Chapter 5: DNA and Chromosomes Flashcards
Base + sugar + phosphate =
nucleotide, the monomer subunit of DNA (polymer of nucleotides)
Base + sugar =
nucleoside
Why is DNA referred to as 5’ -> 3’?
This is the order DNA is replicated
Purines
- 2 ring structures
- Adenine
- Guanine
Pyrimidines
- 1 ring structure
- cytosine
- thymine
How are bases paired?
Adenine and Thymine, can form 2 hydrogen bonds
Guanine and Cytosine, can form 3 hydrogen bonds
DNA vs RNA structure
Both: have nitrogenous base connected to 1’ end, and phosphate attached to 5’ end to lead to the next nucleotide connection
DNA has H on 2’ end, while RNA has OH group on 2’
Describe formation of DNA
- phosphodiester bonds between 3’ (OH) and 5’ (PO3) carbons on deoxyribose
- 2 strands held together in double helix by hydrogen bonds between complimentary base pairs
- strands MUST run antiparallel
- leaves 3’ end OH group and 5’ end phosphate group on opposite ends
How does DNA encode genes?
- DNA is transcribed to RNA
- some RNAs are translated to direct the production of specific protein molecules, some have RNA as the final product
What two reactions are the most frequent spontaneous chemical reactions known to cause DNA damage in cells?
Deamination and Depurination, both involved hydrolysis
Depurination
disruption of bond, causes base to be removed and left with depurinated sugar
purine bases - A or G - removed and leaves phosphate and sugar alone
Deamination
-removal of an amino group
How is DNA packaged?
in heterochromatin and euchromatin
pattern of:
heterochromatin/telomere -> euchromatin -> heterochromatin/centromere -> euchromatin…….
goes on until end, which will be heterochromatin/telomore
Heterochromatin
firmly/densely packed part of chromosomes that is genetically inactive
consists of a telomeres and centromeres
Euchromatin
uncoiled, loosely packed part of chromosomes that is genetically active
Nucleosomes
- basic units of eukaryotic chromosome structure
- contain DNA wrapped around a protein core of eight histone molecules
What are histones?
- contain many basic amino acids (lysine, arginine), have + charge
Explain structure of DNA with histone proteins
- “beads on a string”
- histones wrapped in form of chromatin, with nucleosome including approx. 200 nucleotide pairs of DNA
- each histone has space for linker DNA in between them
- nucleosome released from digestion of linker DNA by nuclease
- ## only exposed linker DNA digested, not those wrapped around nucleosome core
Histone code
a hypothesis which states that DNA transcription is largely regulated by post-translational modifications to these histone proteins
- Different combinations of histone tail modifications can confer a specific meaning on the stretch of chromatin on which they occur
- provides cell memory, has different chemical tags that are put on amino acids of the histones that extend out of the nucleosome
What is cell memory based on?
- chromatin structure
- structure may effect gene expression, inherit when cell divides
- some genes may only act once
Eukaryotic DNA
- packed into multiple chromosomes
- specialized sequences required for DNA replication and chromosome segregation
- can carry ancient sequences
Chromosome Painting
- exposes the chromosomes to collection of single-stranded DNA molecules that have been coupled to comb of fluorescent dyes
- human chromosome pairs can be identified by hybridization to chromosome specific fluorescent probes
- two copies each, maternal and paternal chromosomes (homologous)
Describe the process needed to produce a eukaryotic chromosome that can be duplicated and then segregated at mitosis
Each chromosome has multiple origins of replication, one centromere, and two telomeres.
The DNA replicates in interphase, beginning at the origins of replication and proceeding bidirectionally from each origin along the chromosome.
In M phase, the centromere attaches the compact, duplicated chromosomes to the mitotic spindle so that one copy will be distributed to each daughter cell when the cell divides.
Prior to cell division, the centromere also helps to hold the duplicated chromosomes together until they are ready to be pulled apart. Telomeres contain DNA sequences that allow for the complete replication of chromosome ends.
Mobile genetic elements
- in genomes of nearly all organisms
- ancient viral like sequences
- can move from one position to another by transpostional or site specific recombonation
Viruses and transposable elements
- put themselves into genome
- created genetic variation
- crucial for evolution
Give an example of the histone code
H3K4me^3
it activates transcription
it attracts proteins that make euchromatin
Why do histones have many amino acids that have a net positive charge?
it stabilizes the negative charge on the DNA sugar phosphate backbone; so DNA can be packaged tightly around the histones
