A1.2 HL DNA and Directionality
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What gives DNA strands directionality?
Numbering of carbons in the sugar.
What does the 5′ end have?
A terminal phosphate group.
What does the 3′ end have?
A terminal hydroxyl group.
In which direction is DNA synthesized?
5′ → 3′ (new nucleotides added to 3′ end).
How many bonds form between A–T pairs?
Two hydrogen bonds.
How many bonds form between C–G pairs?
Three hydrogen bonds.
Which base pair is stronger?
C–G (3 bonds).
Which regions of DNA are weaker?
A–T rich regions (e.g., TATA boxes).
Why is DNA’s width uniform?
Purines (A, G) always pair with pyrimidines (T, C).
What proteins package DNA in eukaryotes?
Histones.
What is chromatin?
DNA + proteins (mainly histones).
What is a nucleosome?
DNA wrapped around 8 histones + linker DNA.
What does “beads on a string” describe?
Loosely packed nucleosomes.
When are chromosomes most visible?
Metaphase of mitosis.
What is a chromosome?
Coiled, condensed chromatin during cell division.
What is a chromosome?
Coiled, condensed chromatin during cell division.
How can histones be modified?
By adding methyl, acetyl, or phosphate groups.
How do histone modifications affect DNA?
Loosen or tighten packing, changing gene expression.
What are chromatids?
Identical halves of a duplicated chromosome.
When can genes be expressed?
When DNA is loosely packed (unwound chromatin).
What is the function of promoter regions (e.g. TATA box)?
Signal where transcription begins.