Molecules, Cells and Variation - 1.3 + 1.4 Flashcards
Function of sugar-phosphate backbone in DNA?
Gives strength
Function of the coiling of DNA?
Gives compact shape
Function of the double helix in DNA?
Each strand serves as a template in replication.
Protects genetic code (bases)
Makes molecule more stable
How does DNA being large support its function?
Allows large amount of information to be stored.
How does the high number of hydrogen bonds in DNA benefit its function?
Gives stability
Prevents code being disrupted
Allows chain to unzip easily for replication + transcription
How does the sequence of bases in DNA benefit the function?
Provides genetic code for protein synthesis
How does complementary base pairing benefit the function of DNA?
Enables information to be replicated accurately
DNA chromosome structure in eukaryotes
DNA exists in the nucleus, surrounded by membrane.
Histones associate with a region of DNA to form a nucleosome. DNA is further coiled making chromatin.
DNA is present as indistinct chromatin in the nucleus for most of the cell cycle. During mitosis, DNA condenses and coils further, becoming chromosomes.
Chromatin
Structure formed when DNA is packaged around histones and super coiled. Provides a compact store of genetic information.
Nucleosome
Subunit of chromatin, consisting of sections of DNA wrapped around histones. Provides structural support.
DNA structure in prokaryotes
DNA is smaller, circular and unassociated with proteins. No defined nucleus. During replication, DNA attaches to mesosome.
Semi-Conservative method of DNA replication
1) Strands separate as hydrogen bonds break. Promoted by helicase.
2) Strand act as templates for formation of new complementary strands.
3) DNA nucleotides align next to template strands according to their specific base pairing.
4) Nucleotides join forming a polynucleotide strand using DNA polymerase. Hydrogen bonds reform.
5) Two DNA molecules are identical to each other and to the original DNA.
Why is DNA replication semi-conservative?
Each newly formed DNA molecule contains one of the original polynucleotide strands and one newly synthesised from new individual nucleotides.
Why does the 5’, 3’ direction of synthesis in DNA make replication more complicated?
DNA polymerase can only work in the 3’, 5’ direction. The 3’, 5’ strand is replicated in it’s entirety and is the leading strand, allowing continuous replication.
The 5’, 3’ strand is formed in smaller fragments, making the 3’ available. DNA ligase has to join the fragments together, causing discontinuous replication.
How does RNA differ in structure to DNA?
- Pentose sugar is Ribose.
- Uracil replaces Thymine
- Are single stranded
- Shorter in length, with a lower molecular weight
Ribosomal RNA
- Makes up 80% of the total RNA of the cell. Is synthesised on genes present on DNA.
- rRNA synthesis occurs in the nucleolus.
- It moves to cytoplasm via nuclear pores, it then associates w/ protein molecules, forming ribosomes.
Messenger RNA
- Formed from a single DNA strand during transcription.
- Has a variable length depending on gene.
- Mostly exists for short time. Degrades after protein synthesis.
Transfer RNA
- Acts as intermediate molecule between triplet code of mRNA and amino acid sequence present in polypeptide.
- It transfers amino acids in cytoplasm to ribosome.
- All have same basic structure. Clover leaf shape.
- 5’-end of tRNA always ends in guanine, 3’-end always ends in CCA.
- Anticodon is situated on bottom loop. Directly related to amino acid carried by tRNA.
- Rest of molecule has variable base sequence.
Gene
Region of DNA, whose nucleotide base sequence codes for the production of a specific polypeptide/protein. Chromosome may contain many hundreds of genes.
Locus
Position of the gene on the DNA
Allele
Different forms of a gene, which code for similar polypeptides, and are located on the a similar locus on homologous chromosomes. They carry genes controlling the same characteristics but not necessarily the same alleles.
Transcription
1) DNA uncoils and strands separate.
2) One DNA strand acts as a template strand (non-coding sequence).
3) Single RNA nucleotides line up alongside the template strand (specific complementary base pairing).
4) RNA polymerase joins nucleotides together forming backbone of pre-mRNA.
5) DNA strands in nucleus recoil when mRNA has been produced.
Splicing
Spliceosome removes introns and joins exons together to form mRNA. This leaves the nucleus and attaches to a ribosome allowing translation.
Prokaryotes don’t have introns.
Initiation of translation
1) Ribosome moves along mRNA and finds start codon. 1st tRNA binds at P site on ribosome.
2) tRNA’s anticodon pairs w/ specifc codon on mRNA .
3) 2nd tRNA binds at A site.
4) Bond between 1st amino acids and tRNA is broken. Released energy forms peptide bond between 1st + 2nd amino acid.
5) Dipeptide attached to second tRNA has formed.
6) Disassociated tRNA moves away to gain new amino acid.
7) Dipeptide attached to 2nd tRNA moves to P site.
Elongation of translation
1) Peptide chain continues to grow and ribosome moves along to next new codon.
2) New codon is aligned with A site, allowing the next tRNA to interact with the ribosome.
3) Dipeptide is linked to 3rd amino acid, 2nd tRNA moves away to amino acid pool in cytoplasm.
4) Process continues along mRNA strand until all codons have been ‘read’ and polypeptide has been produced.
Termination of translation
1) When a stop codon is reached, bond between polypeptide and last tRNA is hydrolysed. Ribosome leaves mRNA and can reattach at the start again.
2) As the protein is synthesised it can develop different structures. May undergo further modification in the ER if exported out the cell.
Regulation of transcription
- Gene must be stimulated by transcriptional factors, -
These bind to DNA upstream from specific gene, and stimulate RNA polymerase to begin transcription, allowing protein synthesis. - An inhibitor prevents transcription factors from binding when a gene is not expressed.
Regulation of translation
- Ccan be prevented by breaking down mRNA before it is translated at the ribosome.
- Small interfering RNA (siRNA) carries this out.
- RNA is cut into smaller pieces forming siRNA.
- One strand combines with an endonuclease enzyme.
- siRNA binds via complementary base pairing.
- Signalling endonuclease to cut mRNA where siRNA binds.
- mRNA is now unable to be translated.
Epigenetics
Refers to a change in phenotype without a change in genotype.
Example of changes to gene expression
- Methylation of DNA
- Acetylation of histone tails
- Phosphorylation of amino acids
Gene mutation
- Changes in the nucleotide base sequence of DNA.
- Results in formation of a different polypeptide.
- Altered base sequence codes for different amino acid sequence. Produces new alleles.
- Occur naturally at random, rate varies.
- Can arise due to incorrect pairing in DNA replication.
- Most are harmful and recessive.
Mutagenic agent examples
- High energy radiation e.g. X-rays, gamma rays, U.V. light.
- High energy particles e.g. alpha and beta particles.
- Chemicals such as nitrous oxide or benzene
Substitution as a gene mutation
- Replacement of one or more bases by another.
- Same amino acid may be coded for, (degenerate), so polypeptide remains unchanged.
- Could be one different amino acid, but a functional protein is still produced.
- Different amino acid is coded for, creating a non-functional protein.
- Could form a stop codon, terminating polypeptide, creating a non-functional protein.
Deletion as a gene mutation
- Removal of one or more bases.
- May result in a frame shift. All future codons change.
- Amino acid sequence is altered, non-functional protein formed.
Addition as a gene mutation
- Addition of one or more bases.
- Leads to a frame shift.
- Sequence of amino acids is altered, non-functional protein formed.
How can a gene mutation affect the activity of an enzyme?
- Active site may change, so substrate can’t attach making the enzyme non-functional.
- Slow down enzyme activity.
- Affects tertiary structure and shape of active site.
- Can block a metabolic pathway.