Nucleic Acids 1- Nucleic Acids and chromosomes Flashcards
What are chromosomes made of
DNA and histones- they segregate as cells divide.
What is a nucleotide?
A polymer made up of nucleic acid monomers.
Characteristics of RNA
Single stranded and it can assume a variety of shapes.
What does a nucleotide consist of?
A pentose sugar, a nitrogenous base and one or more phosphate groups.
What is the difference between deoxyribose and ribose?
no -OH group on 2’ C atom in deoxyribose- just a H group.
What ‘ Carbons are bonded to what
1’ C- base
3’C- OH group
5’C- phosphate group.
What are the differences between adenine and guanine
Adenine has an amine group attached to the purine ring- guanine has a carbonyl group and amine group attached to the purine ring.
What is the difference between uracil, cytosine and purine
Cytosine has an amine group attached to the pyrimidine ring, thymine has a carbonyl group attached to the pyrimidine ring, uracil has a methyl group attached to the pyrimidine ring.
What do nucleosides consist of
Pentose sugar and a base.
What are the names of the nucleosides
adenine (deoxy)adenosine guanine (deoxy)guanosine cytosine (deoxy)cytidine uracil (deoxy)uridine thymine (deoxy)thymidine
Describe the structure of a single chain of DNA
A long chain of deoxyribonucleotide monomers linked by phosphodiester bonds.
Describe the bonding between adjacent nucleotides.
The 3’-OH of sugar of one nucleotide is linked to phosphate group, which in turn is joined to 5’-OH of adjacent sugar.
What is the directionality of DNA
5’ end and 3’ end- DNA is asymmetrical.
In what sequence are the nucleotides read
The first nucleotide in a sequence has a free 5’ phosphate, and the last one a free 3’ OH.
How did Watson and Crick determine the secondary structure of DNA?
Analysing X-Ray diffraction patterns from DNA fibres.
Where are the phosphate groups found in the double helix
on the outside-giving DNA a negative charge.
Where are the bases found in the double helix.
They point inwards and the flat planes are perpendicular to the helix. They stack on top of each other- contributing to the stability of the double helix.
How many bases are there per helical turn
10
What is the width of DNA
2nm
Why are the hydrogen bonds between C and G more stable
Because there are more of them
Describe the melting and re-annealing of DNA.
Melting- heat to near boiling point or have a low salt conc- denatures DNA
Re-annealing- cool or high salt conc- hydrogen bonds reform- and due to CBP- original DNA strand reformed- hence it is a reversible process.
What is the genome
The entire DNA coding for the organism
Describe the E.coli genome
has 4.6 x 106 base pairs in a single circular double-stranded molecule. The length of E.coli DNA is 1.4 mm- longer than itself- has to be supercoiled. Less DNA- no chromosomes- DNA is circular.
Describe the human genome
~ 3 x 109 base pairs of DNA divided into chromosomes that each contain a single, linear double-helical DNA molecule of ~ 200 x 106 base pairs.
When are chromosomes visible
Just before cells begin mitosis- hence they are not visible in non-dividing cells.
Describe the difference between mitotic and interphase chromosomes
Mitotic- highly condensed chromosomes
Interphase- more extended form
Chromosomes are not all the same size.
What is the Karyotype
A karyotype is an organised profile of someone’s chromosomes.
A diploid human cell has 46 chromosomes
22 pairs of ‘normal’ chromosomes (autosomes) and
2 sex chromosomes (X and Y)
What can the Karyotype be used for
Can be used to detect abnormalities such as genetic diseases and cancer. Each chromosome is stained with unique fluorescent dye. For example Abnormalities in breast tumour: 48 chromosomes instead of 46, multiple translocations, eg two pieces of chromosome 8 (green) plus piece of chromosome 17 (purple).
Why is it important that the DNA is coiled.
The DNA in a diploid human cell is nearly 2m long- but nucleus is only 5 - 8 m in diameter.
How is DNA packaged in eukaryotic cells
Eukaryotic DNA is tightly packaged, forming a complex with proteins. This complex is called chromatin. During interphase, when cells are not dividing, chromatin is more extended. Further condensation during mitosis produces chromosomes.
What is the lowest level of packaging
A nucleosome- which consists of DNA wrapped around histones.
Describe the structure of the nucleosome
8 histones; 2 each of 2A, 2B, 3 and 4
About 150 base pairs of DNA wrapped around the histones (core DNA)
Histone 1 between the nucleosomes- on the outside.
Linker DNA between each nucleosome.
How do histones and DNA interact.
Histones: positively charged; interact with negatively charged sugar-phosphate backbone of DNA
What fold condensation do nucleosomes give
around 7 fold- 200 bp is 68 nm when not wrapped- but wrapping around each histone octamer is around 10nm.
How are the nucleosomes packed further to give chromatin
Nucleosomes are folded to give 30nm fibre- 40 fold condensation- 6 nucleosomes then coil together to fold into loops. Net result: each DNA molecule has been packaged into a mitotic chromosome that is 10 000 fold shorter than its fully extended length.
Interphase chromosomes will have a lower degree of folding.
