LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I Fhow do we know that genes are parts of chromosomes? Fhow are they arranged on chromosomes? Fare chromosomes

LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I

how do we know that genes are parts of chromosomes?

how are they arranged on chromosomes?

are chromosomes only genes?

how is chromosome number maintained through the generations?


what is the chromosomal basis for Mendel’s 1st law - equal segregation?

what is the chromosomal basis for Mendel’s 2nd law - independent assortment?

how does all the DNA fit into a tiny nucleus?


CH3 key concepts historical development of

the chromosome theory... genetics + cell biology

chromosomes mitosis & meiosis chromosome behavior &

inheritance patterns organelle chromosomes

CHAPTER 3: KEY CONCEPTS

genes are parts of chromosomes mitosis = 1 nuclear division 2 daughter nuclei

identical to original nucleus meiosis = 2 nuclear divisions 4 genetically distinct

daughter nuclei with one set of chromosomes Mendel’s laws of (1) equal segregation & (2)

independent assortment are based on (1) separation of each member of a chromosome pair & (2) the independent movement of chromosome pairs

CHAPTER 3: KEY CONCEPTS

chromosomes can be identified microscopically using various visible landmarks

a chromosome (chromatid) contains a single DNA molecule

DNA winds around protein spools, spooled units then coil, loop & supercoil to form a chromosome

much of eukaryotic DNA is present in multiple copies most multiple copy DNA has no known function

OVERVIEW

GENETICS + CELL BIOLOGY

mechanisms of character transmission (so far studied) unknown until chromosomes were accessible by more advanced microscopy

behavior of Mendel’s particles parallel chromosome movement – merger of cytology and genetics – Sutton-Boveri Theory (1902)


cell biology findings leading to understanding of remarkable correlation with transmission: nuclear divisions – initially ‘aberrations’ in cell physiology 2 gametes fusion during fertilization discovery of chromosomes discovery that somatic chromosome # constant for given

species; # did not seem to correlate with species complexity (e.g., ant with 1pr)

chromosomes present in pairs* – diploid number gametes formed have half as many – haploid number each member of diploid pair are derived from each parent;

each parent contributes a haploid complement to zygote


Sutton-Boveri theory: genes are located on chromosomes

Mendelian genes and observed chromosomes... occur in pairs segregate equally into gametes assort independently into gametes

evidence is convincing... but correlative only...


Sutton-Boveri theory: genes are located on chromosomes

problems? do chromosomes retain their physical integrity

through interphase? some chromosome pairs look the same... how can

you tell that they assort independently?

CHROMOSOME THEORY - SEX LINKAGE

a grasshopper sp. has unpaired & heteromorphic pair observed 2 patterns with = frequency (counted) non-homologous chromosomes assort independently ... but not random... mechnanism?


a thistle sp. has 12 chromosome pairs 12 variants identified each variant had a 3rd copy of one chromosome chromosomes all carry genetically distinct material


meiosis in a beetle Tenebrio segregation of heteromorphic

chromosomes X and Y in gametes receive different

material s don’t have Y


chromosome complement in 2 insect spp. gender correlates with 2 patterns, s have 2 Xs, no Y s have 1X and either 0 or 1 Y


X and Y segregate like homologues in s s make 2 kinds of gametes in equal proportions responsible for determination of gender


in some species... are heterogametic WZ are homogametic ZZ distinguish system from XY found in some birds, reptiles, fish, and some

insects (moths and butterflies) same rules apply as in XY but in reverse


compare this pattern with X-linked inheritance in flies (discussed in lecture 03)


compare this pattern with X-linked inheritance in flies (discussed in lecture 03)

CHROMOSOME THEORY - SEGREGATION

XX XY XXY X0 Flies

Mammals


in flies... X:autosome ratio 1, mRNA on X:autosome rato < 1, mRNA on XXY = X0 = fertility is determined by Y , X0 = sterile

in humans... gender determined by Y


sometimes observe “exceptional” progeny


proof of (... ok, strong evidence for) the chromosomal theory of inheritance

CHROMOSOME THEORY

2 important points to remember... not all genes on sex chromosomes have to do with

gender or sexual differentiation many autosomal genes are important for gender

CHROMOSOMES

levels of genetic organization:

1. ploidy – chromosome sets

2. n – how many of each type

3. size – arbitrary / relative

4. centromere – position

5. landmarks – chromomeres, puffs, abnormalities

2n = 6

CHROMOSOMES

text order of ideas for this section of chapter 3

very smallDNA

very largegenome

mediumchromosometopography

smallchromosome

structure

my order of ideas for this section of chapter 3

very smallDNA

very largegenome

mediumchromosometopography

smallchromosome

structure

CHROMOSOMES - n

n

CHROMOSOMES - CENTROMERE

also... (d) acentric and (e) dicentric ... later!

CHROMOSOMES - n, SIZE, CENTROMERE

CHROMOSOMES - n, SIZE, CENTROMERE

CHROMOSOMES - TOPOGRAPHY

human nucleus, metaphase, n = 23 centromeres - note positions chromomeres - bead-like thickenings


tomato nucleus, prophase I, n = 12 centromeres - orange heterochromatin - green: dense, note position... later euchromatin - white: less dense, active genes... later


Drosophila melanogaster chromosomes, n = 4 heterochromatin - blue: dense, note position... later euchromatin - gold: less dense, active genes... later


mouse nucleus, satelite DNA, different G+C content multiple tandem DNA sequence repeats much of it in centromeric heterochromatic regions


tomato chromosome-2 nucleoli:

nuclear organelles containing rRNA 1 or more / nucleus (spp.)

nucleolar organizers (NO): genes encoding rRNA redundant - high copy


telomeres no visible “structure” FISH (fluorescent in situ

hybridization) telomere-specific DNA tandem arrays of non-coding

sequence overcomes replication

problem... more on this later


human nucleus, metaphase, n = 23, Giemsa stain banding patterns - consistent, chromosome-specific G-dark & G-light... significance of bands?


Drosophila chromosomes centromeres telomeres euchromatin heterochromatin polytene chromosomes chromocenter


why is this why is this significant?significant?


Zea maize, n = 10

CHROMOSOMES - GENES

CHROMOSOMES - GENES

single gene ID with FISH

CHROMOSOMES - GENES

meaningful relationships ?

CHROMOSOMES - GENES

meaningful relationships ?

CHROMOSOMES - GENES

repetative genes - duplication ?

CHROMOSOMES - STRUCTURE

honeybee metaphase chromosomes

continuous “fiber” 30 nm wide


D. melanogaster DNA molecule from 1 chromosome* single continuous “fiber” 15 mm (5 x longer than the

fly!) a chromosome contains only 1 DNA molecule



1. nucleosome (10 nm)

2. solonoid (30 nm)


3. scaffold loop (? nm)4. supercoil

(chromatid)


where should the genes be ?

TUTORIAL #1: T.9.19 or R.9.21

go to the TUTORIAL page on the genetics web site download the file tut1-06F.pdf follow instructions and attempt as many of the

problems as possible you will continue to work on these in tutorial #1, with

the assistance of your TA and your peers

Documents

LECTURE 04: CHROMOSOMAL BASIS OF INHERITANCE I Fhow do we know that genes are parts of chromosomes? Fhow are they arranged on chromosomes? Fare chromosomes