Strand4LifeScience

Strand 4 Life Science

Source:  AZ Dept of Education

AZ Concept 2 Reproduction and Heredity

For some reason the State of AZ honed in on melanin and skin pigmentation.  Melanin is a small molecule derivative of the amino acid tyrosine. Wikipedia is a good starting point for finding a gene that plays a role in skin color.  Genes do not code for small molecules like melanin, but they do code for proteins that synthesize the small molecules. Wikipedia will direct the student to an overview of different genes that can result in ocular/cutaneous albinism.   Top on the list is an enzyme called tryosinase.   The name of this gene is OCA1 Let’s take a tour of websites that will teach your student AZ life science requirements with a protein of his or her choice.  In the following examples, the choice is the OCA1, aka TYR, gene that codes for tyrosinase.

Grades 6-8

LS3: Heredity: Inheritance and Variation of Traits
 Genes are located in the chromosomes of cells, with each chromosome pair containing two variants of each of many distinct genes. Each distinct gene chiefly controls the production of a specific protein, which in turn affects the traits of the individual (e.g., human skin color results from the actions of proteins that control the production of the pigment melanin).

Genecards is an excellent resource for finding information on genes.

the OCA1 gene is located on chromosome 11
The student types in the gene name, clicks on go, and learns that the OCA1 gene is located in the q14.3 band of chromosome 11

The student visiting the site on tyrosinase will learn that mutations in this enzyme albinism.  Tyrosinase also uses copper ions as cofactors.
Changes (mutations) to genes can result in changes to proteins, which can affect the structures and functions of the organism and thereby change traits.

Or some genetic changes may result in an amino acid substitution in the protein that does not affect the protein function.  UniProt.org offers an excellent resource that the 6-8 student can glean for this objective.  Instruct the student to type the name of his/her protein followed by “human.”  On this site we learn that albinism is autosomal recessive, meaning that one can get by with just one good copy of OCA1.  UniProt has all sorts of interesting information on the enzyme tyrosinase that the student may explore as much as he/she feels motivated.

LSc2c
A collage of images from Uniprot.org and image searches on the Internet. We learn that tyrosinase is an enzyme  located in melanosome membranes. Ethnic differences in skin pigmentation may be due to due to something other than the sequence of the tyrosinase enzyme. Note the amino acid sequence of tyrosinase is in single letter code.

Have the student paste the amino acid sequence of his/her protein in the report.  Adding a short diagram of the topology domains of the protein is helpful.  Amino acids 19-476 are on the inside (lumen) of the melanocyte.  Amino acids 477-497 go through the membrane.  Amino acids 498-529 are in the cytoplasm.   UniProt has a table with “natural variants” of human tyrosinase.  Many of the mutations are referenced by publications in the peer reviewed literature.

LSc2f
Mutations in the OCA1/TYR gene can alter the amino acid sequence. Changes in the amino acid sequence can alter tyrosinase activity and melanin production.

The other images show that ethnic differences in skin pigmentation are not solely explained by amino acid sequence of tyrosinase. Ask the student to search for images that guide his/her further understanding of the protein.  Questions don’t have to be answered at this stage in my opinion.  The goal is to get the student to form mental images of how she thinks work and to get asking more questions to form better mental images.

Sexual reproduction provides for transmission of genetic information to offspring through egg and sperm cells. These cells, which contain only one chromosome of each parent’s chromosome pair, unite to form a new individual (offspring). Thus
offspring possess one instance of each parent’s chromosome pair (forming a new chromosome pair).

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One would hope that this concept was introduced before the students to looking for the location for the gene of a particular protein of interest.  To reinforce this concept have the student put a diagram like this in the report and an arrow pointing to the approximate location of the gene of interest.  Students wishing to take things to the next level may visit Oline Mendelian Inheritance in Man  (OMIM.org.)  This site has a good discussion on the OCA1 mutation phenotypes.  Intuitively, a single amino acid substitution could slow tryosinase activity or totally obliterate it.  The degree of albinism in an individual is not as clear cut as one would think in a brief discussion of the experiments of Gregor Mendel.  Going beyond the state requirements teaches the student to think critically.
 Variations of inherited traits between parent and offspring arise from genetic differences that result from the subset of chromosomes (and therefore genes) inherited or (more rarely) from mutations. (Boundary: The stress here is on the impact of gene transmission in reproduction, not the mechanism.)

 

Have the students perform an image search of their gene and Mendelian inheritance.  In autosomal (non-sex  chromosome) recessive disorders, one can get by with just one good copy of a gene.
In sexually reproducing organisms, each parent contributes half of the genes acquired (at random)

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Grades 9-12

LS3: Heredity: Inheritance and Variation of Traits
 In all organisms the genetic instructions for forming species’ characteristics are carried in the chromosomes. Each chromosome consists of a single very long DNA molecule, and each gene on the chromosome is a particular segment of that DNA. The instructions for forming species’ characteristics are carried in DNA.

Let’s continue to look at the OCA1/TYR gene on Chromosome 11 to drive home how long each chromosome really is, ornl.gov is a good resource for diseases by various chromosomes.  It might be hard at first for a high school student to immediately comprehend what 134 million base pairs is all about.

LSc2_ChromosomeSize
Left Disease loci on Chromosome 11 from Oak Ridge National LaboratoryRight: Zooming in on the OCA1/TYR gene. Note the large number of allele variations in this short segment of DNA

Let the student zoom in and out of the gene region on NCBI just to drive home how much information is really contained in our chromosomes.  The allele variations (mutations) in just a short segment of chromosome 11 is rather thought provoking.  The overlap of the OCA1/TYR and GRM5 genes on the complement strand is also rather thought provoking.  https://www.ncbi.nlm.nih.gov/gene is a good place for anyone interested in biology to explore the human or other genomes.  It is very easy to end up spending hours on this site just browsing and asking questions.  Have the student prepare a report of what he/she has discovered in an allotted amount of time just browsing.

 All cells in an organism have the same genetic content, but the genes used (expressed) by the cell may be regulated in different ways. Not all DNA codes for a protein; some segments of DNA are involved in regulatory or structural functions, and some have no as-yet known function.

Regulatory elements may be promoters near the transcription start site and enhancers further upstream and enhancers up to one million upstream or down stream from the gene.

 

NCBI is calling the OCA1 gene TYR for tyrosinase.  Note that the complementary strand of the TYR/OCA1 gene is the promoter of the GRM5 gene.  U6 small nuclear 16 is a site for the mRNA splicing proteins to assemble.
 The information passed from parents to offspring is coded in the DNA molecules that form the chromosomes. In sexual reproduction, chromosomes can sometimes swap sections during the process of meiosis (cell division), thereby creating new genetic combinations and thus more genetic variation.

Wikipedia has a good pieces on chromsome translocation. and chromosome cross over.  Chromosome translocation is swapping of non homologous chromosomes and crossing-over is swapping of homologous chromosomes.
Although DNA replication is tightly regulated and remarkably accurate, errors do occur and result in mutations, which are also a source of genetic variation. Environmental factors can also cause mutations in genes, and viable mutations are
inherited. Environmental factors also affect expression of traits, and hence affect the probability of occurrences of traits in a population. Thus the variation and distribution of traits observed depend on both genetic and environmental factors

At this point it is no longer profitable to get at the TYR/OCA1 gene.  To really drive home this point might require testing environmental chemicals and the Ames/Reverse Mutation test.  When I was at the School of Public Health some of the professors had tested all sorts of house hold products using the Ames Test.  Smoke flavor and black hair dyes were strong mutagens.  Having the kids bring in stuff from home might drive home how fast bacteria “evolve.”  Positive results might generate repeat customers for the company performing the test.  I know of at least one company in Tucson that has the infrastructure to perform the Ames Test.