Chapter 13 Flashcards
13.1
What do chromosomes consist of?
primarily of DNA and associated proteins
13.1
Where is the genetic info in chromosomes?
resides in the DNA
13.3
What allows us to make comparisons between organisms?
their complete genome sequences
13.3
Why is gene number not a good predictor of biological complexity?
as different genomes were sequences and annotated, it came as a surprise to find that humans have about the same number of protein-coding genes as many organisms with much smaller genomes
13.3
What does differential gene expression allow?
the same protein-coding genes to be deployed in different combinations to yield a variety of distinct cell types
13.3
How can a single genes yield multiple proteins?
either because of alternative splicing or posttranslational modification
13.3
What is alternative splicing?
different exons are spliced together to make different proteins
13.3
What is posttranslational modification?
proteins undergo biochemical changes after they have been translated
13.3
How is genome size measured?
in number of base pairs
13.3
What has the complete sequence of small bacterial genomes allowed researchers to do?
to define the smallest genome (therefore, the minimal set of proteins) necessary to sustain life
13.3
What is the significance of ~500 genes?
current findings suggest that the minimum number of genes necessary to encode all the functions essential to life is ~500
13.3
What does it mean that genomes of bacteria and archaeons are information dense?
most of the genome has a defined function
- 90% or more of their genomes consist of protein-coding genes (protein usually has unknown function)
13.3
What do bigger genomes have?
more genes, allowing these bacteria to synthesize small molecules that other bacteria have to scrounge for, or to use chemical energy in the covalent bonds of substances that other bacteria cannot
13.3
Describe the relation of genome size and organismal complexity in eukaryotes.
just as the number of genes does not correlate well with organismal complexity, the size of the genome is unrelated to the metabolic, developmental, and behavioural complexity of the organism
13.3
What is the C-value paradox?
disconnect between genome size and organismal complexity
- C-value is amount of DNA in a reproductive cell
- paradox is the apparent contradiction between genome size and organismal complexity, leading to the difficulty of predicting one based on the other
13.3
Why are some eukaryotic genomes so large?
polyploidy: having more than two sets of chromosomes in the genome, especially prominent in many groups of plants
13.3
How has polyploidy played an important role in plant evolution?
- many agricultural crops are polyploid
- 30-80% flowering plants have polyploidy in evolutionary histories, either because of duplication of the complete set of chromosomes in a single species or because of hybridization between related species followed by duplication of the chromosome sets in the hybrid
13.3
What is the principal reason for large genomes among some eukaryotes?
their genomes contain large amounts of DNA that do not code for proteins, such as introns and DNA sequences that are present in many copies
13.4
What is a nucleoid?
structure with multiple loops formed by supercoils of DNA in bacteria
- supercoil loops are bound together by proteins
13.4
Describe the bacterial genomes.
they are circular and the DNA double helix is underwound, which means that it makes fewer turns in going around the circle than would allow every base in one strand to pair with its partner base in the other strand
13.4
What is topoisomerase II?
an enzyme that causes underwinding, which breaks the double helix, rotates the ends to unwind the helix, and then seals the break
13.4
What does underwinding create?
creates strain on the DNA molecule, which is relieved by the formation of supercoils in which the DNA molecule coils on itself
13.4
What does supercoiling allow?
all the base pairs to form, even though the molecule is underwound
13.4
What are negative supercoils?
supercoils that result from underwinding
13.4
What are positive supercoils?
supercoils that result from overwinding
13.4
What kind of supercoils are there in most organisms?
most DNA is negatively supercoiled
13.4
What happens when DNA in a loop is nicked?
supercoils in that loop unwind and DNA duplex forms a relaxed double helix
13.4
How do bacterial cells package their DNA?
as a nucleoid composed of many loops
13.4
How do eukaryotic cells package their DNA?
as one molecule per chromosome
13.4
Describe the similarity between bacterial and eukaryotic cell packaging.
- have topoisomerase II
- DNA is usually negatively supercoiled
13.4
Describe eukaryotic DNA.
- linear
- each DNA molecule forms a single chromosome
13.4
How is DNA packaged in a chromosome?
with proteins to form a DNA-protein complex called chromatin
13.4
Describe histone proteins.
- found in all eukaryotes
- interact with any double-stranded DNA
- are evolutionarily conserved, which means that they are very similar in sequence from one organism to the next
- histone proteins from one eukaryote can associate with DNA from another
- form the core of a nucleosome (8 proteins)
- each nucleosome also includes a stretch of DNA wrapped twice around the histone core
- rich in positively charged amino acids lysine and arginine, which are attracted to the negatively charged phosphate groups in each DNA strand
13.4
What is the first level of chromatin packaging also called?
- beads on a string (nucleosomes are the beads and the DNA is the string_
- 10-nm fiber (reference to its diameter, which is about five times the diameter of the DNA double helix
13.4
Describe the second level and onwards of chromatin packaging.
- occurs when chromatin is more tightly coiled, forming a 30-nm fiber
- as chromosomes in nucleus condense in preparation for cell division, each chromosome becomes progressively shorter and thicker as the 30-nm fiber coils onto itself to form a 300-nm coil, a 700-nm coiled coil, and finally a 1400-nm condensed chromosome in a manner that is still not fully understood
13.4
What is chromosome condensation?
progressive packaging
an active, energy-consuming process requiring the participation of several types of proteins
13.4
When is greater detail of the structure of a fully condensed chromosome revealed?
when the histones are chemically removed
13.4
What happens to DNA without histones?
DNA spreads out in loops around a supporting protein structure called the chromosome scaffold
13.4
What is the size difference between the volume of a fully condensed human chromosome and a bacterial cell?
volume of a fully condensed human chromosome is five times larger than the volume of a bacterial cell
13.4
What are the 6 levels of chromosome condensation?
- DNA duplex (2 nm in diameter)
- nucleosome fiber (210 nm in diameter)
- chromatin fiber (30 nm)
- coiled chromatin fiber (300 nm)
- coiled coil (700 nm)
- condensed chromatid (1400 nm)
13.4
Describe the first level of chromatin packaging.
eukaryotic DNA winds around histone proteins
