Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
4.1
| Question | Answer |
|---|---|
| With improvements in the microscope, biologists of the late nineteenth century saw that cell division is immediately preceded by nuclear division, | and during nuclear division, the chromosomes split neatly in two and distribute their halves to the two daughter cells. |
| Biologists of the 19th Century | They came to suspect that the nucleus was the center of heredity and cellular control, and they began probing it for the biochemical secrets of heredity. |
| Biologists of the 19th Century | They came to suspect that the nucleus was the center of heredity and cellular control, and they began probing it for the biochemical secrets of heredity. |
| Swiss biochemist Johann Friedrich Miescher (1844–95) | studied the nuclei of white blood cells extracted from pus in hospital bandages, and later the nuclei of salmon sperm, since both cell types offered large nuclei with minimal amounts of contaminating cytoplasm. |
| In 1869, he discovered an acidic, phosphorous-rich substance he named nuclein. | He correctly believed this to be the cell’s hereditary matter, although he was never able to convince other scientists of this. We now call this substance deoxyribonucleic acid (DNA) and know it to be the repository of our genes. |
| By 1900, biochemists knew the basic components of DNA—sugar, phosphate groups, and organic rings called nitrogenous bases —but they didn’t have the technology to determine how these were put together. | That understanding didn’t come until 1953, in one of the twentieth century’s most dramatic and important stories of scientific discovery (see Deepen Insight 4.1). The following description of DNA is largely the outcome of that work |
| DNA | is a long threadlike molecule with a uniform diameter of 2 nm, although its length varies greatly from the smallest to the largest chromosomes. |
| Most human cells have 46 molecules of ---- totaling 2 m in length. | DNA |
| This makes the average DNA molecule about ---- (almost 2 in.) long | 43 mm |
| To put this in perspective, imagine that an average DNA molecule was scaled up to the diameter of a utility pole (about 20 cm, or 8 in.). | At this diameter, a pole proportionate to DNA would rise about 4,400 km (2,700 mi.) into space—far higher than the orbits of the International Space Station (408 km) and the Hubble Space Telescope (600 km). |
| At the molecular level, DNA and other nucleic acids are polymers of -----. | nucleotides |
| A nucleotide consists of a sugar, a phosphate group, and a single- or double-ringed | nitrogenous base |
| Two of the bases in DNA—------)—have a single carbon–nitrogen ring and are classified as pyrimidines (py-RIM-ih-deens). | cytosine (C) and thymine (T) |
| The other two bases—----- —have double rings and are classified as purines | adenine (A) and guanine (G) |
| The structure of DNA, commonly described as a double helix, resembles a spiral staircase (fig. 4.2). | Each sidepiece is a backbone composed of phosphate groups alternating with the sugar deoxyribose. |
| The steplike connections between the backbones are pairs of | nitrogenous bases |
| The bases face the inside of the helix and hold the two backbones together with | hydrogen bonds |
| Across from a purine on one backbone, there is a pyrimidine on the other. | The pairing of each small, single-ringed pyrimidine with a large, double-ringed purine gives the DNA molecule its uniform 2 nm width. |
| The structure of DNA, | commonly described as a double helix, resembles a spiral staircase (Fig. 4.2). |
| Each ----- is a backbone composed of phosphate groups alternating with the sugar deoxyribose. | sidepiece |
| The steplike connections between the backbones are pairs of | nitrogenous bases |
| The bases face the inside of the helix and hold the two backbones together with | hydrogen bonds |
| Across from a purine on one backbone, there is a pyrimidine on the other. | The pairing of each small, single-ringed pyrimidine with a large, double-ringed purine gives the DNA molecule its uniform 2 nm width. |
| A given purine cannot arbitrarily bind to just any | pyrimidine |
| Adenine and thymine | form two hydrogen bonds with each other, and guanine and cytosine |
| Therefore, where there is an A on one backbone, there is normally a T across from it, and every C is normally paired with a G. | A-T and C-G are called the base pairs. |
| The fact that one strand governs the base sequence of the other is called the | law of complementary base pairing |
| law of complementary base pairing | It enables us to predict the base sequence of one strand if we know the sequence of the complementary strand. |
| Credit for determining the double-helical structure of DNA has gone mainly to James Watson and Francis Crick (fig. 4.3). | The events surrounding their discovery form one of the most dramatic stories of modern science—the subject of many books and at least one movie. |
| When Watson and Crick came to share a laboratory at Cambridge University in 1951, both had barely begun their careers. Watson, age 23, had just completed his Ph.D. in the United States, and Crick, 11 years older, was a doctoral candidate in England. | Yet the two were about to become the most famous molecular biologists of the twentieth century, and the discovery that won them such acclaim came without a single laboratory experiment of their own. |
| What was the purpose of Franklin's research on DNA, and how did it contribute to our understanding of genetics? Others were fervently at work on DNA, including Rosalind Franklin and Maurice Wilkins at King's College in London. | Using a technique called X-ray diffraction, Franklin had determined that DNA had a repetition helix structure with sugar and phosphate on the outside of the helix. |
| Without her permission, Wilkins showed one of --- said, "The instant I saw the picture my mouth fell open and my pulse began to race." It provided a flash of insight that allowed the Watson and Crick team to beat Franklin to the goal. | Franklin's best X-ray photographs to Watson |
| Combining what they already knew with the molecular geometry revealed by Franklin's photo, they were quickly able to piece together a scale model that fully accounted for the known geometry of DNA. | They rushed a paper into print in 1953 describing the double helix, barely mentioning the importance of Franklin's 2 years of painstaking X-ray diffraction work in unlocking the mystery of life's most important molecule. |
| Franklin published her findings in a separate paper back to back with theirs. | For this discovery and the ensuing decade of research on DNA that it opened up, Watson, Crick, and Wilkins shared the Nobel Prize for Physiology or Medicine in 1962. |
| Nobel Prizes | are awarded only to the living, and in the ironic finale of her career, Rosalind Franklin had died in 1958, at the age of 37, of a cancer possibly induced by the X-rays that were her window on DNA architecture. |
| What would be the base sequence of the DNA strand across from ATTGCATCG? If a DNA molecule was known to be 20% adenine, predict its percentage of cytosine and explain your answer. | The DNA strand across from ATTGACTCG is TAACGAGC. If there is 20% adenine, then there is also 20% thymine. Cytosine and guanine each make up 30%. |
| The essential function of DNA is to carry instructions, called genes, for the synthesis of proteins | . At this point in the chapter, we will provisionally regard a gene as a segment of DNA that codes for a protein. |
| Humans are estimated to have about 22,300 genes. These constitute only about 2% of the DNA. | The other 98% doesn't code for proteins, but plays various roles in chromosome structure and regulation of gene activity. |
| DNA doesn’t exist as a naked double helix in the nucleus of a cell, | but is complexed with proteins to form a fine filamentous material called chromatin. |
| . In most cells, the chromatin occurs as 46 long filaments called ---. | chromosomes |
| There is a stupendous amount of DNA in one nucleus—about 2 m (6 ft) of it in the first half of a cell’s life cycle and twice as much when a cell has replicated its DNA in preparation for cell division. | It is a prodigious feat to pack this much DNA into a nucleus only about 5 μm in diameter—and in such an orderly fashion that it doesn’t become tangled, broken, and damaged beyond use. |
| In order to achieve such packing, DNA is extensively coiled and ------ . | supercoiled |
| The DNA first winds around spools of proteins called histones to form the | little granules (core particles) |
| . The chromatin then folds into successive zigzags, loops, and coils, getting thicker and shorter as it does so | Ultimately the DNA, itself 2 nm in diameter, is consolidated into chromatin strands 150 times thicker and 1,000 times shorter than the naked DNA. |
| Finally, each chromosome is packed into its own spheroidal region of the nucleus, called a chromosome territory. | A chromosome territory is permeated with channels that allow regulatory chemicals to have access to the genes. |
| This is the state of the DNA in a nondividing cell. | It’s not a static structure, but changes from moment to moment according to the genetic activity of the cell as individual genes are turned on and off. |
| Whole chromosomes migrate to new territories as a cell develops—for example, moving from the edge to the core of a nucleus as its genes are activated for a certain developmental task, or back to the nuclear lamina to silence some genes. | This allows genes on different chromosomes to partner with each other in bringing about developmental changes in the cell. |
| When a cell is preparing to divide, it makes an exact copy of all its nuclear DNA by a process described later, increasing its allotment to about 4 m of DNA. | Each chromosome then consists of two parallel filaments called sister chromatids. |
| In the early stage of cell division (prophase), these chromatids coil some more until each one becomes, | at its most compact, 10,000 times shorter but 350 times thicker than the DNA double helix. |
| Only now are the chromosomes thick enough to be seen with a light microscope. | This compaction not only allows the 4 m of DNA to fit in the nucleus, but also enables the two sister chromatids to be pulled apart and carried to separate daughter cells. |
| Despite all this intricate packaging, the DNA of the average mammalian cell is damaged an astonishing 10,000 to 100,000 times per day! | The consequences would be catastrophic were it not for DNA repair enzymes that detect and undo most of the damage. |
Created by:
Russells3709