What is the structure of DNA?
Double helix
How many base pairs are there for every complete turn of the DNA helix?
10
How are nucleotides in DNA held together?
By the sugar phosphate backbone
What type of bond forms between individual nucleotides on the sugar phosphate backbone?
Covalent bonds between 5’ phosphate and 3’ -OH of deoxyribose
What type of bond forms between complementary base pairs?
Hydrogen
In what direction do the two strands of DNA run?
Antiparallel
- 5’ to 3’ on one strand
- 3’ to 5’ on the other strand
What is a nucleoside?
Pentose sugar and a nitrogenous base
What is a nucleotide?
Pentose sugar, nitrogenous base and a phosphate group
What are the three functions of DNA?
Gene expression
Replication
Store genetic information
Describe the process by which a gene is expressed.
- Promoter regions on DNA establish the gene to be expressed (RNA polymerase II)
- Transcription - RNA polymerase moves along the template strand synthesising an mRNA molecule containing the code for the gene to be expressed
- Translation - mRNA is used as a template to assemble a sequence of amino acids which further folds to produce a protein
Describe the process of DNA replication.
- Complementary strands are fully separated
- Each strand acts as a template
- Free bases form hydrogen bonds with the exposed strand
- semi conservative replication
What is a chromosome?
A string of DNA combined with proteins
Are humans haploid or diploid?
Diploid
How many chromosomes do humans have?
46
- 22 pairs of autosomes
- 1 pair of sex chromosomes
What is the structure of a chromosome?
Telomere P arm Centromere Q arm Telomere
Where is the telomere found and what is its function?
Found at either end of the chromosome
Protects the chromosome and the genetic information carries
How are telomeres affected during DNA replication and why?
Short pieces of the telomere are lost
This ensures no genetic information is lost during DNA replication
What is the function of the centromere?
Holds metaphase chromosomes together
Attaches to mitotic spindle during cell division
How are bands numbered on chromosomes?
From the centre out
What type of dyes are used to stain chromosomes?
Fluorescent
Describe the process by which DNA is transformed into a chromosome.
- DNA coils around a histone to form a nucelosome
- H1 protein binds to the outside of the nucelosome to form a chromatosome
- Chromatosomes fold to produce 30nm fibre
- 30nm fibre forms loops 300nm in length
- Loops are compressed and folded to produce 250nm wide fibre
- Tight coiling of 250nm wide fibre produces chromatids
- Chromatids are joined by centrosomes to form chromosomes
What is a nucleosome?
DNA coiled around a histone octamer
What is an octamer?
2 molecules of each of 4 different histones = H2A, H2B, H3 and H4
How many times does DNA wrap around a histone octamer?
1.65
