Genes, Cell Divison ans Genetics

Briefly describe the structure of DNA

• genetic material of an organism

• Codes for all the proteins needed within an organism

• Double helix

• 2 helices held by complimentary hydrogen bonding

• It is a polynucleotide chain

  • Nitrogenous Base (adenine,thymine,guanine,cytosine)

  • Phosphate group

  • Sugar (deoxyribose sugar)

What does a chromosome consist of?

1. DNA

2. Histones (proteins)

3. and trace amounts for chromosomal RNA

How are chromosomes organized ?

DNA is wound around 8 globular proteins called histones - this is known as a nucleosome

Nucleosome pack together forming a chromatin fiber of about 6 nucleosome per turn taking thr shape of a solenoid

Further supercoiling occurs and a tightly packed structure - chromosome arises

Karyotype

• complete set of chromosomes, differs by species

• Eg humans usually have around 46 chromosomes

Karyogram

visual image where the chromosomes of an organism are neatly organized by length and centromere location.

This allows for one to determine the species type, or any genetic defects

DNA replication

• occurs during s phase of interphase before meiosis and mitosis

• This is when a chromosome replicates itself

• These then join to form sister chromatids

Cohesion proteins

Keep sister chromatids together

Kinetochore

DNA- Protein complex serving as an attachment for micro tubules

Microtubules

Made of tubulin dimers, form the spindle during meiosis and mitosis

Centromere

Region where kinetochore and cohesion proteins are located, serving to keep sister chromatids together

Sister chromatids

2 identical DNA molecules

homologous chromosomes

• a pair of chromosomes with similar but not identical genetic information (Have different alleles)

• cells with pairs of homologous chromosomes are called diploid.

Advantages of more than 1 set of chromosomes

1. More likely to survive a mutation → chromosomes usually have similar genes so in the case of a mutation the unaffected gene can act as a backup

2. Polyploidy - 3 or more sets of chromosomes associated with advantageous features such as increased size, resistance to disease/env conditions

3. Genetic Variation → mixture of genes and characteristics from both parents

Meselson and Stahl Experiment - Describe 

bacteria was grown on a medium containing N15 

→ lowest density after centrifugation with caesium chloride 

then after one generation transferred to an N14 containing medium and allowed to replicate 

→ one band of intermediate density bcs of N14 and N15

the sample was then again transferred to an N14 medium and allowed to replicate 

→ one light band bcs of n14 and N14 and one intermediate n15 n14

why does this prove semi-conservative replication 

• if it was conservative there would be no intermediate strands only one light vs one heavy

• if it was dispersive it would have been HALF the density of parental DNA

Steps of DNA replication

1. Initiation

2. Elongation

3. Termination

Initiation

Helicase unwinds the two complimentary strands at the origins of replication producing a replication fork

Single stranded binding proteins stabilize the newly separated Strands (due to their complimentary nature they want to rejoin)

Gyrase prevents the supercooling of double stranded dna outside the replication fork

Elongation

• RNA primase → rna primer from which dna polymerase III begins to synthesize DNA from (operates in 5’ to 3’ direction)

• Two strands are synthesized at the same time one is the leading strand the other the lagging

• Leading strand - Continuous replication

• 3’ - 5’ parent strand (therefore the daughter strand is 5’ - 3’)

• Phosphate is removed from dnTPs and the energy released is used for dna replication

• Lagging strand - Discontinuous replication

• Occurs on the 5-3 parents strand (hence replicated strand will be 3 - 5)

• because dna polymerase III only works in 5-3 it needs to complete replication in short segments called Okazaki fragments - here several rna primers are required at the start of each Okazaki fragment

Termination

• end of replication DnA polymerase I removed rna and replaced it w dna

• DNA ligase joins the Okazaki fragments and seals gaps by forming phosphodiester bonds

• DNA polymerase III proofreads.

Proofreading Mechanism

DNA polymerase I and III proofread. When recognize a mispairing of bases, it removes the incorrect nucleotide and tries again

Mismatch Repair Mechanism

• a second set of enzymes post replication survey the newly replicated molecules and looks for remaining mismatched base pairs

• detects wrong base bcs dna strand is chemically modified after replication - methylation. The new strand would not be methylated yet so the enzyme knows which strand to check and which base is incorrect (the A or the G)

• mismatch repair enzymes cut mismatched base/adjacent bases

• DNA polymerase and ligase synthesize and seal up a new base sequence

Excision Repair Mechanism

• this is when the dna of the cell is damaged by radiation/chemicals

• enzymes constantly inspect DNA and cut any defective bases and a few other adjacent bases.

• DNA polymerase and ligase synthesize and seal up a new base sequence