Skip to main content
Social Sci LibreTexts

2.4: Genetics, Cellular Biology, and Variation (Part 2)

  • Page ID
    6657
  • [ "article:topic", "authorname:aschoenberg" ]

    DNA

    Deoxyribonucleic Acid is the chemical name for DNA we use to talk about its chemical structure. It comes in packages, called chromosomes which we can see in the microscope when it's all natted up as big dreadlocks inside the cell nucleus, when the cell is about to divide. Try not to confuse the doubling-thing in DNA and chromosomes because there are four very different kinds of doubling that happen at different times and scales, listed here from big to small:

    1. First, the biggest scale of doubling is the pairing up of duplicated homologous chromosomes at the beginning of meiosis (metaphase I), but this doesn't last long. Karyotype pictures are often taken during this phase, so you see what looks like four chromosomes for each of the 23.
    2. Second, for most of cell's life it has two of each of the 23 chromosomes, in what are called homologous pairs. The pair comes from fertilization, you get one from your mom and one from your dad; this is what Mendel was studying. Another pairing at this same scale is the duplicated sister chromatids after replication, and before the beginning of meiosis mentioned above.
    3. Third, if you compare the upper and lower part of each of the 23 in a karyotype, you see the sister chromatids are joined in the center by a centromere, and the upper and lower parts of each side are called arms. The centromere is never exactly in the middle, so every chromosome has a shorter arm, p (for petit), and the longer arm, q. So, the X chromosome was named because after replication it has two sister chromatids joined in the middle by a centromere, and it looks like an "X", a dot with four arms.
    4. Fourth, the smallest scale of doubling is at the structural level: the complementary strands of DNA wound together in a double helix. So if you have an A on one side, you have a T on the other; if you have a G on one side, you have a C on the other. These complementary strands are like the two sides of a zipper that come apart for protein synthesis and replication.

    Replication

    DNA likes to make copies of itself.

    Protein Synthesis

    There are two main stages in protein synthesis: transcription and translation. It starts in the nucleus with transcription, where enzymes take the message from the DNA and transcribes it into messenger RNA (mRNA). The mRNA takes the message out of the nucleus into the cytoplasm to the ribosome.

    Translation occurs when the ribosome reads the message and puts the right amino acids in the right order. The ribosome needs help to gather and place the amino acids and uses transfer RNA (tRNA). The tRNA has an amazing property that a combination of three bases (codon) will stick to a particular amino acid, and as the ribosome reads the message from the mRNA it use the tRNA to transfer the correct amino acid in the correct order to make a protein.

    After protein synthesis, the protein can leave the cell and do whatever it needs to do to keep the individual alive.

    Check out this groovy example of hippy science from Stanford University in 1971: protein synthesis reenactment (the trip kicks-in at around 3:13)

    A gene is a discrete sequence of DNA nucleotides.

    Often one gene makes one protein, but not always. The expression of genes is influence by the enivorment. Some proteins require many gene. Some genes produce more than one protein.

    Polygenic Traits

    Polygenic traits are determined by a combination of many genes. For example, the hemoglobin protein takes 4 genes (6 for fetuses) and we'll look at a tiny change in one of the four genes that causes Sickle Cell anemia.

    Pleiotropic Genes

    Pleiotropy is where a single gene can effect multiple traits.

    Locus > Gene > Allele

    The words locusgene, and allele can be very confusing, especially since people (including myself) get lazy and use them interchangeably, but technically they are distinct and you may as well get in the good habit of using them correctly. Locus has the same root as "location", and it refers to a place on a chromosome where a string of bases will fit. If that string of bases codes for a protein (a functional product), then that spot is also a gene. But in many cases, there can variations as to what bases fit in that same spot, and each of those possible variations is called a different allele, and often codes for different proteins and change the organism. In summary, if a locus does something it's a gene, all the different versions of the gene are alleles.

    Introns and Exons

    One of the ways to increase variation is have genes modify other genes, and there are many stages of protein synthesis where that can happen, one of them being during transcription where parts of the code are cut out.

    Cells and the Source of Variation

    The origin of all variation is mutation. Mutations can occur at many different scales. The smallest is the Single Nucleotide Polymorphism (SNP, called a "snip") that changes a single base.

    If we think of meiosis as sorting and delivering genetic information to our kids in packets that we call chromosomes, there are many parts of the process where variation can occur, including: 1) meiosis shuffles the packets of informations in recombination as Mendel proved in his Principle of Independent Assortment, 2) meiosis moves traits from packet to packet in crossing over, and 3) meiosis can deliver an extra packet in non-disjunction.

    Another possibility is that two different packages can stick together, like the chromosomal shifting that fused the greater ape chromosome 2a and 2b, into Homo sapiens chromosome 2. The same genes are there, just on one chromosome instead of two. These big changes are important for macroevolution (speciation), and makes it impossible for humans to reproduce with other apes (apologies to Jerry Springer and the Weekly World News).

    The use of the word recombination can be confusing because most of the recombination that makes you different from your siblings is Mendel's principle of independent assortment, the shuffling of chromosomes, but there is also a type of recombination called crossing over that is a very different specific process on a smaller scale where genes jump between homologous chromosomes. Also, recombination is technically not a separate force of evolution, it's an aspect of all of them.

    Because this is an introduction we have skipped many other processes that influence variation, such as genes that don't produce proteins directly, but just effect other genes that do. But, most genes make proteins, so lets get the basics down first.

    for another review read Dennis O'Neil's description of mutation and recombination

    LaPlasmogenia.jpg

    Figure \(\PageIndex{5}\)La Plasmogenia (in Spanish) Alfonso Luis Herrera introduced Darwin to Mexico

    image085.gif

    Figure \(\PageIndex{6}\) - Leonardo DaVinci notebook

    Imagination Activities

    • Extract your own DNA

    Follow the instructions on this video, and take a selfie with your own DNA. 
    For each cell, how many membranes does the soap have to break down to release the DNA?

    Imagination questions

    • Your tax dollars at work

    The Human Genome Project is federally funded. Do you think it's worth it?

    • Exploration

    Take a few minutes to explore the human genome with MapViewer at the National Center for Biotechnology Information. How is exploring our genome different from exploring unknown jungles, or the bottom of the sea, or outer space?

    • Life

    Is a virus alive? A DNA molecule?

    What of humans is mechanistic? Where does free-will exist?

    • The whole is greater than the sum of its parts

    What is a person? A bunch of cells. Each individual is made up of about a trillion cells (1,000,000,000,000). Most of those cells have 46 chromosomes. Each chromosome has about a million and a half base pairs, and the human karyotype has about 3.2 billion base pairs total. The information in the 3,200,000,000 base pairs makes sure the 1,000,000,000,000 cells all work together. 3.2 billion seems like a lot of base pairs, but if you take computer memory as a metaphor it's not that much. If you think of a base pair like something between a bit and a byte, then all the genetic information fits on a 3.2 gigabyte (3.2 GB) thumbdrive. It's like the basic install of a video editing program. Not something you could attach to an email, but it wouldn't take that long to download. So what makes people so complicated, if their code is so small?

    • Multiple identities

    Here's an introduction to chimerism, the idea most of us are conjoined twins to a small extent. Does this change how you think of yourself?

    Genetics and Ethics

    Remember that the anthropological imagination avoids scientism: putting science on a pedestal protected from all criticism, and valuing science and scientific knowledge above people. Anthropology is the science of humans, so there is no way to avoid humanistic questions. The field of genetics has grown in a political context where world economic systems have loosened many ethical guidelines. It's like the wild, wild, west.

    Identity and Ownership

    If you think the government reading your emails is bad, think of having scientists steal your genetic code without your consent and owning you in your afterlife.

    Exercise \(\PageIndex{1}\)

    One of the great things about Obamacare is that it headed-off a growing problem of insurance companies using genetic information as a way to screen out members with a propensity for expensive medical conditions.

    Our reliance on genetics in forensic anthropology (using anthropology for legal, usually police, situations) can lead to problems such as this case which seems straight out of the Jerry Springer show: * Man Fails Paternity Test Because Unborn Twin Is The Biological Father Of His Son

    Stem Cells

    Stem cells are undifferentiated, that means they divide and grow into many different kinds of cells. When you think of yourself as a trillion cells, once upon a time, you used to be one cell: a tiny, cute, little zygote (what happened!??!!). Stem cells are left over from that transition from one cell to billions of cells. They are great for medicine because if you are missing cells in your body, you tell the stem cells to become them.

    The controversy was much worse a decade ago when the major source of stem cells was aborted fetuses, but today, they can be harvested from your own baby teeth, your blood, even from your left over liposuction.

    Notes

    Cloning

    image087.gif

    Figure \(\PageIndex{7}\)stuffistumbledupon.com

    To me, human cloning is no big deal. We're already dealing with the ethical issues that cloning raises.

    Genetically identical humans? It's what we get with identical (monozygotic) twins, when the zygote divides and then separates into two embryos who become two individuals. They may be identical genetically, but variations in how they interact with their environment will make them physically distinct, and most importantly, their different cultural experiences will make them different people.

    Exploiting people to harvest their organs? The demand for kidneys has led to transplant tourism and black markets in places like India and the Philippines, and a legal kidney market exists in Iran.The 1% are already playing God and cannibalizing the bodies of the 99%.

    (Full disclosure: my tolerance for cloning may be biased because I am currently raising several clones of my own of various ages on a secret organic ranch in southern Utah which I plan to use to for parts as I get older. Here's a home video of my favorite "Little Arnie", AS0983-2342, getting in shape for his transplant surgery.)

    GMOs

    I think recombinant DNA, or genetically modified organisms (GMO) are a bit scarier... Will it kill you to eat them? No! Are they poisonous? No! Should you try avoid them? Yeah, it's probably a good idea to eat as much organic produce as as you can afford.

    *Read this article that examines a historic period in agronomy known as the "Green Revolution"and how we might be able to use lessons learned from this technocratic disaster, to avoid potential disaster from the current GMO revolution.

    We look at hackers today as some kind noble outlaw, or social bandit, but we need to keep asking ourselves, what are our limits, and how far we are willing to go to hack ourselves and the code that makes us who we are.

    image089.gif

    Figure \(\PageIndex{8}\) - Frank Miller and Geof Darrow 1993. Hard Boiled

    The mad scientist trope in biology has a long history, from the Jewish golems, the alchemists of the Middle Ages, Mary Percy Shelly's Frankenstein; or, the Modern Prometheus, or some of my recent favorites, the Larry Fessenden 1991 movie, The Telling, and Margaret Atwood's 2003 novel, Oryx and CrakeBut, another worry with the rapidly expanding technology of genetic modification is the chaos theory idea from Jurassic Park, or what I like to call the Homer Simpson effect ­– shit happens – genes can move around out of our control, and I think this next section is even scarier:

    Lateral Gene Transfer

    Lateral gene transfer (LGT), also called horizontal gene transfer (HGT), is distinguished from the kind of up-and-down verticalness of heredity that we represent with a family tree. It's important that you understand the classic Standard Evolutionary Theory of Darwin, Mendel, and the Modern Synthesis, because that's how things work 99.999999% percent of the time, but there are rare exceptions. Some evolutionary scientists have advocated including new research into an Extended Evolutionary Synthesis, others say that the old paradigm works fine.

    Some consider LGT to be a kind of gene flow, but classic gene flow happens within the same species through migration. LGT is about genes that can be moved from one nucleus to another in ways other than meiosis. Humans have learned to do this intentionally as shown in the previous section, but it can also happen naturally.

    I think the scariest thing about all the genetic engineering going on today is the combination of GMOs and LGT. Nature has dealt with LGT for billions of years and you can expect that we've evolved to deal with whatever genes are floating around in the environment. When you create a new gene, you can test how it will influence a certain organism in the laboratory, but when you release the gene into the environment, there is no way to test every possible combination of that gene inserted into every other species. So for example, you make a gene that allows leaf cells to produces their own pesticide and you put it in corn. Fine, the corn is great, no bugs, no need to spray pesticides. But, what happens if that gene moves to bacteria that live in your stomach. Whoops! Now you don't have to bother drinking pesticide, because you're producing it in your stomach already. 

    On the bright side, LGT does make a great back-story for horror movies.

    Notes

    Imagination Questions

    • Designer Babies
      • 1) My niece was screened for chromosomal abnormalities in utero, and I've been avoiding asking my sister if she would have gotten an abortion if the fetus tested positive for Down syndrome. Would you abort your child if you found out it was going to be born with a disease? Would you like time to prepare? Would you even want to know?
      • 2) an article on a sperm donor clinic who filters out "the donor matches with an elevated risk of rare recessive paediatric conditions."
    • Do you think births should be natural? Do you have limits on how far people should go to provide for the future of their children?

    Imagination Actions

    GMOs

    Using what you've learned, find a recent campaign calling for the labeling of genetically modified foods (e.g. Just Label It), and write a letter to the appropriate policy maker, arguing for, or against, the legislation/action.

    Vocabulary

    • allele
    • autosome
    • cell
    • centromere
    • chromatid
    • chromosome
    • codon
    • crossing-over
    • double helix
    • gamete
    • gene
    • individual
    • karyotype
    • locus
    • meiosis
    • mitochondria
    • mitosis
    • nucleotide
    • nucleus
    • ovum
    • population
    • somatic cell
    • species
    • sperm
    • zygote