3.4.1 and 3.4.2 DNA, genes and chromosomes and DNA and protein synthesis

What is a gene?

A section of DNA that contains coded information for polypeptide synthesis and functional RNA (tRNA and rRNA), in the form of a specific sequence of bases along the DNA molecule.

Difference between DNA in prokaryotic and eukaryotic cells?

In prokaryotic cells DNA is short and circular and not associated with proteins, however in eukaryotic cells DNA is long, linear and associated with proteins called histones.

What two molecules are chromosomes made of?

DNA which contains information carrying nucleic acid, and histones which provide a scaffold for DNA

What is a locus?

The particular position of a gene on a DNA molecule

What is an allele?

Different forms of a gene

How does a base sequence change on a gene result in a different enzyme structure?

Produces a new allele, and results in different amino acids being coded for, resulting in production of a different polypeptide and a different protein. This could change the tertiary 3D structure of an enzyme stopping it from functioning.

What was the scientists’ reasoning as to why there must be a minimum of 3 bases to code for each amino acid?

Only 20 amino acid regularly occur in proteins, each amino acid must have its own code of bases on the DNA. There are only 4 bases, and if each one coded for a different amino acid, there would only be 4 amino acids. With 2 bases, 16 amino acids are possible, and 3 produce 64 codes which is enough for 20.

What are introns?

Non coding bases within genes

What are exons?

Coding regions in a gene coding for an amino acid sequence

What is the genome?

All the genes in the cell

What is a proteome?

All the proteins coded for by genes

What are the 3 forms of RNA?

Messenger RNA (mRNA) - transports genetic code from DNA to ribosome.

Ribosomal RNA (rRNA) - forms the ribosome

Transfer RNA (tRNA) - transports amino acids to the ribosome

What are the features of mRNA?

Single stranded, all bases unpaired, linear molecule

Contains codons

Variable in length (dependent on gene length)

What are codons?

Set of 3 bases coding for individual amino acids

Features of tRNA?

Single stranded in a clover leaf shape and a standard length

Amino acid binding site called an anticodon that binds to a codon

It has intramolecular base pairing (within the same strand)

Why is genetic code denegenerate?

Most amino acids are coded for by more than one triplet

What are some of the features of the genetic code?

A few amino acids are coded for by only a single triplet, and the remaining amino acids are coded for by between 2 and 6 triplets.

The triplet is read in one direction along the DNA strand

Code is non overlapping, so each base sequence is only read once

The code is universal to all organisms

What does universal code mean?

Same triplet always codes for same amino acid

What does non overlapping code mean?

Each base is only part of one triplet

What are stop codes?

Triplets that don’t code for any amino acids and mark the end of a polypeptide chain

How are chromosomes dispersed during cell division, and normally?

Only visible as distinct structures in cell division. Rest of the time they are widely dispersed throughout the nucleus.

Structure of a chromosome?

Highly folded DNA, and at the start of cell division appear as two threads held by a single point. Each thread is called a chromatid.

How is DNA coiled in a chromosome?

First wound around histones, then this complex is coiled and further coiled before being packed into the chromosome. It only contains one DNA molecule, but this is very long and contains many genes

What are homologous pairs?

One chromosome of each pair is from the father, and one is from the mother due to the fusion of the egg and sperm. The pair is always two chromosomes carrying the same genes but not necessarily the same alleles for them. 2 chromosomes carrying the same genes - short definition

What are the 2 general stages of protein synthesis?

Transcription - DNA code is transcribed to the pre-mRNA which is spliced to form mRNA.

Translation - the mRNA code, carrying the instructions for primary polypeptide structure is read and the polypeptide is assembled.

Stages of transcription including splicing?

1. Hydrogen bonds between base pairs break and DNA unzips. Only one strand acts as a template.

2. RNA nucleotides base pair with the DNA bases, following complementary base pairing rules.

3. RNA polymerase moves along the strand and joins adjacent nucleotides together by forming phosphodiester bonds to produce pre-mRNA.

4. Pre-mRNA contains both introns and exons. Splicing removes introns from pre-mRNA so only the exons remain.

Stages of translation?

mRNA attaches to the ribosome, and attaches to start codon. ATP is used when amino acid attaches. The sequence of mRNA bases determines order of amino acids. Each tRNA molecule brings a specific amino acid (as it binds to a specific amino acid) to the ribosome. Codons on mRNA bind to tRNA anti codons. Adjacent amino acids joined by condensation forming peptide bonds using energy provided by ATP. tRNA is released, then the ribosome moves along to form the polypeptide until it reaches the stop codon.