SCNC1112 Chap.10 DNA, Genomics and the Rules of Inheritance
DNA, Genomics and the Rules of Inheritance
DNA is a double-stranded molecule whose sugar and phosphate backbone carries four bases, each pairing with one specific partner on the opposite strand. That single rule does two jobs: it holds the strands together, and it means either strand specifies the other exactly, which is what makes faithful copying possible.
Four bases read three at a time give sixty-four possible triplets, comfortably more than enough to specify the twenty amino acids plus instructions to start and stop, so information capacity comes from length rather than from the size of the alphabet. Two metres of this molecule has to fit inside a nucleus a few micrometres across, and the filing system has names that questions test in order.
DNA wraps around histone proteins; eight histones with their DNA form a nucleosome; nucleosomes with linking DNA make chromatin; chromatin condenses into a chromosome. Each term names a level of compaction rather than a different molecule. How tightly a region is packed also affects whether the machinery that reads genes can reach it, so the filing system doubles as a control system.
How the molecule is copied was settled by an experiment worth studying for its design. Three models were available: conservative copying leaving the original intact, semi-conservative copying separating the strands and building a partner for each, and dispersive copying producing patchwork strands. Bacteria were grown so their DNA carried a heavy isotope of nitrogen, then switched to a light one.
After one round of copying two of the three models predicted the same result, so the experimenters ran a second round, where the predictions finally diverged, and the observed pattern left only semi-conservative copying standing. Writing down what each model predicts before measuring is the transferable lesson. Information then flows from DNA to RNA to protein.
Transcription copies a stretch of DNA into messenger RNA; translation reads that message three bases at a time at a ribosome, with transfer RNA delivering the matching amino acid. The question that looks trivial and is not concerns why a muscle cell and a nerve cell in the same person behave nothing alike despite identical DNA. The answer is that different genes are being transcribed, so a different set of proteins is present.
Identity of sequence does not imply identity of expression. Inheritance closes the chapter. Most genes come in two copies, one from each parent, and alternative forms are alleles. Two identical alleles make an individual homozygous and two different ones heterozygous, and an allele that masks another in a heterozygote is dominant.
Mitosis produces two cells with the full complement for growth and replacement, while meiosis produces cells with half the complement so that two of them combine into a full set, shuffling parental combinations along the way.
What this chapter covers
- 01
Base Pairing and Information Capacity
- 02
Histones, Chromatin and Chromosomes
- 03
Three Models of Replication Tested
- 04
Transcription and Translation
- 05
Why Identical DNA Gives Different Cells
- 06
Alleles, Dominance, Mitosis and Meiosis
Reading a recessive condition through two generations
- 2Enumerate the combinations. Each parent passes either allele with equal chance, giving four equally likely pairings: one homozygous unaffected, two heterozygous, one homozygous affected.
- 2State the expected proportions. One quarter affected, one half carriers, one quarter neither affected nor carrying.
- 2Interpret the number correctly. One quarter is a probability per conception, describing the expected distribution across many such families rather than a quota any one family must fill. Three unaffected children change nothing, because each conception is independent of the ones before it.
Key terms
- Base Pairing
- The specific pairing of adenine with thymine and cytosine with guanine, which holds the two strands together and makes each one a template for the other.
- Nucleosome
- Eight histone proteins with the length of DNA wound around them, the first level of packaging above the bare double helix.
- Semi-Conservative Replication
- Copying in which the two strands separate and each acts as a template, so every new molecule keeps one original strand.
- Codon
- A group of three bases in messenger RNA that specifies one amino acid or signals the start or end of a chain.
- Allele
- One of the alternative forms of a gene, of which most organisms carry two copies, one inherited from each parent.
- Meiosis
- The division producing cells with half the usual complement of chromosomes, shuffling parental combinations so that two such cells can combine into a full set.
DNA, Genomics and the Rules of Inheritance FAQ
If every cell has the same DNA, why are cells so different from each other?
Because having a gene and reading it are different things. Different regulatory proteins are present in different cell types and different regions of chromatin are accessible, so a different set of messenger RNAs and therefore a different set of proteins is made. Almost every question about development or tissue specialisation turns on that distinction.
What made the replication experiment convincing rather than merely suggestive?
Two things. The experimenters wrote down what each of the three models predicted before measuring, and they continued to a second round of copying because one round could not separate two of the models. A design that keeps going until the predictions diverge is what turns a measurement into a decision.
Does having three unaffected children change the risk for a fourth?
No. Each conception is an independent event, so the one-in-four figure applies again. The proportion describes what is expected across many families rather than a sequence that has to balance out, and treating it as a quota is one of the most common errors in genetics questions.
Exam move
Practise the packaging sequence in order and be able to say what each level is for. Then reconstruct the replication experiment from the three predictions rather than from the result, and finish by writing out allele combinations for two or three crosses so that the proportions come from counting rather than from memory.
