AQA A-level Biology Variation A* Notes

Genetic Info
Variation
Relationships
between orgisms

, CHROMOSOMES: Short thick bundles
of DNA




CENTROMERE: Area where the
chromatids of a chromosome are
attached




LOCUS: a specific, fixed position on a
chromosome where a particular gene
or genetic marker is located




HOMOLOGOUS PAIRS: A pair of
chromosomes of the same type, one
from each parent.

Same genes at same alleles

, Nucleic acid
• Deoxyribonucleic acid (DNA)
• Ribonucleic acid (RNA) Bases
Purine bases (A+ G)
made up of monomers - nucleotides Pyramidine bases (C+T+U)
3 components:
• phosphate group DNA= A,T,G,C
RNA= A,U,C,G
• pentose sugar
• nitrogenous base

DNA ruure
• Antiparallel chains Each one of the 2 DNA strands has
one original stand and one newly
• width of helix is constant synthesised one
• no restrictions on sequence of bases
• sequence in one chain determines the other
• Complementary chains
• Sugar-phosphate backbone
• Bases paired in centre of helix
• DNA is capable of replication Semi conservative replication

DNA: The Triplet Code

• DNA provides code for organisms proteins
• Wide variety of proteins - due to seq of AA
• Approx. 20 essential AAs (regularly occur in proteins)
• Each AA must have own code on DNA


Why A Triplet Code?

• produces 64 codes
(more than enough to satisfy requirement of 20 AAs)

The Triplet Code: A seq of 3
bases codes for 1 amino acid


* DNA code transcribed onto mRNA

, RNA
Struure
• Ribose sugar
• Uracil base (instead of thymine)
• Single stranded nucleic acid
• No double helix
• Smaller + less complex
& It can pass out of the nucleus via the nuclear pores whereas
DNA cannot

Types
• Ribosomal RNA (rRNA) - structural component of ribosomes
• Messenger RNA (mRNA) - protein synthesis
• Transfer RNA (tRNA) - protein synthesis

mRNA
• single strands
• Made in nucleus via transcription , passes out via nucleus pores
• nucleus , into cytoplasm , to ribosomes
Each triplet of bases form a codon

* mRNA code
CODON: Sequence of 3 bases that complementary to
codes for a particular amino acid DNA code



tRNA

Struure *ROLE: trans specific AAs
• single stranded to mRNA mol at ribosome
• 'Clover-leaf' shaped
• Held by hydrogen bonds
• Bases in the anticodon are complementary to
bases in codon on mRNA
• Each type of tRNA can match + bind to
particular amino acid
(at amino acid binding site)
• tRNA - the intermediate between triplet codon
on mRNA and the amino acids
• Involved in translation during protein synthesis