UNIT 3 LESSON 2 DNA AND CHROMOSOMES

Question 1

before dna discovery

Answer 1

fredirich miescher isolated the nuclei of white blood cells and extracted an acdiic molecule he called nuclein which is renamed as nucleic acids

Question 2

what abt nucleic acids

Answer 2

in 1919, phoebus levene isolated two types of nucleic acids: rna and dna
- he proposed that both were made up of individual units called nucleotides.
- each nucleotide was composed of one of four nitrogen containing bases, a sugar, and a phosphoate

Question 3

the sugar diff

Answer 3

dna nucleotides: deoxyribose sugar group where h is bonded instead of oh to carbon #2
rna nucleotides:
ribose sugar group where oh is at carbon 2

Question 4

the nitrogen bases

Answer 4

dna has nucleotide adenine, guanine, cytosine and thymine
rna has the same purine bases and the cytosine base and uracil is present instead of thymine

Question 5

intermolecular bonds

Answer 5

nucleic acids are made up of nucleotides held by covalent bonds
the nitrgoen base is attached to the i’ carbon by a glycosidic bond
the phosphate group is attched to the 5’ carbon by a phosphodiester bond

Question 6

whats chargaffs rule

Answer 6

an austrian american biochem chargaff launched a research progran to study the chemistry of nucleic acids
by late 1940s, he concludeded:
- there is variation in the composition of nucleotides in diff species
- a=t and g=c

Question 7

RECAP: by 1940s, scientific community knew

Answer 7

• dna is the hereditary material (hershey and chase)
• dna is a polymer of nucleotides and each of the 4 types of nucleotides contain a po4, a deoxyribose sugar and 1 of the 4 bases ( a g c or t) - (levene)
• dna is composed of repeating units of nucletodies in fixed roportions that vary with diff species (chargaff)

Question 8

what did pauling discover

Answer 8

in 1951, pauling discovered that many proteins have helix-shaped structures
watson and crick used this info when deducing the structure of dna

Question 9

franklin - a helix structure of dna

Answer 9

rosalind franklin and maurice wikins used x ray diffraction to analyze dna samples in 1950
- franklin got high resoltuion photos and using math theroy she determiend:
- dna has helical structure
- the nitrogen bases are on the inside of the dna helix and the sugar phosphate bckbone is on the outside

Question 10

what did watson and crick do

Answer 10

• n 1953, waatson and crick published a paper that proposed a dna structure that;
• had a twisted ladder which called a double helix. the sugar phosphte molecules make up the sides or handrails of the latter and the bases make up the rungs of the ladder by potruding inwards.

Question 11

watson and crick model

Answer 11

• the distance between the sugar phosphate backbones remains constant over the length of a molecule in dna.
  to remain this constain:
  – a is always with t
  – g is always with c

Question 12

whats the modern dna model

Answer 12

• its composed of two polynucleotide strands that twist around each other to form double helix.
• the twos trands are complementary due to complementary base pairing of a with t and c with g
• hydrogen bonds link each complementary base pair.
  – two h bonds between a and t
  – three h bonds between c and g

Question 13

what else abt the modern dna model

Answer 13

each strand had a 5 prime end to a 3 prime end. the 5’ to 3’ come from the numbering of carbons in the deoxyribose sugar.
- the two strands are antiparallel, where the 5 prime end from one strand is across from the 3 prime end of the complementary strand.
- the sequence of a dna strand is written in the 5’ to 3’ direction

Question 14

eukaryotic chromosomes

Answer 14

THE DNA HELIX IS COILED AND LONG SEGEMENTS (2OO nucleotides long) are wrapped around a complex of 8 histones (proteins)
– the bead like structure is called a nucleosome
- dna is negtive (bcuz of phosphate groups) and so is attrcted to the positively charged histones.
- the histones protect the dna from breaking & detangling

Question 15

what is chromatin

Answer 15

nucleosomes are packed as a solenoid which is a 30 nm fiber that has at least 5 nucleosmes strands together wound helically.
- this arrangment of dna maeks it more compact and easier to accomodate in relatively small nucleus

Question 16

chromosomes

Answer 16

• the 30 nm solenoids form loops, which are attached to a supporting protein scaffold.
• the scaffold folds further to condense the dna into chromosomes.
• the chromosomes are duplicated during replication, forming pairs of identical chromatids join by a centromere.