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Chapter 6

Chapter 6. DNA Structure and Function. Why learn about DNA?. Solving the mystery to the structure of DNA in the 1950’s was only the first step. As we learn more and more about the function of DNA, we may be able to relieve countless diseases and miseries that currently afflict us.

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Chapter 6

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  1. Chapter 6 DNA Structure and Function

  2. Why learn about DNA? • Solving the mystery to the structure of DNA in the 1950’s was only the first step. • As we learn more and more about the function of DNA, we may be able to relieve countless diseases and miseries that currently afflict us.

  3. Why learn about DNA? • For example, adult cloning…. • Takes hundreds of attempts to obtain one viable embryo • To be successful, researchers must reprogram the DNA (of the adult cell) to function like the DNA of an egg cell • Results in animals that develop health problems associated with aging, such as arthritis, lung diseases, etc. due to shorter telomeres • However, …..

  4. Why learn about DNA? • There are benefits of cloning: • Helps scientists to unravel the molecular mechanisms of human genetic diseases • May give scientists the ability to form replacement tissues of organs for people with incurable diseases • Could save endangered animals from extinction or bring extinct animals back • Brings us closer to the possibility of cloning humans • If we can clone a lost pet for a grieving owner, why not clone a lost child for a grieving parent??

  5. In fact, livestock and pet animals are already being cloned commercially. • See Figure 6.1 in your textbook. • What do you think??? • Learning about the structure and function of DNA will help you to understand how cloning is done and how it works, as well as the possible pros and cons of this technique so that you can form your own opinions about the ethical issues surrounding cloning.

  6. Chromosomes • Inside every cell, DNA is organized into a structure called a chromosome. • While prokaryotes have a single, circular molecule of DNA as their single chromosome, eukaryotes have a number of chromosomes. • During most of a cell’s life, its chromosomes consist of a single copy of DNA. • However, just before cell division, the cell duplicates each chromosome, so that each chromosome consists of two identical copies of DNA. • This ensures that each new cell that is produced will get an identical copy of all of the DNA.

  7. Chromosomes • Each chromosome then consists of identical copies of DNA, called sister chromatids. • These sister chromatids are attached at a constricted region somewhere near the center called the centromere. • This gives the chromosome the appearance of an X shape.

  8. Chromosomes • In a human cell there are 23 pairs of chromosomes, or 46 chromosomes in total. • If stretched out end to end, the 46 chromosomes in a human cell would be about 2 meters long! • That’s a lot of DNA to fit into a nucleus that is so small small that it can only be seen on high power under a light microscope. • Therefore, the DNA in a chromosome associates with proteins that organize it into a tightly packed (or condensed) package that can fit into the microscopic nucleus.

  9. Chromosomes • In its most condensed (tightly packed) state, a duplicated chromosome consists of two long, tangled filaments (sister chromatids) that form a characteristic X shape. • If you look more closely however, you will see that each sister chromatid is actually a hollow tube formed by coils, like a phone cord. • The coils consist of a twisted fiber of DNA and protein. • The DNA molecule wraps twice at regular intervals around “spools” of proteins called histones. • In a micrograph, the DNA with its associated histones look like beads on a string. • Each bead is called a nucleosome, the smallest unit of chromosomal organization in eukaryotes.

  10. Chromosomes

  11. Chromosome Number • The sum of all chromosomes in a cell of a given type is called the chromosome number. • Each species has a characteristic chromosome number.

  12. Chromosome Number • Since the human chromosome number is 46, this means that humans have 46 chromosomes. • Also, since humans have 2 of each type of chromosome (which equals 23 pairs of chromosomes), their chromosome number is said to be diploid (or 2n). • The 23 pairs of chromosomes are like two sets of books numbered 1 to 23. There are 2 versions of each book (a pair). • The two members of each pair (except for the sex chrom0somes) have the same length and shape, and they hold information about the same traits. • Your father gave you one set of “books” (chromosomes), while your mother gave the the other set. Your father’s “books” say slightly different things than your mother’s “books” about the same “topics” (traits).

  13. Chromosome Number Your set of chromosomes from your father (blue) are the same size and shape as those from your mother (pink) but they contain slightly different information about the same traits.

  14. Types of Chromosomes • Every pair of chromosomes in each cell are the same in males and in females, except for one pair. • Those that are the same in both males and females are called autosomes. In humans there are 22 pairs of autosomes. • The one pair that differ between males and females are called sex chromosomes. In humans, the sex chromosomes in males are XY, while in females, they are XX.

  15. Types of Chromosomes

  16. Types of Chromosomes • In fruit flies, mammals, and many other animals, females have two identical sex chromosomes (XX), while males have two different sex chromosomes (XY). • However, this is not the case in all species. • In butterflies, moths, birds, and certain fishes, for example, males have two identical sex chromosomes, while females have two different sex chromosomes. • In addition, in certain species of invertebrates, turtles, and frogs, environmental factors determine the sex of the animal. • For example, the temperature of the sand in which sea turtle eggs are buried determines the sex of the hatchlings.

  17. Types of Chromosomes • In humans, the sex chromosomes that the individual inherits from each parent determines whether that individual will be male or female. • Every egg (female) contains an X chromosome. • Half of the sperm from the male contain an X chromosome, the other half contain a Y chromosome. • So, which parent determines the sex of the child in humans?

  18. Types of Chromosomes • A karyotype is a picture of an individual’s diploid complement of chromosomes. • A karyotype is made by taking cells from an individual and treating them to make their chromosomes condense. • The chromosomes are then stained so that they become visible under a microscope. • Then, a micrograph of the chromosomes in a single cell is digitally rearranged so that the images of the chromosomes are lined up by centromere location, and then are arranged according to size, shape, and length. • This image can then be compared to a standard to reveal any extra or missing chromosomes, and some structural abnormalities.

  19. Karyotypes Normal=Standard

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