dna genes and chromosomes

Question 1

describe the structure of dna in eukaryotes

Answer 1

dna molecules are long linear and associated with proteins called histones
dna is very long so must bewound up to fit into the nucleus
histones help support the dna
together histones and dna are coiled tightly together to form compact chromosomes

Question 2

describe the structure of DNA in prokaryotes

Answer 2

DNA molecules are short , circular and not associated with histones

Question 3

describe genes , locus and alleles

Answer 3

gene - is a sequence of bases on DNA which determines the amino acid sequence of a poly peptide
locus - a fixed position in which a gene occupies on a particular DNA molecule
allele - different versions of a gene. the order of bases in each allele varies slightly so they code for slightly different versions of the same polypeptide

Question 4

describe the genetic code

Answer 4

a sequence of three DNA bases called a triplet code
one triplet codes for a specific amino acid
the complementary 3 bases on mrna is called a codon
the code is universal (same for all living things) non overlapping (each triplet is read separately) and degenerate - some amino acids have more than one triplet code.
there are 20 amino acids and only 4 bases so a triplet code is needed with each amino acid in a polypeptide is coded for by 3 bases

Question 5

descibe introns and exons

Answer 5

exons are a sequence within a gene that code for amino acid sequences
introns are non coding sequences
prokaryotic DNA doesnt contain introns

Question 6

describe the structure of mrna and trna

Answer 6

mrna -
single stranded
linear chain
more nucleotides
complementary to the base sequence of a gene
unpaired bases
made up of codons
broken down after use

trna -
single stranded folded into a clover shape
hydrogen bonds
anticodon
specific amino acid attachment site
all similar lengths
re-used

Question 7

what is a genome and a proteome

Answer 7

genome - complete set of genes in a cell
proteome - full range of proteins that a cell is able to produce

Question 8

describe how mRNA is formed by transcription in eukaryotes

Answer 8

hydrogen bonds between DNA bases break
one DNA strand acts as a template
free floating RNA nucleotides align by complementary base paring
in RNA uracil base pairs with adenine in replace of thymine
rna polymerase joins adjacent rna nucleotides
by phosphodiester bonds
pre-mRNA is spliced and introns are removed to form mature mRNA

Question 9

describe how a polypeptide is formed by translation of mRNA

Answer 9

mrna attaches to ribosome or rough endoplasmic reticulum
tRNA anticodons bind to complementary mrna codons
trna brings a specific amino acid
amino acids join by peptide bonds
with the use of atp - condensation
trna is released after amino acid is joined to its polypeptide
the ribosome moves along the mRNA to form polypeptide

Question 10

describe the different gene mutations

Answer 10

substitution- one base is changed for another during DNA replication
only the codon with the mutation is affected
deletion - one base is left out during dna replication . this results in a frame shift and all the codons after mutation are affected
addition - one extra base is added during DNA replication . this results in a frame shift as all the codons after the mutation are affected

Question 11

why do mutations not always cause a change in protein structure

Answer 11

• mutation may result in a codon which codes for the same amino acid (degenerate) and the amino acid sequence stays the same
• mutation may be an intron
• mutation may be a recessive allele
• mutation may code for an amino acid which has the same properties as the original one and so the protein structure remains unchanged