DNA structure and function

Genetic material requirements (4):

• stable.

• Contain info for structure development reproduction.

• Replicated and inherited accurately.

• Capable of change.

Purines are…

double ringed

- guanine and adenine

Pyrimidines are…

single ringed

- cytosine thymine and uracil

DNA overall structure…

• deoxyribose and phosphate backbone

• with phosphodiester bonds with nitrogen base pairs.

• 5’-3’ direction and antiparallel.

• Forming a double helix.

DNA pairs

A=T.

G=-C

Measuring size of DNA

in basepairs (bp) kb Mb

RNA overall structure…

• ribose and phosphate backbone.

• Nitrogenous base.

• Single stranded.

• A=C. G=-U

Alleles

different versions of the same gene (A/a)

The genome is

• the complete set of genetic material of an organism.

• It contains biological instructions to build and maintain the organisms.

• Mitochondrial DNA and nuclear DNA

Prokaryotic genomes

• no nucleus -> genome is nuclei.

• Haploid → single circular chromosome.

• Plasmids → ‘extra’ DNA

DNA gyrase role in prokaryotic genomes…

• binds to DNA loops.

• ATP enters.

• DNA is cut and conformational change and allows DNA to pass through.

• Gyrase rejoins the DNA ends.

• DNA gyros and ADP+Pi is released.

• DNA loop is supercoiled.

DNA topoisomerase II is also known as…

DNA gyrase

Eukaryotic genomes…

• have a true nucleus.

• Are diploid.

• Organelle DNA in mitochondria and chloroplasts

When does supercoiling occur I eukaryotic genomes?

• during metaphase.

• The nucleus dissolves and chromosomes condense and undergo supercoiling.

Chromatin is…

a mixture of DNA and proteins that from chromosomes

Chromosome anatomy:

• 2 identical chromatids- they each contain 1 DNA molecule

• p arm-short arm structure.

• Centromere- connecting point.

• q arm-long arm structure.

• Telomeres-ends of DNA

Homologous chromosomes

one inherited from each parent

Locus

location of particular gene on chromosome

Allele

one version of a gene inherited from each parent

Homozygous

alleles are same version of gene

Heterozygous

alleles are different versions of gene

Why replicating DNA accurately is important…

• produce gametes.

• Development for different cells.

• Cell maintenance like replacing skin.

• Hereditary

When DNA replication occurs?

(interphase)

MITOSIS steps :

(interphase.)

Prophase.

Prometaphase.

Metaphase.

Anaphase.

Telophase.

MEIOSIS steps:

(interphase).

Prophase I.

Metaphase I.

Anaphase I.

Anaphase ii

DNA replication semi-conservative:

each new molecule has 1 strand of og.

DNA and 1 new strand

What does ori mean?

• specific sites called origins on DNA molecule.

• Prokaryotes have one.

• Eukaryotes have multiple.

• Ori has specific DNA sequence that is recognised by initiators(special proteins)

DNA gyrase/ topoisomerase

unwinds DNA

DNA helices

binds to initiator proteins and ‘unzips’ DNA

DNA primase

synthesises short RNA primer

DNA polymerase

add free nucleotides to make new complimentary DNA strand and removes RNA primer

Leading strand

continuous replication 5’-3’ direction.

DNA polymerase

Lagging stand

• discontinuos replication.

• 3’-5’ direction.

• DNA polymerase creates Okazaki fragments.

• DNA ligase seals gaps.

What are telomeres?

• short repetitive sequence at ends of chromosomes.

• They shorten every time a DNA divides.

• In somatic cells this leads to cell death.

• Shorten faster with unhealthy lifestyle.

• Related to age breakdown of our cells

What does telomerase do?

• replicates ends and seals gaps between fragments.

• Bc ends of DNA on lagging strand is not replicated.

• Primer has nothing to bind to.

• Allowing telomere length to be maintained

Gene defintion

segment of DNA/nucleotide sequence that produces a product (always RNA and sometimes a protein)

Gene expression definition

process where info from gene is used to synthesise a functional gene product with an end product that can affect phenotypes

Transcription and translation in prokaryotes

transcription and translation can be coupled.

Translation can start before transcription

3 RNA products:

• mRNA to encode proteins.

• tRNA to carry proteins.

• rRNA to form the ribosome

Promotors are…

a action of DNA that is where transcription starts - start of an RNA coding sequence

Terminators are…

a section of DNA where transcription stops - end of an RNA coding sequence

RNA process protects ends and splicing

• Introns and exons.

• DNA -> precursor mRNA.

• Splicing occurs - removing introns.

• Producing mature mRNA with 5’ cap & 3’poly A tail.

What is a lariat structure?

formed by slicing rxns.

Occurs in spliceosome (enzyme complex)

Alternative splicing

can remove different exons producing different proteins.

• Eg. If there are 5 exons - mRNA 12345 can be produced or MRNA 1245 or mRNA 1235. They will all produce different proteins.

3 necessities for translation

• rRNA (ribosomes)

• Transfer RNA (tRNA)

• Amino acids

Ribosome structure:

• small and large sub unit.

• 3 binding sites -A (aminoacyl - tRNA) -P (peptide-tRNA) -E (exit site)

tRNA structure:

• made of single-stranded RNA.

• Secondary structure= cloverleaf.

• Consists of paired stems and unpaired loops.

• Amino acid attachment site at 3’ end.

• Anticodon

Charged vs. Uncharged tRNA

bound a.a. / no bound a.a.

aminoacyl=tRNA synthetase is the enzyme involved in charging tRNA

Genetic code general info…

• triplet of bases on mRNA= codons.

• Degenerate = more than 1 codon per a.a. start and stop codons.

Translation mechanism (3step):

1. Initiation.

2. Elongation.

3. Termination.

Initiation stage during translation

• ribosome assembles around target mRNA -> ribosomal complex.

• First tRNA attaches at start codon (AUG)

Elongation stage during translation

• peptide bond created between a.a. attached to its tRNAs by peptide transferase.

• A.a added to growing polypeptide chain

Termination stage during translation

• stop codon reached and recognised.

• Ribosomal complex releases polypeptide