genet 302 lec 1 & 2
What signal tells RNA polymerase to stop making mRNA?
A transcription termination signal (terminator)
What stops the ribosome during protein synthesis?
A stop codon (UAA, UAG, or UGA)
What protects the ends of an mRNA molecule?
The 5′ cap and the poly-A tail
If a mature mRNA has 4 exons, how many introns were removed?
3 introns
What is the name of the region between the 5′ cap and the start codon?
The 5′ untranslated region (5′ UTR)
Why were chromosomes named “chromosomes”?
It means “colored bodies” because they stain strongly in cells (discovered in the 1800s).
What is the chromosome theory of heredity?
Genes are located on chromosomes, and chromosomes explain Mendel’s inheritance patterns.
(Boveri & Sutton ~1900; confirmed by Stevens & Morgan ~1910)
What is a genome?
All the DNA in an organism’s cells (nuclear + mitochondrial).
What is C-value?
The amount of DNA in a haploid G1 cell — the smallest amount of DNA in a species.
Examples:
E. coli = 5 Mb
Drosophila = 180 Mb
Humans = 3,100 Mb
Paris japonica = 150,000 Mb (huge!)
What is the C-value paradox?
Genome size does NOT correlate with organism complexity.
Plants can have far more DNA than humans.
What is DNA reassociation used for?
To measure how repetitive a genome is by seeing how fast DNA strands find partners again.
Steps:
Isolate DNA
Shear it
Heat → single strands
Cool → strands re-anneal
What is Cot½?
The time when 50% of DNA has re-annealed — a measure of repetition.
Why does bacterial DNA re-anneal slowly?
Because it is mostly unique DNA, so each piece has only one partner.
What three DNA types are in eukaryotic genomes?
Highly repetitive (satellite DNA)
Moderately repetitive
Non-repetitive (single-copy genes)
Fast → medium → slow reannealing
Which organisms have the most repetitive DNA?
Plants (like corn) and mammals have far more repetitive DNA than bacteria.
What is moderately repetitive DNA?
It includes transposable elements and multi-copy genes.
What are transposable elements?
DNA sequences that copy and insert themselves into new locations.
Why are some genes multi-copy?
To make large amounts of:
rRNA
tRNA
histone proteins
Why do histone genes exist in many copies?
Cells need huge amounts of histones to package DNA into nucleosomes.
Histones: H1, H2A, H2B, H3, H4
How can multicopy genes be arranged?
Dispersed (C. elegans)
Clustered (Humans)
Tandem arrays (Drosophila)
How are human histone genes arranged?
In clusters, mainly at:
6p21 (54 genes)
1q21 (6 genes)
What does the 45S rRNA gene produce?
It makes 18S, 5.8S, and 28S rRNAs after cleavage.
How are rRNA genes organized in humans?
Multiple copies in tandem arrays (hundreds of copies per cluster).
What is satellite DNA?
Highly repetitive tandem repeats (2–200 bp units, up to 1000 repeats).
Where is satellite DNA located?
Centromeres
Telomeres
What sequence makes telomeres?
(TTAGGG)n repeated thousands of times.
How do centrifuge experiments identify satellite DNA?
DNA with unusual GC content forms separate bands in CsCl gradients.
Cot-curve experiments showed satellite DNA is what?
Highly repetitive
Classify eukaryotic DNA
Type What it contains
Non-repetitive Single-copy genes
Moderately repetitive TEs + multi-copy genes
Highly repetitive Satellite DNA (centromeres, telomeres)
What was Carl Rabl testing?
Whether chromosomes disappear during interphase.
What did Rabl observe?
The same chromosomes seen in telophase reappear in prophase of the next division.
What did Rabl conclude? Why this matters
Chromosomes persist through interphase — they are not destroyed and re-formed. It proved chromosomes are continuous physical structures, not temporary.
What did Boveri study?
Sea urchin embryos with different numbers of chromosomes.
What did boveri find? What did this show? Why it mattered
Only embryos with the correct chromosome number developed normally. All chromosomes are required for normal development. It showed chromosomes carry essential information (genes), not just DNA mass.
What did Sutton study? What did he observe? What did Sutton conclude?
Grasshopper chromosomes during meiosis.
Adults have pairs of chromosomes
Gametes have one of each pair
Chromosome behavior matches Mendel’s laws → chromosomes carry genes.
What did Stevens discover?
Sex is determined by chromosomes (X and Y).
XX = female, XY = male
This was the first proof that a specific chromosome controls a trait
What did Morgan show using fruit flies? Why was this important?
The eye-color gene is on the X chromosome. It proved genes are physically located on chromosomes.
Why was “Anaphase I of spermatogenesis” correct
In the image:
One chromosome is large
One is small
That matches the X and Y chromosomes separating.
In Anaphase I:
Homologous chromosomes separate
X and Y go to opposite poles
In oogenesis, both sex chromosomes are X and equal in size → you would not see a big and small chromosome.
So it must be male meiosis (spermatogenesis) and Anaphase I.
What chemicals are in the centrifuge tube?
Cesium chloride (CsCl) → makes a density gradient
Ethidium bromide (EtBr) → makes DNA fluoresce
What happens when mouse DNA is centrifuged?
DNA separates into bands based on base composition.
Fragments with different G+C content form satellite bands.
What G+C values were seen?
Mouse DNA formed bands around:
~30% GC
~42% GC
DNA with unusual GC% separates from the main band.
What was satellite DNA?
Satellite DNA = DNA that formed a separate band because it has a different base composition.
When satellite DNA was isolated and tested, what was it?
A. Highly repetitive
Because it re-annealed very fast in Cot-curve experiments.
What does it mean that transposable elements are “capable of independent replication”?
It means they can copy themselves and insert into new DNA locations without needing the host cell to copy them.
They are self-replicating DNA parasites inside the genome.
What does dispersed mean?
Copies are scattered throughout the genome with no pattern.
Example: C. elegans histone genes
How to identify:
Copies appear on different chromosomes or far apart
No repeating block
What does clustered mean?
Copies are close together in one region but not in a repeating order.
Example: Human histone genes
54 genes at 6p21
6 genes at 1q21
How to identify:
Many copies in one location
Random order (H2A, H4, H3, H2B mixed)
What is a tandem array?
Copies are arranged back-to-back in repeating units.
Example: Drosophila histone genes, human rRNA genes
How to identify:
Same sequence repeated:
ABC ABC ABC ABC
Looks like cloned blocks
What is in a eukaryotic ribosome?
An 80S ribosome made of:
4 rRNAs
~80 ribosomal proteins
How are human rRNA genes organized?
They are multiple-copy tandem arrays.
Two types:
45S rRNA genes → make 18S, 5.8S, 28S
5S rRNA genes → separate tandem array
So humans use:
Multiple copy – tandem array
On chromosome diagrams, what do the colors mean?
Color Meaning
Yellow Genes = euchromatin
Blue Satellite DNA =. heterochromatin