Chapter 6 Flashcards
Within an organism’s DNA are the instructions….
necessary to build all the proteins it requires
DNA is passed from generation to generation in the form of _____________. ELABORATE
Chromosomes
- In eukaryotes, chromosomes may be visible in the cells during cell division. –> appear as the familiar X-shaped objects that are split during mitosis or meiosis.
- The size and # of chromosomes in a cell are specific to each species.
The largest known plant has _______ chromosomes
about 1200
How is a protein synthesized from DNA?
- When a particular protein is needed, the portion of DNA (the gene) that codes for this protein is activated.
- The nucleotide sequence is copied (transcribed) into a molecule of RNA (ribonucleic acid).
-The RNA then moves to cytosol, where its sequence is translated by the ribosomes into amino acid chains called polypeptides. - Elsewhere in the cell, polypeptides are further modified to form functional proteins.
- How the message in DNA is decoded to make proteins is central to the development of all life
Gregor Mendel overview: who was he & what did he do?
- He was an Austrian monk who studied pea plants (Pisum sativum)
- Conducted cross-pollination experiments over 7 years in late 1800s
- Analyzed inheritance patterns in 28,000+ plants
- His analysis clearly showed how certain traits were expressed in the next gen from each cross-pollination experiment. –> Proposed that traits pass from parent to offspring via “factors”
- cuz of his work, we now know that a hereditary molecule does exist.
- Over past 100 years, many experiments, using increasingly sophisticated methods, revealed that DNA = the carrier of the hereditary information
All new cells arise from ________________________, & all the information that is needed for optimal cell functioning is coded in a cell’s ________
division of existing cells, DNA
Eukaryotes Vs Prokaryotes: location of DNA
Eukaryotes = DNA is stored in the nucleus.
Prokaryotes= stored in the cytosol.
- Regardless of the location, all forms of life use DNA in the same way to build proteins & grow new cells.
Mendel proposed the concept of “factors” for each specific trait inherited by organisms. ELABORATE
- These factors, inherited from parents, determine an organism’s measurable characteristics like size, color, and markings.
- Today, we know these “factors” as genes= DNA segments with instructions for making proteins that express inherited traits.
-Genes exist in different forms called alleles, leading to variations in traits. - Genes are scattered along an organism’s DNA strands & can vary greatly in length.
Gene similarities within a species and with other species.
- In a species, the gene for a particular characteristic is always found in the same location on a particular chromosome.
- As scientists continue to study, they are discovering that many fundamental genes (such as those that direct the development of an embryo or the building of cell membranes) are shared by almost every form of life.
A histone is
a special protein molecule that is the core around which the DNA strand wraps
An organism’s genome is
the complete set of hereditary information that is contained in an organism.
- genome of eukaryote is usually spread over many chromosomes, which often occur as pairs of
homologous chromosomes. –> Each pair represents 2 copies of a particular set of genes. –>Ex, humans have
23 homologous pairs of chromosomes of diff sizes, 1 pair of which are special sex chromosomes
- Each set of 23 chromosomes contains a total
of more than 20 000 different genes, which r combined to form roughly 3 billion base pairs.
About ____________ ago, Mendel hypothesized the existence of a hereditary molecule that passes genetic information through the generations. Less than _________ ago, scientists determined that this hereditary material is, in fact, DNA.
150 years, 70 years
Frederick Meischer & his work
- 1868=Frederick Meischer studied cell nuclei to understand their composition.
- At the time, proteins were thought to be the hereditary material cuz they were known to be complex & carry out numerous biological functions
- he collected pus (mainly white blood cells) from the bandages of his patients & extracted a substance, rich in phosphorus and acidic, from white blood cells, naming it “nuclein” since it was found in the nucleus.
- At that time, heredity was poorly understood, with theories suggesting a blending of parental traits. –> they could not explain why a short woman and a tall man produced children with a variety of heights, not a height that was an average of the two parents’ heights
–> Although Mendel was crossbreeding his peas at
about the same time (1865), his work was generally unnoticed until the early 1900s. - function & composition of nuclein (later identified as DNA) were not understood until >50 years later, and its structure was not determined until decades after that.
Frederick Griffith & Experiment (not conclusion)
- THE TRANSFORMING PRINCIPLE
-role of Meischer’s nuclein (DNA) was still unknown when Frederick Griffith began studying pneumonia epidemic in europe during World War I. - in 1928, Griffith, a medical officer for British Military, carried out an experiment that accidentally shed light on the function of DNA in inheritance.
- Griffith used 2 strains of pneumonia bacteria:
–> Smooth strain/S-strain: had capsule that surrounded each cell and caused the bacterial colonies to look smooth & glossy when grown on agar. WAS HIGHLY VIRULENT (PATHOGENIC)
–> Rough strain/R-strain: Lacked a capsule, forming rough colonies and WAS NON-VIRULENT. - Injecting mice with the S-strain caused pneumonia and death in few days, while the R-strain caused no signs of pneumonia & lived.
- Griffith concluded that capsule surrounding the S-strain was responsible for the virulence.
- He heated the S-strain cells, destroying capsule that surrounded the cells and killing the bacteria. The dead S-strain no longer caused an infection in the mice.
- when he mixed the heat-killed S-strain with the live non-virulent R-strain, many of the mice got pneumonia and died.
- Griffith then isolated living bacteria that appeared like S-strain bacteria from the dead mice. –> Somehow, the living R-strain bacteria acquired some factor from the heat-killed S-strain that made them virulent.
–> The newly virulent R-strain bacteria even formed smooth colonies when cultured, just like the living S-strain bacteria. - We now know that bacteria can take up genetic
material from nearby bacteria and use this DNA as their own
Griffith’s conclusion
- Although Griffith couldn’t identify exact material involved in inheritance, he understood that some hereditary substance had passed from the dead S-strain cells to the live R-strain cells.
- When the R-strain bacteria acquired this material,
they were effectively transformed into infectious S-strain bacteria. - He called this process transformation, and he called the factor that was responsible the transforming principle.
- At the time, the most likely candidates for the transforming principle were proteins and DNA, but further experiments were required to determine its identity
Transformation is
a change in a genotype or phenotype caused by the direct uptake of genetic material by a cell
Avery, McLeod, and McCarty
- DNA TRANSFORMATION CONFIRMED
- In 1944, Oswald Avery, Colin McLeod, and Maclyn McCarty (all researchers) expanded on Griffith’s findings to identify the genetic material.
- They used S- and R-strains of Streptococcus bacteria, which differ in their disease-causing ability, isolating each strain and growing them in separate cultures
- Avery’s team hypothesized that either DNA, RNA, or proteins was responsible for transforming R-strain bacteria into virulent forms.
- They treated heat-killed S-strain extracts with enzymes to selectively destroy DNA, RNA, or proteins, then mixed these extracts with R-strain bacteria to observe the transformation.
- The experiments showed that DNA was the transforming substance, as only the samples where DNA remained intact could induce transformation.
- Despite strong evidence, Avery and his team were cautious to conclude DNA’s role as genetic material due to prevailing beliefs in protein-based inheritance and the need for further validation of DNA’s role. –> If some protein had not been destroyed by the enzymes, their results would be incorrect.
Hershey & Chase
- DNA IS THE HEREDITARY MATERIAL
- In 1952, Bacteriologists Alfred Hershey and Martha Chase sought to identify whether DNA or proteins served as genetic material.
- They used a bacteriophage (viruse that infect bacteria) with DNA and a protein coat to infect E. coli bacteria. –> The bacteriophage they used had both DNA & a protein coat –>they knew that bacteria could be transformed by viruses, but they did not know which part of the virus—the protein coat or the DNA (or RNA)—did the transforming
- When bacteriophage infects bacterium, it inserts its genetic material into the bacterium & uses the bacterium’s cellular processes to produce new bacteriophages.–> Note: at the time, no one knew what part or parts of a virus entered cells during an infection or even what a virus looked like. –> too small to be seen with the most powerful microscopes
- They employed radioisotopes to label molecules:
- 32P (Phosphorus-32): Tagged DNA, as DNA contains phosphorus and proteins only contain a tiny amount
- 35S (Sulfur-35): Tagged proteins, as proteins contain sulfur, while DNA does not.
- Process: Separate bacteriophage groups were labeled with either 32P or 35S and then used to infect E. coli colonies to trace which molecule entered the bacterial cells.
- This experiment provided crucial evidence that DNA, not protein, is the genetic material that viruses transfer to host cells. –> Radioactivity was only detected within bacterial cells that wre infected
by viruses containing DNA labelled with 32P. The radioactive protein coats had remained outside the bacterial cells, while the radioactive DNA had entered the cells.
Phoebus Levene
In 1920s, Phoebus Levene reported that each DNA molecule contained three major components: deoxyribose sugars, phosphate groups, and nitrogenous bases.
- A DNA molecule is a polymer made of nucleotide subunits. Each nucleotide subunit consists of a nitrogenous base attached to one deoxyribose sugar, which is connected to a phosphate group
- By 1949, the four nitrogenous bases had been identified. –> Adenine (A) & guanine (G) are double-ring structures= purines, while thymine (T) and cytosine (C) are single-ring structures= pyrimidines.
Edward Chargaff
- an organic chemist
- did not agree with researchers who suggested that DNA contained equal amounts of the 4 nitrogenous bases.
- In 1950, Chargaff found that these bases always occur in definite ratios.
- He also found that the quantities of T & A always matched, as did the quantities of G & C.
- Ex, Human DNA was estimated to contain 30.9 % A, 29.4 % T, 19.9 % G, & 19.8 % C.
- Scientists now knew DNA’s chemical composition & its role as the molecule of
inheritance.
X-ray Crystallography
- 1 new tech at that time was X-ray crystallography & this tech involves X-rays bombarding a sample of a compound, which is usually in the form of solid crystal.
- The atoms deflect the X-rays in a specific way, creating a pattern on a photographic plate.
- The pattern is then analyzed to help determine the molecular structure of the original sample.
Wilkins & Franklin
- Franklin and Wilkins used X-ray crystallography to investigate DNA’s structure at King’s College, London, working somewhat independently on two crystal forms.
- had a strained relationship as colleagues
- Wilkins had produced some preliminary crystallographs of DNA that suggested its helical
structure, but Franklin was unconvinced. –> Wilkins’ initial DNA samples were impure, resulting in unclear crystallographs.
Franklin’s Achievements:
- Prepared purer DNA samples, producing high-quality crystallographs with a distinctive “X” pattern.
- Suggested that DNA’s sugar-phosphate backbone was on the outside of the molecule, contrary to the prevailing belief.
- Proposed that DNA was a double helix that rotated clockwise with a 2 nm diameter and a 3.4 nm helix turn length.
- Though aware of Chargaff’s rules, Franklin could not determine the arrangement of nitrogenous bases within the helix and was hesitant to publish her findings due to these uncertainties.
Watson & Crick
- BUILDING A MODEL OF DNA
- In 1952, the team of James Watson and Francis Crick were building models of the DNA molecule, incorporating everything they knew about DNA. They
had a wealth of information available to them: - four different nitrogenous bases (A, T, C, and G)
- Chargaff ’s ratios of the nitrogenous bases
- the phosphate and sugar backbone
- Without her consent, Maurice Wilkins shared Rosalind Franklin’s findings with Watson, providing them with her concept of a double helix with inward-facing bases & her measurements of DNA’s dimensions.
- Watson & Crick realized that the double helix could incorporate all of the facts
Structure of the Model:
- DNA was modeled as a double helix with 2 phosphate-sugar backbones twisting clockwise.
- Nitrogenous bases faced inward, connected to those one the opposite strand by H-bonds, with each purine pairing with a pyrimidine (A-T with 2 H-bonds, G-C with 3 H-bonds) according to Chargaff’s rules= complementary base pairing
- The symmetry is key to the structure of DNA
& its ability to divide itself accurately & convey genetic information
- model showed that DNA molecule could only be stable if the strands ran antiparallel= if they ran in opposite directions.
–>1 DNA strand must have the hydroxyl of the 3’ C attached to the deoxyribose sugar at one end and the phosphate attached to the 5’ C of the last sugar
at the other end. The other strand must wind around the first with its 5’ end opposite the 3’ end of the first strand - A great deal of research was needed to understand DNA replication, but Watson and Crick’s initial model of the structure of DNA gave scientists a place to start
DNA molecules can be __________ of nucleotides in length, and replication has to occur ____________ with
few _____________.
millions, very quickly, (if any) errors
( SLIDESHOW) there are 3 proposed potential mechanisms of DNA replication: List them
- Conservative Model –>the 2 parental strands would act as templates for replication, but then recombine afterwards.
- Semiconservative Model (scientifically proven one): —-> the 2 parental strands would act as templates for replication & remain separated from each other, incorporated into two new molecules.
- Dispersive Model (this one was not included in the textbook) –> each strand of both daughter molecules contains a mixture of old & newly synthesized parts
Semiconservative replication is
a mechanism of DNA replication in which each of the 2 strands of parent DNA is incorporated into a new double-stranded DNA molecule
- involve separating the 2 parent strands and building a new, complementary replacement strand for each.
- The new molecules would consist of 1 parent strand & 1 new strand
STEPS
- Complementary base pairing in the DNA double helix: G pairs with C, A pairs with T.
- The two chains unwind and separate.
- Each “old” strand is a template for the addition of bases according to the base-pairing rules.
- The result is two DNA helices that are exact copies of the parental DNA molecule with one “old” strand and one “new” strand
Matthew Meselson & Franklin Stahl (Experiment only)
- In 1958, Matthew Meselson & Franklin Stahl carried out an experiment that demonstrated that DNA replication is semiconservative
- Meselson & Stahl used isotopes to label the parent DNA strands before replication –> they used was “heavy” nitrogen, 15-N.
- E. coli was grown for 17 gens in a medium with ^15N, ensuring full incorporation of ^15N into DNA.
- Bacteria were then transferred to a medium with “light” nitrogen (^14N).
- Allowed to undergo 1-2 rounds of replication (~20 min per round).
- Any new DNA produced should have lighter 14-N incorporated into its structure, thus making it less dense than the parent 15N DNA. —> To determine density, the DNA was isolated and placed in a centrifuge tube that contained a mixture that, when centrifuged, produces a density gradient
from most dense at the bottom to least dense at the top. DNA migrates to a level in the
tube with a density similar to its own.
Matthew Meselson & Franklin Stahl (Results & conclusion)
- before replication, upon centrifuging, the original heavy 15N DNA was in a single band. SEE DIAGRAM ON PAGE 283
- 1st replication, there was a single band of DNA at a density of hybrid DNA= DNA with equal parts of 15N and 14N –> If replication was conservative, 2 bands would be seen, 1 containing the parental 15N DNA and one containing only new 14N DNA.
- single band of hybrid DNA confirmed that DNA
replication was indeed semiconservative. - 2nd replication confirmed this conclusion. 2 bands of DNA were detected
–> 1 had a density consistent with hybrid DNA & 1 corresponded to DNA built only with light 14N nucleotides –> Each double strand of hybrid DNA had been separated and used to build complementary 14N strands, producing 1 band of hybrid DNA & 1 with only 14N. - The clarity of the results of Meselson and Stahl’s elegant and innovative experiment left little doubt. DNA replication is semiconservative
Most of our understanding of the process of DNA replication comes from the study of _____________________. Elaborate
the bacterium E. coli
- DNA replication in eukaryotes resembles that in prokaryotes, though eukaryotic replication is more complex due to linear DNA structure and higher DNA volume.
