Ch 5: DNA and Chromosomes Flashcards
What is one of the most important things that life depends on?
The stable storage, maintenance, and inheritance of genetic information
General description of each DNA molecule
A double helix, composed of antiparallel complementary DNA strands, which are held together by hydrogen bonds between G-C and A-T base pairs
The nucleotide consists of a phosphate, a sugar, and a base
How is the genetic material of a eukaryotic cell kept?
it is contained in a set of chromosomes, each from a single long strand of DNA molecule that contains many genes
Besides genes, what must eukaryotic (linear) DNA contain?
- many replication origins
- one centromere
- two telomeres
What do the centromeres and telomeres do?
they help ensure that each chromosome is duplicated efficiently, and can be separated out to the two daughter cells
How is DNA folded?
It is tightly bound to histone and non-histone chromosomal proteins, this combination of DNA and protein is called chromatin
Histones pack the DNA into a repeating array of DNA protein particles called nucleosomes, which then associate with each other to form more compact chromatin structures
Cells can regulate their chromatin by temporarily condensing or decondensing particular regions
What are the different types of chromatin and what do they do?
Heterochromatin packs DNA so tightly that gene expression is often turned off. It can spread along a chromosome, and it can be inherited from one daughter cell to the next, estabilshing heritable gene expression from one cell to the next
Euchromatin is less compact, and allows access to proteins involved in transcription
cell cycle
the orderly sequence of events by which a cell duplicates it’s contents and divides into two
interphase: chromosomes are duplicated
mitosis: duplicated chromosomes are distributed to the daughter nuclei
centromere
specialized DNA sequence that allows duplicated chromosomes to be separated during M phase; can be seen as the constricted region of a mitotic chromosome
Chromatin
complex of DNA and proteins that makes up the chromosomes in a eukaryotic cell. The DNA carries the genetic information, the proteins package and control that information
chromosome
long, threadlike structure composed of DNA and proteins that carries the genetic information of an organism. It becomes visible as a distinct entity when a plant or animal cells prepares to divide
Epigenetic inheritance
the transmission of a heritable pattern of gene expression from one cell to it’s progeny that does not involve altering the nucleotide sequence of the DNA
Gene
Unit of heredity that directs the production of a particular protein or functional RNA molecule. Includes introns
Gene expression
The process by which a gene makes a product that is useful to the cell or organism by directing the synthesis of a protein or an RNA molecule with a characteristic activity
Genetic code
set of rules by which the information contained in teh nucleotide sequence of a gene and its corresponding RNA molecule is translated into the amino acid sequence of a protein
Genome
The total genetic information carried bya ll the chromosomes of a cell or organism.
In humans, the total number of nucleotide pairs in the 22 autosomes plus the X and Y chromosomes
Heterochromatin
Highly condensed region of an interphase chromosome; generally gene-poor and transcriptionally inactive
Histone
One small group of abundant, highly conserved proteins around which DNA wraps to form nucleosomes, structures that represent the most fundamental level of chromatin packing
Karyotype
An ordered display of the full set of chromosomes of a cell, arranged with respect to size and number.
Each pair should have two copies that are the same size and shape. Translocations between chromosomes often result in cancer
nucleolus
Large structure within the nucleus where ribosomal RNA is transcribed and ribosomal subunits are assembled
nucleosome
bead-like structural unit of a eukaryotic chromosome composed of a short length of DNA wrapped around an octameric core (eight proteins) of histone proteins
Replication origin
Nucleotide sequence at which DNA replication is initiated
Telomere
repetitive nucleotide sequence that caps the ends of linear chromosomes. Counteracts the tendency of the chromosome to otherwise shorten with each replication
Why is it important to study DNA and everything that interacts with it?
The biotech revolution started with the discovery of DNA structure. Pretty much anything that interacts with DNA is important to the biotech world
What are all the key components within a nucleic acid?
1) phosphate backbone
2) sugar
3) base
How do the different nucleotides bond?
When a helix forms, it is always the bonds between the base pairs that form the helix
The bond using hydrogen bonds: 2 bonds for A-T pairs, and 3 for G-C pairs, which means that it takes more energy to break apart strands with a high proportion of C-G pairs
How does the length and the %CG concentration of the DNA affect the melting temperature?
longer strands and higher C-G concentrations mean they need a higher temperature to break apart. It scales almost linearly with more nucleotides
What are the major and minor grooves in DNA?
They are the grooves where the base pairs of DNA are exposed. The proteins need to be able to get into contact with the base pairs in order to read the DNA
How many chromosomes for human DNA?
Human DNA has 23 pairs of chromosomes: 22 autosomes plus the x and y chromosome
What does a gene contain?
1) the exon: the part that becomes a protein
2) the intron: gets spliced out
3) regulatory sequences
4) “junk sequences” secquences that we are slowly figuring out, may contain regulatory sequences
Explain an example of chromosome translocation
The Philadelphia Chromosome: causes leukemia, a blood cancer (especially B-cells). A translocation between chromosome 9 and 22.
ABL: when turned on, causes growth, so regulation of this gene is important
When ABL is translocated to be next to BCR, it loses it’s regulatory mechanism and starts uncontrollably expressing BCR, which leans to uncontrolled cell growth. (The BCR/ABL fusion gene)
Kinase
takes ATP and transfers a phosphate group to another protein.
This has implications for treating things like BCR/ABL fusion, because blocking this mechanism of the BCR/ABL protein can stop it’s uncontrolled growth. However it has lots of side effects because it is not very specific, and lots of relapses
When is chromosomal translocation most likely to happen?
During mitosis, because the genome is exposed for a long duration, and the extreme force needed to pull the chromosomes apart is most likely to cause breaks
Euchromatin
less compact than heterochromatin
Contains the majority of the genes
Active and allows for transcription to occur
What is an important mechanism for turning genes on an off?
Chromatin remodelling: transitions between chromatin states, generally requires ATP
Histone codes
certain histone tails affect genes in specific ways (by promoting either DNA condensation or decondensation). Deciphering the histone code can help us figure out gene expression and regulation and epigenetics
How many chromosomes does a normal human skin cell have? How many in a sperm or egg cell?
skin cell: 23 pairs
sperm or egg: 23
Which sex chromosomes does a human egg cell have?
X chomosome
What are somatic cells vs germ cells?
Somatic cell shave the full complement of chromosomes
germ cells (sprem/egg) do not have the full complement
What elements of the genome are required for proper transmission of genetic information to the off-spring?
3 elements:
1) replication origin, telomere, centromere
How does a genome evolve?
Mutations during gene replication may be selected during adaptation
How is the DNA backbone formed?
The nucleotides in the chain are linked through sugars and phosphates, which form a backbone of alternating sugar-phosphate groups, which are held together by phosphodiester bonds linking the 5’ end to the 3’ end
In their 1953 paper in the double-helical structure of DNA, Watson and Crick famously wrote: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism of the genetic material.” What did they mean?
Each strand in a DNA double-helix contains all the information needed to produce a complementary partner strand
When are chromosomes in their most compact form?
During mitosis: it creates chromosomes as discrete bodies that are relatively easy to separate and organize
What is true about the association of histone proteins and DNA?
Histone proteins have a high proportion of positively charged amino acids, which bind tightly to teh negatively charged DNA backbone
Where is heterochromatin not commonly located?
chromosomal regions carrying genes that encode ribosomal proteins. These genes are active in gene transcription, so are unlikely to be turned off by being tightly compacted into heterochromatin.
