Applied Sciences

Genetics: Reproducing Life and Producing Variation

CLARK SPENCER LARSEN

E S S E N T I A L S O F PHYSICAL ANTHROPOLOGY SECOND EDITION

CHAPTER

3

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Copyright ©2013 W.W. Norton, Inc.

Genetics: Reproducing Life and Producing Variation

  • Questions addressed in this chapter:
  • What is the genetic code?
  • What does the genetic code (DNA) do?
  • How does understanding genes help us understand variation?

The last chapter ended with a brief introduction to DNA. But, what is DNA? What is it made of? And how can a small molecule like DNA ‘code’ for all of the traits in a living organism? We will address these and other questions in this chapter. Ultimately, what we are doing in this chapter is understanding how the genetic code (DNA) results in variation, because it is this variation that natural selection can act upon and lead to evolutionary changes. We will start by looking at the fundamental unit of all life on Earth: the cell. Inside each cell, the DNA code is structured into packages known as chromosomes. We will see how the DNA molecule can copy itself so that each cell in an organism’s body contains the same DNA information. We will then look at how DNA codes for proteins, which all living organisms are made of. Finally, we will look at a concrete example of how DNA impacts our lives by examining human blood types. Though we have to dive into the microscopic world, do not lose sight of the big picture: DNA is a code for making proteins, and we are made of proteins. If the DNA slightly changes (through mutation, which we met in the last chapter), the protein changes, and thus the organism can change.

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Copyright ©2013 W.W. Norton, Inc.

The Cell: Prokaryotes

  • Prokaryotes
  • 3.5 billion years old
  • Single-celled bacteria
  • No nucleus or organelles

All living organisms are made of cells; they are the basic units of life. There are many, many organisms that are made of just one cell, and many (including you) that are made of trillions of cells. All of life can be divided into two big categories, depending on the kind of cell they have. The first kind are organisms called prokaryotes. Prokaryotes are single-celled bacteria without nuclei or any special structures called organelles. They often have structures shown here in this image, like a cell wall, an outer membrane, a cytoplasm within which the DNA resides, and they often have locomotor structures like a flagellum. On this slide is a microscopic image of a prokaryotic cell that we have all heard of: Escherichia coli (E. coli), which lives in the guts of many mammals, including humans. Though prokaryotic cells live within us, and have been instrumental factors in driving human evolution, we will turn now to the cells we are made of: eukaryotic cells.

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Copyright ©2013 W.W. Norton, Inc.

The Cell: Eukaryotes

  • Eukaryotes
  • 1.2 billion years ago.
  • Some single-celled; all multicellular organisms (including humans)
  • DNA contained in a nucleus
  • Organelles

All animals, plants, fungi, and many single-celled organisms called protists are made of eukaryotic cells. These cells have a nucleus that contains DNA, and often have membrane-bound parts of the cell called organelles. These include chloroplasts (found in plants) and mitochondria, which help produce the molecular energy that powers cellular processes. Notice in this image that the eukaryotic cell is a bit more complicated than a prokaryotic cell. The microscopic image here is of kidney cells, which clearly have a nucleus, a membrane keeping the components of the cell contained, and a fluid within the cell called a cytoplasm.

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Copyright ©2013 W.W. Norton, Inc.

The Cell: Somatic Cells and Gametes

  • Somatic cells
  • Body cells
  • Full DNA (humans: 46 chromosomes)
  • Mitosis
  • Gametes
  • Eggs (ova) and sperm
  • Half DNA (humans; 23 chromosomes)
  • Meiosis

There are two types of eukaryotic cells in all animals and plants: somatic cells and gametes. Somatic cells, also called body cells, are found all over the body. Shown in the above image are the somatic cells found in the (clockwise from top left) brain, blood, bone, and skin. Somatic cells all contain a complete copy of the organism’s DNA. For example, in humans, somatic cells have all of the DNA packed in 46 chromosomes. Somatic cells also replicate through a process called mitosis, which we will learn about in just a moment. At the bottom right is an image of the other kind of eukaryotic cells: gametes. The large round cell is called an egg, or an ova. The small wiggly structures surrounding the egg are sperm. These are gametes. They contain only half of the organism’s DNA (23 chromosomes in humans) and replicate through a process known as meiosis.

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Copyright ©2013 W.W. Norton, Inc.

Chromosomes

  • DNA packaged in chromosomes
  • Chromosome number varies by species
  • Number of chromosomes does not correlate with complexity

Since we just mentioned chromosomes, it is worth examining chromosome number in a bit more detail. Humans have 46 chromosomes in our somatic cells. 23 of these came from our mother, and 23 from our father, for a grand total of 46. But, this number, 46, is not special at all. Other apes, like chimpanzees, have 48 chromosomes. Some primates have fewer chromosomes, like the colobus monkey which has 44. Some organisms we would consider to be less complex than us have fewer chromosomes, like the house fly with 12 or the salamander with 24. But, plenty of organisms have more than we have, like the potato with 48, the camel with 70, or algae, which has 148 chromosomes. Classifying organisms by the number of chromosomes they have would be like organizing books in a library based on the number of pages they have, or by the color of its jacket cover. It wouldn’t make sense. What matters are not the number of chromosomes an organism has, but the similarity in DNA that is packaged in the chromosomes. For instance, humans and chimpanzees share about 98% of their DNA. This is remarkable, and, in some ways, indicates how important 2% of a difference can be.

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Copyright ©2013 W.W. Norton, Inc.

DNA: The Blueprint of Life

• DNA

Genes

Chromosomes

Genome

• Nuclear DNA:

homoplasmic

• Mitochondrial DNA:

heteroplasmic

Most likely, you have all heard of DNA, and have probably heard that it is the “blueprint,” or “recipe,” or “code” for life? But, how does this work? It helps first to understand the structure of DNA, and to understand how it is packaged in your cells. It is estimated that there is six feet worth of DNA in every cell in your body. Six feet!? If cells are microscopic, how can this be? As shown in this image, the DNA molecule is wound up into compact structures that we have already encountered: chromosomes. Sections of that DNA specifically code for a specific protein in the body: These are called genes. The genome is all of the genes put together in all of the chromosomes. The DNA that is in the nucleus of our cells is called homoplasmic, meaning it is more or less the exact same in every cell in our body. But, the nucleus is not the only place in a cell that contains DNA. An organelle called the mitochondria also contains DNA. Mitochondrial DNA (mtDNA) is much, much smaller; it only contains 37 genes. And these genes are only inherited from your mother, meaning they can be used to trace one’s maternal lineage (called a matriline). Unlike nuclear DNA, mitochondrial DNA can differ from cell to cell, making it heteroplasmic.

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Copyright ©2013 W.W. Norton, Inc.

DNA: The Blueprint of Life

  • DNA structure
  • Sugar
  • Phosphate
  • Nucleotide base
  • Adenine (A)
  • Thymine (T)
  • Guanine (G)
  • Cytosine (C)
  • A with T
  • C with G
  • CAAAT
  • GTTTA

We are finally ready to discuss what DNA actually is. It is a molecule; in fact, a very simple one. DNA is made of three things: a type of sugar, a phosphate group, and a nucleotide base pair. The sugar and phosphate form the backbone of the long DNA molecule and these do not vary along the chain. What varies along the chain are the nucleotide base pairs. These bases can be one of four types: adenine (A), thymine (T), guanine (G) and cytosine (C). You can think of DNA as a ladder with the sugar and phosphates forming the uprights, and the bases forming the rungs. The rungs are made of two base pairs that cling together using hydrogen bonds. Critical to understanding DNA is the fact that the base pairs do not randomly cling to each other. Instead, A only clings to T, and C only clings to G. These are called complementary bases. What this means is, if you know one side of the DNA chain, you know the other. If a DNA sequence is CAAAT, the other side MUST be GTTTA. Though any two humans may have over 99% of their DNA base pair order identical, there are, of course, differences. Differences in these single nucleotide regions are called SNPs (pronounced “snips” and short for single nucleotide polymorphisms). These are some of the areas of the genome that can be used to solve crimes using DNA evidence since they can vary from one individual to another.

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