DNA structure

Question 1

DNA-

Deoxyribonucleic Acid

Answer 1

* 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

Question 2

Dna Strands

Answer 2

consists of two long polynucleotide chains

Question 3

DNA strand

Each strand made of 4 types of nucleotide subunits

Answer 3

( linked by phosphodiester bonds —> sugar-phosphate backbone with N-bases sticking out)

Question 4

DNA Structure-

Two strands held togeher by hydrogen bonds between the bases -

Answer 4

purines ( adenine, guanine) and pyrimidines ( cytosine, thymine)

Question 5

DNA

The opposing sugar- phosphate backbones have opposite polarity ( antiparallel)

Answer 5

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

Question 6

DNA structure

2 sugar-phosphate backbones twist around one another —-> form a double helix with 10 bases per helical turn

Answer 6

Base pairs fit together when the two strands are antiparallel ( opposite polarity)

Question 7

Each strand of DNA molecule has a nucleotide sequence exactly complementary to its partner strand-

Answer 7

critical for copying DNA

Question 8

DNA

Answer 8

carrier of genetic information

Question 9

Genome

Answer 9

a complete set of genetic information in a cell

Question 10

Genes-

Answer 10

fragments of DNA molecule coding for proteins + many non- coding sequences –> extremely long sequence of nucleotides ( message written in 4 letter code)

Question 11

Human cell contain s- ____ ___ of DNA?

Answer 11

2 meters

Question 12

Eucaryotic Cell Nucleus

-Provides a compartment in which the DNA and RNA-

Answer 12

-dependent functions are sequestered

Question 13

Eucaryotic Cell Nucleus-

Enclosed by the ____ ____

Answer 13

nuclear envelope

Question 14

Eucaryotic Cell Nucleus

May contain one or more ________? the site of ____

Answer 14

nucleoli

rRNA synthesis and ribosome assembly

Question 15

Eucaryotic Cell Nucleus-

Nuclear Pores-

Answer 15

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

Question 16

Functions of the nucleus:

Answer 16

* 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

Question 17

Eucaryotic Cell Nucleus

supported internally by the nuclear lamins ( intermediate filaments)

Answer 17

surrounded externally by a network of intermediate filaments

Question 18

DNA packing - Chromosomes

name derived from Greek ( chroma + soma ) “ colored body”

Answer 18

composed of DNA + proteins = chromatin

Question 19

DNA packing

Human genome- 3.2 x 10^9 nucleotides distributed over 24 chromosomes

Answer 19

Each cell contains 2 copies of each chromosome ( with a few exceptions) - one from the mother and one from the father

Question 20

ploidy

Answer 20

number of sets of chromosomes per cell

Question 21

haploid-

Answer 21

one set of chromosomes per cell

Question 22

diploid

Answer 22

two sets of chromosomes per cell

Question 23

tetraploid-

Answer 23

four sets of chromosome per cell

Question 24

Identification of Chromosomes

Chromosomes can be specifically identified in various ways

Answer 24

Karyotype-display of the full set of mitotic chromosomes

Banding patterns allows identification of human chromosomes

Giemsa dye-stains regions rich in A-T

Question 25

Chromosome functions- \* carry genes

Answer 25

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

Question 26

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

Answer 26

solution: DNA is packaged into a series of higher order structures by specialized proteins that coil and fold the DNA.

Question 27

Interphase-

Answer 27

DNA 1,000 times more compact than in the extended form

Question 28

Metaphase-

Answer 28

compaction ratio ~10,000 fold

Question 29

DNA Packing DNA + proteins= chromatin Proteins:

Answer 29

2 classes - Histone and nonhistone chromosomal proteins

Question 30

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

Answer 30

\* positively charged \* the most highly conserved of all known eucaryotic proteins

Question 31

Nucleosomes

Answer 31

-fundamental packing units of DNA

Question 32

Nucleosomes \* made of-

Answer 32

- 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.

Question 33

Formation of Nucleosomes

Answer 33

- 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"

Question 34

Formation of 30 nm fiber

Answer 34

\* 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

Question 35

Levels of DNA packing:

Answer 35

1. Beads on a string - nucleosomes linked by linker DNA 2. 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 3. Looped domain- Current model - Loops od 30 nm fiber attached to proteins that form the chromosomal axis 20,000-100,000 bp per loop 4. Metaphase chromosome- final level of packing

Question 36

DNA packing - Chromosomes Interphase cells Chromatin in an interphase chromosome is not in the same packing state throughout the chromosome

Answer 36

\* 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.

Question 37

Forms of chromatin in interphase cell: \* Heterochromatin ( 10% of chromatin)

Answer 37

- highly condenced transcriptionally intactive - most of the heterochromatin does not contain genes \*concentrated around centromere and telomeres

Question 38

Forms of chromatin in Interphase cell: \* Active euchromatin (10% of chromatin)

Answer 38

\* least condedsed \* histone h1 les tightly bound \* nucleosomal histones chemically modified

Question 39

Forms of chromatin in interphase cell: Inactive euchromatin ( 80% of chromatin)

Answer 39

\* 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

Question 40

Miotic cell Metaphase chromosomes

Answer 40

very condensed chromatin

Question 41

sperm cells \* sperm head-

Answer 41

most condensed form of chromatin

Question 42

DNA packing Nucleosome Replication and Assembly:

Answer 42

- 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

Question 43

Dna PAcking Nucleosome

Answer 43

-binding to DNA affected by: \_ base sequence ( AT- rich regions easier to bend) -Binding of other proteins - may displace nucleosomes

Question 44

DNA packing Eucaryotic cells have mechanisms to adjust the local structure of chromatin \* chromatin remodling complexes-

Answer 44

- 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

Question 45

DNA packing Eucaryotic cells have mechanisms to adjust the local structure of chromatin \* reversible modification of histone tails

Answer 45

\* 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)

Question 46

DNA packing Eucaryotic cells have mechanisms to adjust the local structure of chromatin

Answer 46

Chromatin remodeling complexes and histone tails modifying enzymes may work in concert allowing rapid changes in chromatin structure according to cell needs.