Gene replication

Monomers of Nucleic Acids

Nucleotides:
1. nitrogenous bases
2. Organic phosphate group
3. Pentose sugar (deoxyribose for DNA, ribose for RNA)

How does phosphate attach to the ribose sugar?

Carbon prime 1 in the sugar attaches to the nitrogenous base, carbon prime 5 attaches to the phosphate.

Pyrimidines

1 ring nitrogenous base

Thymine, Cytosine, Uracil

Purines

2 ring nitrogenous base

Guanine, Adenine

Chargaff’s law

Nitrogenous bases are always complimentary

C=G
T=A

Order of DNA

5’ = Phosphate end, 3’= Hydroxyl end

G1

Cell prepares for replication
1. It must grow in size
2. It must receive a stimuli (signal) to initiate replication

Synthesize phase

DNA replication

G2

Cell prepares for mitosis
1. Ensure that DNA replication was done properly, look for errors in replicated code

Direction that enzymes read

3’-5’

Origin

Portions of DNA indicating where enzymes can bind (special sequences of DNA). Eukaryotes have many origin sites, bacteria only have one.

Helicase

Unzips your genes, creates a replication bubble

Topoisomerase

Alleviates the strain caused by the replication fork, breaks one strand, lets it unwind, then seals it again.

Single stranded binding proteins

Prevent re-annealing of complementary strands near the replication fork

DNA polymerase 3

Attaches to primer set by primase and adds complementary DNA by reading 3’-5’, creating a 5’-3’ strand for leading, opposite for lagging. 

Adds deoxynucleotide triphosphates to the 3’ (hydroxyl). Energy comes from the nucleus itself 

Primase

Sets down an RNA primer for DNA polymerase 3 to dock onto. Adds nucleotides 5’-3’, while reading 3’-5’ off of the conservative strand.

Lagging strand

1. Primer gets deposited multiple times

2. DNA polymerase 3 attaches to a primer, builds until it reaches another primer, drops off, and attaches to the next primer, thus making okazaki fragments.

Leading strand

1. Only one primer is deposited, DNA polymerase 3 works steadily along

DNA polymerase 1

Binds to regions just upstream to the primer and removes ribose and uracil with deoxyribose and thymine. Ligase, distributed via DNA ligase, will seal these nucleotides together via phosphodiester bonds

Mutations

• Every million base pairs that DNA polymerase 3 replicates, it creates a mistake, leading to a mutation that can be corrected by other enzymes. However, most of DNA is non-coding (introns), therefore there is rarely a phenotypic mutation

Erosion

Primase adds to the end, but polymerase 1 can’t bind to regions upstream of it (if it is at the very end bit of DNA), so each replication gets shorter due to a couple of nucleotides not being replicated.

Telomeres

Special end bits of non-coding DNA regions (introns) that are meant to be eroded to combat DNA erosion to protect coding DNA.

Produced by telomerase, found in gametes, stem cells, and tumour cells

Telomerase activity

In muscle or brain cells (cells that don’t divide), telomerase is turned off, but tumour cells actively turn on telomerase to divide nonstop

How to count chromosomes

Count centrosomes! Chromosomes can be single or double copied

Haploid

1N=23, a cell with 1 complete set of chromosomes of the entire genome

Diploid

2N=46, a cell with two complete sets of chromosomes (entire genome)

Diploid or haploid?

1. Count (odd is 1N, even is 2N)

2. Pairs (absent= 1N, present= 2N)

3. Double/single copy (count centromeres)

Homologous pairs of chromosomes

Chromosomes that code for the same genes, but have possible differing alleles.