lecture 6-DNA and chromosomes

chromosomes

single piece of coiled DNA with many different genes, each gene codes for one single PROTEIN

DNA

• DeoxyriboNucleic Acid

• Watson and Crick made first model of DNA

• made of nucleotides

• draw it

nucleotide

• contains 5 carbon sugar deoxyribose, phosphate group, and nitrogen base

• Double alpha helix

• order of nucleotides=encodes info

nitrogen bases

Purines:

• adenine

• guanine

Pyrimidines:

• thymine

• cytosine

double alpha helix structure

• phosphate backbone OUTSIDE and nucleotides face INSIDE

• Adenine and thymine paired together with H bond

• guanine and cytosine paired together with H bond

• antiparallel chains

DNA replication steps

1. 2 nucleotide strands unzip by DNA helicase at base pairs

2. each strand builds its opposite strand by base pairing with nucleotides that float freely in the nucleus

3. each new DNA has 1 nucleotide strand from OG molecule and the other strand is from the free nucleotides in nucleus

DNA polymerase

• starts at replication origin

• ONLY 5’ to 3’ direction

• leading strand: forward (continuous)

• lagging strand: backwards (discontinuous, makes Okazaki fragments)

DNA primase

• makes RNA primer from DNA template - allows DNA replication to begin

• RNA primer is degraded

• single strand end on template

telomerase

• adds 500-3000 6 base repeats after replication

• genetic info isnt lost

• place for primer to bind

• lengthens 5’ end of lagging

telomere replication

1. Extension: Telomerase binds to 3' of the parental DNA strand and uses its internal RNA to synthesize complementary, repetitive DNA sequences

2. Completion: DNA polymerase and primase can lay down a new primer and synthesize the complementary strand

3. Resetting the Overhang: An enzyme later removes the terminal RNA, leaving a protective single-stranded 3' overhang at the end of the newly synthesized chromosome

chromatin

loosely packed DNA around proteins

histones

protein which the DNA wraps around

nucleosomes

grouped histones together

heterochromatin vs euchromatin

heterochromatin: tighter packed chromatin, not transcribing, non expressed

euchromatin: looser packed chromatin, transcribing, expressed

chromatin regulation

1. histone acetylation

2. DNA methylation

Histone acetylation vs DNA methylation

HA: allows transcription factors to bind to DNA for transcription, creates euchromatin

DM: occurs after DNA synthesis has occurred, lower transcription rates, one X in females is highly methylated

levels of DNA packing

smallest → biggest

1. DNA double helix

2. chromatin fiber

3. looped domains

4. metaphase chromosome

RNA

• single nucleotide chain

• blueprint for primary protein structure

• sequence for amino acids

• RiboNucleic Acid

RNA polymerases

I: transcribes RNA (ribosomal)

II: transcribes mRNA (messenger)

III: transcribes tRNA (transfer)

regulation of transcription

• RNA polymerase II attaches to promoter sequence to start transcription

• non coding DNA is where the transcription factors attach

introns vs exons

Exons are the "expressed" coding sequences that exit the nucleus to form mRNA and are translated into proteins.

Introns are intervening, non-coding sequences that are spliced (cut) out of the RNA transcript and remain inside the nucleus

removing introns

• the spliceosomes will splice the intron at a specific DNA sequence, releasing a “lariat” DNA

• spliceosomes are made of snRNP

methionine, valine, glycine

meth- AUG (start codon)

val- GUU

gly- GGA

three primary stop codons

1. Ochre: UAA

2. Opal: UAG

3. Amber: UGA