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Genes & the genetic code
What is a gene?
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A section of DNA that contains the coded info for making polypeptides &
functional RNA
Coded info in form of a specific sequence of bases along the DNA molecule
Genes (along with environment factors) determine the nature + development
because pp make proteins, enzymes are proteins, control chemical reactions
Located at a fixed position (locus) on a DNA molecule

The genetic code
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3 bases (triplets) code for each amino acid
‘Degenerate code’- most amino acids are coded for by more than one triplet / 64
possible triplets but only 20 amino acids
‘Non-overlapping’- each base in the sequence is read only once
‘Universal’- each triplet codes for the same amino acid in all organisms
Exons: code for amino acid sequences
In eukaryotes, much of the nuclear DNA does not code for polypeptides
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Codon: three bases on the mRNA that codes for a single amino acid
DNA & Chromosomes
Prokaryotic cells

Eukaryotic cells

DNA molecules shorter, circular

Longer, linear

Not associated with protein molecules- no
chromosomes

Associated with proteins called histones to
form chromosomes

Chromosome structure
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Only visible when a cell is dividing
When first visible, appear as two threads (each thread = chromatid)
The DNA in chromosomes held by histones
Helix is wound around histones to fix it in position
The DNA-histone complex is then coiled
Coil= looped + further coiled = packaged into the chromosome
A lot of DNA is condensed into a single chromosome

Homologous chromosomes
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Sperm & egg contribute to one complete set of chromosomes to the offspring
One of each pair = maternal chromosomes / paternal chromosomal es
Homologous pairs: a pair of chromosomes, one M & one P, that have the same
gene loci & therefore determine the same features

Alleles
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One of a number of alternative forms of a gene
Each individual inherits one allele from each of its parents

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When diff, each allele has a diff base sequence, amino acid sequence, produces
a diff polypeptide

Genome: the complete set of genes in a cell
Proteome: full range of proteins that a cell is able to produce
Structures & functions of ribonucleic acid
DNA

mRNA

tRNA

Double nucleotide
chain

single

Single

Largest molecule
out of 3

Smaller than DNA,
larger than tRNA

smallest

double-helix

single-helix

Clover shaped- one end of the chain extends
beyond the other; the part which an amino acid
attaches

Pentose sugar
deoxyribose

ribose

ribose

A, T, C, G

A, U, C, G

A, U, C, G

Messenger RNA (mRNA)
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It transfers the DNA code from the nucleus to the cytoplasm
Base sequence determined by the sequence of bases on a length of DNA in a
process called transcription
Once formed, leaves nucleus through nuclear pores to enter the cytoplasm,
where it associates with ribosomes
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DNA provides the instructions in the form of a long sequence of bases
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The production of polypeptides from the sequence of codons carried by mRNA
TRANSLATION
The process of Transcription:
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The nucleotide bases on the template strand pair with complementary free
nucleotides in the nucleus
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2) A tRNA molecule with an anticodon complementary to the first codon on the mRNA
attaches itself to the mRNA by specific base pairing

3) A second tRNA molecule attaches itself to the next codon on the mRNA in the same
way
4) The two amino acids attached to the tRNA molecules are joined by a peptide bond
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Its amino acid binds to
the first two and the second tRNA molecule moves away
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• Change the sequence of amino acids -ps
• Change hydrogen/disulfide bonds
• Alters ts/active site
• Esc cannot be formed
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No effect because the codon “” still codes for glycine

Comparing the base sequence of a gene provides more information than
comparing
the amino acid sequence for which the gene codes
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Explain why

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