3.4 Genetic information, variation and relationships between organisms

what is a gene

A gene is a base sequence of DNA that codes for:

• the amino acid sequence of a polypeptide

• a functional RNA (including ribosomal RNA and tRNAs).

where are genes located

genes are located at a fixed position, called a locus, on a DNA molecule

what is a triplet + what does it do

a sequence of 3 DNA bases that codes for a specific amino acid

the genetic code is….

the genetic code is universal, non-overlapping and degenerate.

what does it mean for the genetic code to be universal

Each triplet codes for the same amino acid in all organisms. (indirect evidence for evolution)

what does it mean for the genetic code to be non-overlapping

each base in the sequence is read only once + do not overlap

eg six bases numbered 123456 are read as triplets 123 and 456, rather than as triplets 123, 234, 345, 456

what does it mean for the genetic code to be degenerate 

More than one triplet codes for each amino acid (Triplets code for same amino acid)

→ reduced no. of mutations

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide.

Give two reasons why.

• Triplets code for same amino acid/ genetic code is degenerate

• Occurs in introns /non-coding sequence

what are exons 

base sequences that code for amino acid sequences/ polypeptides

how are genes separated

genes are sepearted by non-coding regions

what are introns

 non-coding sequences

Define ‘non-coding base sequences’ and describe where the non-coding multiple repeats are positioned in the genome.

• DNA that does not code for protein/polypeptides /sequences of amino acids

• Positioned between genes

how many amino acids are there

20 amino acids

compare DNA molecules in prokaryotic cells and eukaryotic cells 

DNA molecules in prokaryotic cells (eg bacteria) : shorter, circular and not associated with proteins (so do not have chromosomes), dont have introns

DNA molecules in eukaryotic cells : longer, linear and associated with proteins called histones to form chromosomes, have introns

what is the mitochondria and chloroplasts (in eukaryotic cells) DNA like

The mitochondria and chloroplasts (in eukaryotic cells) DNA is like DNA of prokaryotic cells (short, circular and not associated with proteins-> so do not have chromosomes)

what is a homologous pair

2 chromosomes that carry the same genes but not always the same alleles of the genes

-> 1 maternal chromosome and 1 paternal chromosome 

what is an allelle 

a variant form of a gene

what does a change in the base sequence of a gene produce

a change in the base sequence of a gene produces a new allele of that gene (mutation) -> results in difrerent amino acids sequence being coded for -> production of a different polypeptide -> different protein

if protein produced is enzyme, may have a different shape -> new shape may not fit enzyme's substrate (cant form enzyme-substrate complex) -> enzyme cant function

genome def 

the complete set of genes in a cell (including those in mitochondria/ chloroplast)

proteome def

full range of proteins that a cell is able to produce.

describe RNA (ribonucleic acid)

• a polymer

• forms single strand 

• made of repeating mononucleotide sub-units

each nucleotide is made up of:

• pentose sugar ribose

• one of the organic bases adenine (A), guanine (G), cytosine (C) and uracil (U)

• a phosphate group

2 examples of RNA INVOLVED IN PROTEIN SYNTHESIS

• messenger RNA (mRNA)

• transfer RNA (tRNA). 

codon def 

the sequence of 3 bases on mRNA that codes for a single amino acid. 

structure of mRNA (messenger RNA)

• Consisting of many mononucleotides

• Long strand arranged in a single helix

• Created during transcription (base sequence of mRNA determined by the base sequence of DNA

• 
  Each codon (set of 3 bases) are complimentary to a triplet on DNA -> mRNA base sequence complimentary to DNA) 

structure of tRNA (transfer RNA)

• relatively small molecule

• made up of around 80 nucleotides

• single-stranded chain folded into a clover­ leaf shape, with one end of the chain extending beyond the other

• This is the part of the tRNA molecule where an amino acid attaches (amino acid binding side)

• At the opposite end of tRNA molecule is an anticodon (a sequence of three other organic bases) which is specific to the amino acid the tRNA attaches to

• has hydrogen bonds

what shape is tRNA

clover leaf shape

Describe differences between the structure of a tRNA molecule and the structure of an mRNA molecule

• tRNA is 'clover leaf shape', mRNA is linear;

• tRNA has hydrogen bonds, mRNA does not;

• tRNA has an amino acid binding site, mRNA does not;

• tRNA has anticodon, mRNA has codon

what are the complimentary base pairing

• guanine with cytosine

• adenine with uracil (in RNA ) or thymine (in DNA).

Transcription

Transcription is the process of making pre-mRNA using part of the DNA as a template

• An enzyme acting on a specific region of DNA causes hydrogen bonds between complementary bases to break

• so DNA uncoils, separating the two strands + exposing the organic bases

• One of the DNA strands used as template strand to make mRNA molecule (template= antisense strand)

• the nucleotide bases on the template strand are complementary base paired with free nucleotides

• exposed guanine base on DNA complementary base pairs to cytosine base of a free nucleotide

  → cytosine complementary base pairs to guanine

  → thymine complementary base pairs adenine

  → adenine complementary base pairs to uracil (rather than thymine)

• As enzyme RNA polymerase moves along strand + joins adjacent nucleotides together by catalysing formation of phosphodiester bonds to form a pre-mRNA molecule, the DNA strands rejoin behind it.

• As a result, only about 12 base pairs on the DNA are exposed at a time (to reduce chance of damage to DNA)

• When RNA polymerase reaches a 'stop codon' on the sequence of bases, it detaches, + the production of pre-mRNA is complete.

In prokaryotic cells, transcription directly results in production of mRNA from DNA

In eukaryotic cells transcription results in the production of pre-mRNA, which is then spliced to form mRNA.

→ The DNA of a gene eukaryotic cells is made up of exons (code for proteins) and introns (do not code for proteins)

These intervening introns would prevent the synthesis of a polypeptide.

The base sequences corresponding to the introns r removed

the functional exons r joined together during a process called splicing.

As most prokaryotic cells do not have introns, splicing of their DNA is unnecessary

mRNA molecules are too large to diffuse out of the nucleus so, once they have been spliced, they leave via a nuclear pore

Outside rhe nucleus, the mRNA is attracted to the ribosomes where it becomes attached, ready for the next stage of the process: translation

Describe how mRNA is formed by transcription in eukaryotes

• Hydrogen bonds (between DNA bases) break;

Ignore DNA helicase.
Reject hydrolysing hydrogen bonds

• (Only) one DNA strand acts as a template

• (Free) RNA nucleotides align by complementary base pairing

• (In RNA) Uracil base pairs with adenine (on DNA) OR (In RNA) Uracil is used in place of thymine;

• RNA polymerase joins (adjacent RNA) nucleotides

• (By) phosphodiester bonds (between adjacent nucleotides)

• Pre-mRNA is spliced (to form mRNA) OR Introns are removed (to form mRNA);

Describe how a polypeptide is formed by translation of mRNA

• (mRNA attaches) to ribosomes/ rough endoplasmic reticulum

• Ribosome moves finds the start codon
  

• (tRNA) anticodons(bind to) complementary (mRNA) codon

• tRNA brings a specific amino acid

• Ribosome moves along to next codon

• Amino acids join by peptide bonds using ATP

• tRNA released (after amino acid joined to polypeptide)

• The ribosome moves along the mRNA to form the polypeptide

In a eukaryotic cell, the structure of the mRNA used in translation is different from the structure of the pre-mRNA produced by transcription

Describe and explain a difference in the structure of these mRNA molecules.

• mRNA has fewer nucleotides/ Pre-mRNA more nucleotides

  OR

  mRNA has no introns/has (only) exons/ Pre-mRNA has (exons and) introns;

  Accept mRNA is shorter OR pre-mRNA is longer

• (Because of) splicing

Give the two types of molecule from which a ribosome is made.

One of RNA / ribonucleic acid(s) / nucleotide(s)/nucleic acid(s) / rRNA / ribosomal RNA / ribosomal ribonucleic acid
and
one of protein(s) / polypeptide(s) / amino acid(s) / peptide(s) / ribosomal protein;

Describe the role of a ribosome in the production of a polypeptide. Do not

include transcription in your answer.

1. mrna binds to ribosome

2. Idea of two codons / binding sites;

3. (Allows) tRNA with anticodons to bind / associate;

4. (Catalyses) formation of peptide bond between amino acids

   (held by tRNA molecules);

5. Moves along (mRNA to the next codon) / translocation

   described;

   Assume ‘it’ refers to ribosome.

Describe the structure of proteins.

Polymer of amino acids;
Joined by peptide bonds;
Formed by condensation;
Primary structure is order of amino acids;
Secondary structure is folding of polypeptide chain due to hydrogen bonding;

Accept alpha helix / pleated sheet

Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide bonds;
Quaternary structure is two or more polypeptide chains.

Describe how phosphodiester bond formed between 2 nucleotides within dna molecule

Condensation reaction

Between phosphate and deoxyribose

Catalysed by dna polymerase

Explain how a chromosome mutation in the second meiotic division could

result in an extra chromosome 18.

In your answer, name the type of chromosome mutation which would result

in the extra chromosome.

non dysfunction so chromosomes/chromatids do not separate