DNA structure Flashcards
DNA-
Deoxyribonucleic Acid
* relatively simple molecule
* carrier of genetic information
* Genes of every cell on Earth are made of DNA
* Cells maintain, replicate and express genetic information carried in DNA
* information in genes:
- copied and transmitted to daughter cell multiple times
- contains primarily instructions for making proteins
Dna Strands
consists of two long polynucleotide chains
DNA strand
Each strand made of 4 types of nucleotide subunits
( linked by phosphodiester bonds —> sugar-phosphate backbone with N-bases sticking out)
DNA Structure-
Two strands held togeher by hydrogen bonds between the bases -
purines ( adenine, guanine) and pyrimidines ( cytosine, thymine)
DNA
The opposing sugar- phosphate backbones have opposite polarity ( antiparallel)
complementary base pairing :
* enables the base pairs to be packed in the energetically most favorable arrangment ( same width 1- ring base pairs with 2 ring base –> same distance between sugar
*provides the basis for replication of nucleic acids
DNA structure
2 sugar-phosphate backbones twist around one another —-> form a double helix with 10 bases per helical turn
Base pairs fit together when the two strands are antiparallel ( opposite polarity)
Each strand of DNA molecule has a nucleotide sequence exactly complementary to its partner strand-
critical for copying DNA
DNA
carrier of genetic information
Genome
a complete set of genetic information in a cell
Genes-
fragments of DNA molecule coding for proteins + many non- coding sequences –> extremely long sequence of nucleotides ( message written in 4 letter code)
Human cell contain s- ____ ___ of DNA?
2 meters
Eucaryotic Cell Nucleus
-Provides a compartment in which the DNA and RNA-
-dependent functions are sequestered
Eucaryotic Cell Nucleus-
Enclosed by the ____ ____
nuclear envelope
Eucaryotic Cell Nucleus
May contain one or more ________? the site of ____
nucleoli
rRNA synthesis and ribosome assembly
Eucaryotic Cell Nucleus-
Nuclear Pores-
Nuclear envelope is penetrated by numerous nuclear pores
* allow passage of molecules and large particles:
from the nucleus to the cytosol and from the cytosol to the nucleus
* movement through the pores is regulated
Functions of the nucleus:
* DNA replication
* DNA packing -chromosomes
* DNA transcription —-> mRNA,rRNA and tRNA
* processing of mRNA
*mRNA transport
*ribosome assembly
* dissolution and reformation of the nuclear envelope during mitosis and meiosis
Eucaryotic Cell Nucleus
supported internally by the nuclear lamins ( intermediate filaments)
surrounded externally by a network of intermediate filaments
DNA packing - Chromosomes
name derived from Greek ( chroma + soma ) “ colored body”
composed of DNA + proteins = chromatin
DNA packing
Human genome- 3.2 x 10^9 nucleotides distributed over 24 chromosomes
Each cell contains 2 copies of each chromosome ( with a few exceptions) - one from the mother and one from the father
ploidy
number of sets of chromosomes per cell
haploid-
one set of chromosomes per cell
diploid
two sets of chromosomes per cell
tetraploid-
four sets of chromosome per cell
Identification of Chromosomes
Chromosomes can be specifically identified in various ways
Karyotype-display of the full set of mitotic chromosomes
Banding patterns allows identification of human chromosomes
Giemsa dye-stains regions rich in A-T
Chromosome functions-
* carry genes
Genes- segments of DNA containing instructions for making a specific protein or set of closely related proteins, or directing the RNA production
~30,000 genes in human cell + non coding sequences of DNA
Chromosomes undergo morphological changes during cell life.
Interphase chromosomes -tangled treats- cannot be distinguished in light microscope,
Metaphase chromosomes - highly condensed - easy to identify in light microscope
Problem: extremely long DNA molecules in the cell must be packed into a very small volume and in such a way as to -
*prevent tangling
* be reversible
* allow rapid, localized, on demand access to DNA
solution:
DNA is packaged into a series of higher order structures by specialized proteins that coil and fold the DNA.
Interphase-
DNA 1,000 times more compact than in the extended form
Metaphase-
compaction ratio ~10,000 fold
DNA Packing
DNA + proteins= chromatin
Proteins:
2 classes - Histone and nonhistone chromosomal proteins
Histones
* large quanities ( ~ 60 million molecules/cell, total mass in chromatin about equal to DNA mass)
* responsible for the first level of chromatin packing
- Nucleosomal histones= H2A, H2B, H3 & H4 small, highly conserved proteins responsible for coiling of DNA into Nucleosomes
- H1 histomes - pack the DNA + nucleosomes into a coil
* positively charged
* the most highly conserved of all known eucaryotic proteins
Nucleosomes
-fundamental packing units of DNA
Nucleosomes
* made of-
- protein core- complex of 8 histone proteins ( histone octamer-2 of each H2A, H2B,H3 & H4)
- double stranded DNA- ~146 nucleotide pairs wrapped twice around octamer
- linker DNA- up to 80 nucleotides.
Formation of Nucleosomes
- 1st level of packing
- converts a DNA molecule into a chromatin tread- ~ 1/3 of its initial length
* beads on a string “ formation of chromatin”
Formation of 30 nm fiber
* 2nd level of packing
* native form of DNA
*nucleosomes bundle together by H1 histones
* Unclear how nucleosomes are packed in a fiber- most probable zigzag model or selenoid structure
Levels of DNA packing:
- Beads on a string
- nucleosomes linked by linker DNA - 30 nm fiber
- Native form of DNA
- Nucleosomes bundled together by H1 histones
- Unclear how nucleosomes are packed in a fiber- most probable zigzag model - Looped domain- Current model
- Loops od 30 nm fiber attached to proteins that form the chromosomal axis
20,000-100,000 bp per loop
- Metaphase chromosome- final level of packing
DNA packing - Chromosomes
Interphase cells
Chromatin in an interphase chromosome is not in the same packing state throughout the chromosome
* regions with genes that are being expressed are more extended, regions with quiescent genes- more compact —> chromosome structure can difffer from cell to cell during cell life.
Forms of chromatin in interphase cell:
* Heterochromatin ( 10% of chromatin)
- highly condenced
transcriptionally intactive
- most of the heterochromatin does not contain genes
*concentrated around centromere and telomeres
Forms of chromatin in Interphase cell:
* Active euchromatin (10% of chromatin)
* least condedsed
* histone h1 les tightly bound
* nucleosomal histones chemically modified
Forms of chromatin in interphase cell:
Inactive euchromatin ( 80% of chromatin)
* more condensed than active euchromatin
* can become active euchromatin
- it is not clear which of the levels of chromatin packing is present in euchromatin and in heterochromatin
Miotic cell
Metaphase chromosomes
very condensed chromatin
sperm cells
* sperm head-
most condensed form of chromatin
DNA packing
Nucleosome Replication and Assembly:
- nucleosomes must be moved out of the way to permit DNA to replicate or be translated
- New nucleosomes must be assembled as DNA is replicated
- New histones are synthesized at the same time as DNA replication
- New nucleosomes assemble on the daughter DNA helices shortley after the DNA is replicated
Dna PAcking
Nucleosome
-binding to DNA affected by:
_ base sequence ( AT- rich regions easier to bend)
-Binding of other proteins - may displace nucleosomes
DNA packing
Eucaryotic cells have mechanisms to adjust the local structure of chromatin
* chromatin remodling complexes-
- protein machines
- use ATP to change nucleosome structure
- make DNA more accessible to specialized proteins ( e.g. those involved in replication, gene expression, and DNA repair)
- inactive during mitosis -helps maintain tightly packed chromosome structure
DNA packing
Eucaryotic cells have mechanisms to adjust the local structure of chromatin
* reversible modification of histone tails
* N- terminal tails function in regulating chromatin structure
*undergo covalent modifications after nucleosomes assembly
* modified tails bind and attract specific proteins to diffrent chromatin regions ( some facilitate further chromatin condensation, some facilitate access to DNA)
*Histone modifying enzymes are strongly regulated
* diffrent combinations of tail modification and diffrent sets of histone binding proteins give diffrent signals ( e.g. for genes expression, replication)
DNA packing
Eucaryotic cells have mechanisms to adjust the local structure of chromatin
Chromatin remodeling complexes and histone tails modifying enzymes may work in concert allowing rapid changes in chromatin structure according to cell needs.
