent, the offspring could only get the constricted allele. Since the round allele is
dominant, all of the offspring will have the identical phenotype (appearance) of a round seed pod and the
same genotype (Rr, heterozygous).
R
R
r
RR
Rr
1
r
Rr
2
rr
3
4
This Punnett Square represents the results for the F2 generation for round seed pods (dominant, R) and
constricted seed pods (recessive, r). There is a 50% chance of getting a round allele from each parent and
a 50% chance of getting a constricted allele from each parent. Thus ¼ of the offspring will be homozygous
dominant with the RR genotype (box 1); 2/4 or ½ of the offspring will be heterozygous dominant with the Rr
genotype (boxes 2 & 3). ¾ will thus exhibit the dominant phenotype of round seed pods (boxes 1 – 3); ¼
of the offspring will be the rr genotype or homozygous recessive and will exhibit the recessive phenotype of
constricted seed pods (box 4). Hence Mendel’s 3:1 ratio in his F2 generation.
You try it. Do a Punnett Square for a cross between a homozygous recessive constricted seed pod and a
heterozygous round seed pod. What phenotypic and genotypic ratios will be present in the offspring?
r
R
r
Rr
rr
r
Rr
rr
The phenotypic and genotypic ratios will be the same 1:1 Rr : rr and round : constricted.
http://www.kumc.edu/gec/lessons.html
A clearing house web site on genetics – lots of lessons & links
e) genetic variation (mutation, recombination, deletions, additions to DNA)
We have already covered variation introduced by recombination (see section on gamete
production, b above). When chromosomes are copied during the synthesis phase of the cell cycle,
mistakes can occur resulting in changes to the genetic material. Mistakes can also occur during crossing
over, as well as in the separation of sister chromatids. If these changes occur in sex cell lines, they may be
inherited and affect future generations.
Common chromosomal abnormalities include deletion – when a segment of DNA is left out.
Example – if the sequence of genes is 1 2 3 4 5, a deletion would be 1 2 4 5. These deletions can be a
segment of a chromosome and can include one or more genes, or it may be a portion of a gene or even a
single nucleotide. Another error is a duplication: 12345 becomes 1232345. Many segments of DNA in
large genomes are duplicated. There are many copies of key genes. This can be an advantage in
response times of a genome to an external signal demanding the activation of a duplicated gene. This has
obviously occurred successfully many times during evolution. Entire chromosomes can be duplicated.
This happens when sister chromatids do not separate during meiosis, resulting in a gamete with two copies
of one chromosome instead of only one copy. The fertilized zygote will then have an extra copy of a
chromosome. This is called trisomy. Trisomy 21 (an extra chromosome 21) is responsible for most cases
of Down Syndrome. There are also inversions in which a sequence is reversed (during crossing over)
14325 might be an example, as well as translocation, when a segment from another chromosome is
inserted into a different chromosome: 12345 may become 1238945.
All of these constitute mutations. The simplest form of mutation, and most common, is a mistake in
copying in which one base pair is substituted for another. This is called a point mutation. Chromosomal
abnormalities are observed thorough karyotyping, in which cells with condensed chromosomes are
photographed and the homologous chromosomes are matched up. The web page below involves
interactive karyotyping. Please go to this site and do some of the karyotypes.
http://www.biology.arizona.edu/Human_Bio/activities/karyotyping/karyotyping.html
f) the structure, function, and replication of nucleic acids (DNA & RNA)
As described earlier BIO.3, nucleic acids consist of chains of nucleotides. A DNA nucleotide
consists of a 5 carbon sugar, deoxyribose, a phosphate attached to the number 5 Carbon, and a
nitrogenous base attached to the number 1 carbon. The nucleotide below is linked through the phosphate
to the number 3 carbon above.
phosphate
C5
O
C4
C1
C3
base
C2
phosphate
C5
As said earlier, carbon must form four bonds. The lines in this figure represent bonds with carbon atoms.
Where there are not four bonds, the others are to hydrogen atoms. In ribonucleic acid (RNA) one of the
bonds of the number 2 carbon is to a hydroxyl group, not a hydrogen atom. The bases in DNA are
adenine, thymine, cytosine, and guanine. In complementary strands of DNA, adenine is always bonded to
thymine and cytosine to guanine, leading to the base pairs AT and GC. The complementary strand is
“upside down”. If one is oriented 5’ to 3’, then the other is oriented 3’ to 5’. In RNA, uracil replaces
thymine. It is the sequence of base pairs that makes up genes. Three nucleotides correspond to one
amino acid in proteins, the actual cellular expression of genes.
The copying or replication of DNA during the cell cycle is accomplished when one set of enzymes
(DNA helicases) unwinds a portion of the double helix. Another enzyme (DNA polymerase) is responsible
for matching up nucleotides with the proper complementary base and bonding it to the growing new strand.
Both strands are copied. Thus two identical DNA molecules are produced, sister chromatids.
http://pulse.pharmacy.arizona.edu/10th_grade/dawn_new/science/dna_rep.html
An edible DNA replication lesson
g) events involved in the construction of proteins
When it is necessary for a gene to become active, RNA polymerase binds to a promoter site on
one strand of DNA, and begins to assemble RNA nucleotides based on the DNA template. Only one strand
of DNA is transcribed into RNA. The sense strand is the 3’-5’ orientation (the other strand is non-sense in
terms of the genetic code) and the messenger RNA (mRNA) is in the 5’-3’ orientation. Transcription ends
when RNA polymerase encounters a “stop” sequence of nucleotides on the DNA template and the mRNA
molecule (single stranded) is released. Many strands of mRNA are being transcribed at the same time,
each one behind the other.
http://library.thinkquest.org/C004535/dna_transcription.html
The mRNA molecule then makes its way out of the nucleus through a nuclear pore and bonds to a specific
site on a ribosome in the cytoplasm (either free or attached, depending on the function of the protein coded
by the gene). It will attach at the 5’end. It is at this point that translation of the mRNA into a protein
occurs.
http://library.thinkquest.org/C004535/dna_transcription.html
Small segments of RNA with 3 nucleotides are called tRNA (for transfer RNA). Each of these segments
carries a specific amino acid that is coded for by the 3 nucleotide sequence (anticodon). These tRNA
molecules will bond to the mRNA, matching codons (3 nucleotide sequences on the mRNA) from the 5’ to
the 3” direction. When two tRNAs are bonded to the mRNA side by side, their amino acids form a peptide
bond. The first, now empty, tRNA drops off, the ribosome moves to the next codon, and another tRNA
moves in to attach its amino acid to the growing protein chain. As soon as there is enough room, another
ribosome attaches to the end of the mRNA and another protein molecule is initiated. When the ribosome
encounters the stop codon, it drops off and the completed protein chain is released. The three base
sequences on the mRNA are referred to as the genetic code.
http://learn.genetics.utah.edu/units/basics/transcribe/
An interactive web site for transcribing DNA and translating RNA using the genetic code
http://www.dnai.org/a/index.html
An interactive web site relating to DNA, RNA and protein synthesis. It stresses the contributions of
scientists to each part of the molecular genetics synthesis.
h) use, limitations, and misuse of genetic information; and
i) exploration of the impact of DNA technologies.
The human genome project has identified the base sequence and specific genes in the entire
human genome. Since it is now possible through electrophoresis to identify specific alleles and to
determine identity with absolute certainty, a number of issues of privacy have appeared. For example,
should it be legal to take the DNA of anyone who is arrested, whether they are convicted or not, and
maintain it in a DNA registry? Most of us have been exposed to a steady diet of the forensic uses of DNA
technology in the media… everything from paternity to rape or murder convictions. Should it be legal to
subject your DNA, however obtained, to analysis without a search warrant? It is possible to find out if you
are a carrier of a seriously deleterious allele. With genetic counseling and fetal analysis, what moral issues
are raised with regard to reproduction? Under what circumstances is it reasonable to risk giving birth to a
child with a serious genetic disease? If a person is tested for the breast cancer allele or the allele for
Huntington’s disease, should this information be available to insurance companies and/or employers?
Proprietary issues are also at stake. Who owns your genes, you or the company that sequences them?
Suppose you are immune to HIV. Whatever allele codes for that immunity has great financial value. Who
benefits?
Recombinant DNA technologies create new food crops with desirable properties. When should
such products be allowed into the food supply? For example, golden rice was genetically engineered to
contain beta carotene, thus reducing serious vitamin deficiency in large segments of the world population.
Roundup resistant corn was genetically engineered to make it possible to use a herbicide, Roundup, on
corn without damage to the crop. Genes for roundup resistance can (and have) make their way into other
varieties of corn that are used for food.
Stem cell research has received a lot of attention lately. Aside from the moral issues regarding the
acquisition of stem cell lines, testing of gene therapy for various genetic conditions is extremely risky. At
this point, there is no widespread use of gene therapy for any condition, although there are hopeful studies.
Please review the power points on Biotechnology Applications and Genetic Counseling
http://www.kumc.edu/gec/lessonpl.html This web site has hundreds of lessons on this topic.
After reviewing material in your textbook, go to the file labeled BIO.6 Review Response and open it
in Word. Type your answers below each question and make them a distinctive readable color or font. Email this file as an attachment.