DNA Organisation

What is DNA?

• Aka deoxyribonucleic acid

• Nucleic acid

• Carries the genetic information of an organism

What is the monomer for DNA?

Deoxyribonucleotide

What is the structure of deoxyribonucleotides in DNA?

• Deoxyribose (pentose/ 5C sugar)

• Phosphate group

• nitrogenous base

  • Adenine, guanine (purines, 2 rings)

  • thymine, cytosine (pyrimidines, 1 ring)

• Glycosidic bonds hold these components together

What are 7 structural components of DNA? (polymer)

• Sugar-phosphate backbone

• Double-stranded helix

• Over 3 billion base pairs

• 5’ end contains phosphate (-PO₄) attached to the 5th C

• 3’ end contains a hydroxyl group (-OH) attached to the 3rd C

• Strands run antiparallel

  • One strand goes from 5’ to 3’

  • Other strand goes from 3’ to 5’

• 2nm in diameter

• One turn (about 10 base pairs) of the helix is 3.4 nm

• Distance between base pairs is 0.34 nm

• The two strands are not equally spaced, resulting in a major and minor groove due to the asymmetric spacing between the antiparallel backbones

How is DNA’s phosphate sugar backbone formed? (polymer)

By forming phosphodiester bonds (condensation reaction) between the phosphate group of one nucleotide and the sugar of another.

How is DNA’s double stranded helix formed?

Complementary base pairing connects purines to pyrimidines via hydrogen bonds:

• C forms 3 hydrogen bonds with G (cars in the garage)

• A forms 2 hydrogen bonds with T (apples on trees)

Recall: A, G - purines (2 rings), C,T pyrimidines (single ring)

What does karyotype mean?

The number and distribution of chromosomes for a species

How many chromosomes do humans have?

• Somatic cells have 46 chromosomes

  • 23 homologous pairs

    • 22 autosomes

    • 1 sex-determining pair

• Gametes have 23

What is the first step of DNA packing?

• DNA is very long (about 2 m long when fully stretched out) and needs to be condensed

• First, the DNA is coiled around 8 stabilizing proteins called histones

  • Since the positively charged histones are attracted to the negatively charged DNA

• Forming a nucleosome

What is the second step of DNA packing?

• The nucleosomes and the DNA between them coil into chromatin fibers

• This is what occurs in interphase

What is the third step of DNA packing?

• The chromatin fibres fold in a process called supercoiling to form chromosomes

• A chromosome is made up of two sister chromatids held together by a centromere

What are genes?

• Sections of DNA that are located on specific chromosomes that code for specific proteins

• Physical and functional units of heredity (i.e. they are passed from parent to offspring)

• Currently, it is estimated that 30,000 genes exist within the human genome

What are noncoding regions?

• Do not provide instructions for protein synthesis

• More than 95% of the human genome is noncoding

• Filled with variable number tandem repeats (VNTRs) (aka microsatellites)

  • Sequences of repeating base pairs (e.g.TAGTAGTAGTAG)

  • Vary among individuals

    • Useful for DNA fingerprinting & profiling

What are telomeres?

• Cap-like features at the ends of chromosomes

• Long sequence of repetitive, noncoding DNA

• Protect cells from losing valuable genetic material during DNA replication

• Bind to proteins that stop the ends from being degraded and sticking to other chromosomes

• Over time (due to ongoing cell division), telomeres become shorter

• Appears to be an indication of age and the general health of an individual

What are pseudogenes?

• Have nucleotide sequences similar to a functioning gene, but do not seem to express any RNA or protein

• LINEs (long interspersed nuclear elements) and SINEs (short interspersed nuclear elements) are 2 types of pseudogenes that are believed to be crippled copies of known functional genes

• The function of LINEs and SINEs is unclear

What is the shape of DNA in prokaryotes?

Most prokaryotes contain their genetic material in the form of a circular double-stranded DNA molecule floating in the nucleoid