3.2 Chromosomes Flashcards

prokaryote

single-celled
organism whose organelles are
not bound by membranes

nucleoid

region of a prokaryotic
cell where the DNA exists

binary fission

a method of
reproduction whereby a singlecelled organism makes a copy of
itself and splits in two

eukaryote

single-celled
organism that contain organelles
bound by membranes

autoradiography

technique of
capturing images of radioactive
substances on photographic film

radioactive marker

a
radioactive isotope of an element
introduced into an organism and
used to follow how the organism
uses that element

plasmid

small ring of DNA
separate from the bacterial
chromosome often used in
genetic modifi cation

histones

proteins associated with
DNA in eukaryotic chromosomes

nucleosome

structure found
in eukaryotic chromosomes
consisting of a strand of DNA
wrapped around eight histone
molecules

supercoiling

a process in which
intense folding and coiling of a
structure occurs

transcribed

the production of
an mRNA molecule from a DNA
template

polypeptide

polymer of many
amino acids combined by peptide
bonds

homologous

chromosome pairs
that occur at fertilization, one
from the female parent and one
from the male parent

centromere

region where sister
chromatids attach

diploid

a cell which has
chromosomes in homologous
pairs

haploid

a cell that has only one
chromosome of each homologous
pair

karyogram

a diagram showing
the chromosomes of an organism

chromatid

the identical parts
of a doubled chromosomes held
together by a centromere

mitotic metaphase

a stage of
meiosis during which homologous
chromosomes line up and the
nuclear membrane disintegrates

karyotype

the number and
appearance of chromosomes
within the cell of an organism

Down syndrome

a
chromosomal anomaly
characterized by 3 chromosomes
in the 21st pair

trisomy

presence of three copies
of a chromosome in a cell rather
than the usual pair

autosomes

chromosomes that
do not determine sex

Prokaryotes do not possess a nucleus – instead genetic material is found free in the cytoplasm in a region called the nucleoid

The genetic material of a prokaryote consists of a single chromosome consisting of a circular DNA molecule (genophore)

The DNA of prokaryotic cells is naked – meaning it is not associated with proteins for additional packaging

In addition to the genophore, prokaryotic cells may possess additional circular DNA molecules called plasmids

Plasmids are small, circular DNA molecules that contain only a few genes and are capable of self-replication

Plasmids are present in some prokaryotic cells, but are not naturally present in eukaryotic cells

Bacterial cells may exchange plasmids via their sex pili, in a process known as bacterial conjugation

This exchange of genetic material allows bacteria to evolve new features within a generation (horizontal gene transfer)

Overview of Bacterial Conjugation

Micrograph of Bacterial Conjugation via Sex Pili

Organisation of eukaryotic chromosomes can be summarised as follows:

DNA is complexed with eight histone proteins (an octamer) to form a complex called a nucleosome
Nucleosomes are linked by an additional histone protein (H1 histone) to form a string of chromatosomes
These then coil to form a solenoid structure (~6 chromatosomes per turn) which is condensed to form a 30 nm fibre
These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin
Chromatin will then supercoil during cell division to form chromosomes that are visible (when stained) under microscope

Eukaryotic chromosomes are linear molecules of DNA that are compacted during cell division (mitosis or meiosis)

Each chromosome has a constriction point called a centromere, which divides the chromosome into two sections (or ‘arms’)

The shorter section is designated the p arm and the longer section is designated the q arm

Eukaryotic species possess multiple chromosomes that may differ in both their size and the position of their centromere

Staining chromosomes with particular dyes (e.g. Giemsa stain) will additionally generate unique banding patterns

Each chromosome will carry specific genes and the position of a particular gene on a chromosome is called the locus

The region in which a locus is positioned can be identified via three points of reference:

The first point of reference is a number (or letter) which denotes the chromosome (e.g. 7q31 refers to chromosome 7)
The second point of reference is a letter (p or q) to denote which arm the locus is positioned on (e.g. 7q31 is on the q arm)
The third point of reference is a number corresponding to the G band location (e.g. 7q31 is at the longitudinal position 31)

Sexually reproducing organisms inherit their genetic sequences from both parents

This means that these organisms will possess two copies of each chromosome (one of maternal origin ; one of paternal origin)
These maternal and paternal chromosome pairs are called homologous chromosomes

Homologous chromosomes are chromosomes that share:

The same structural features (e.g. same size, same banding patterns, same centromere positions)
The same genes at the same loci positions (while the genes are the same, alleles may be different)

Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation

As sexually reproducing organisms receive genetic material from both parents, they have two sets of chromosomes (diploid)

To reproduce in turn, these organisms must create sex cells (gametes) with half the number of chromosomes (haploid)

When two haploid gametes fuse, the resulting diploid cell (zygote) can grow and develop into a new organism

Diploid

Nuclei possessing pairs of homologous chromosomes are diploid (symbolised by 2n)

These nuclei will possess two gene copies (alleles) for each trait
All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis
Diploid cells are present in most animals and many plants

Haploid

Nuclei possessing only one set of chromosomes are haploid (symbolised by n)

These nuclei will possess a single gene copy (allele) for each trait
All sex cells (gametes) in the organism will be haploid, and are derived from diploid cells via meiosis
Haploid cells are also present in bacteria (asexual) and fungi (except when reproducing)

Haploid versus Diploid

Sex Determination in Humans (the XY System)

In humans, sex is determined by a pair of chromosomes called the sex chromosomes (or heterosomes)

Females possess two copies of a large X chromosome (XX)
Males possess one copy of an X chromosome and one copy of a much shorter Y chromosome (XY)

The Y chromosome contains the genes for developing male sex characteristics (specifically the SRY gene)

In its absence of a Y chromosome, female sex organs will develop
The sex chromosomes are homologous in females (XX) but are not homologous in males (XY)

Hence the father is always responsible for determining the sex of offspring:

If the male sperm contains an X chromosome, the growing embryo will develop into a girl
If the male sperm contains a Y chromosome, the growing embryo will develop into a boy
In all cases the female egg will contain an X chromosome (as the mother is XX)

The remaining chromosomes in the organism are called autosomes (they do not determine sex)

Chromosome Types

Autosomes vs Heterosomes

Chromosomes Organised by Size

Karyotypes are the number and types of chromosomes in a eukaryotic cell – they are determined via a process that involves:

Harvesting cells (usually from a foetus or white blood cells of adults)
Chemically inducing cell division, then arresting mitosis while the chromosomes are condensed
The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids or not

The chromosomes are stained and photographed to generate a visual profile that is known as a karyogram

The chromosomes of an organism are arranged into homologous pairs according to size (with sex chromosomes shown last)

normal aryogram

Down syndrome

Summary of the Process of Autoradiography

Autoradiography

Cells are grown in a solution containing radioactive thymidine (tritiated thymidine – ³H-T)
The tritiated thymidine is incorporated into the chromosomal DNA of the cell (³H-T is used as thymidine is not present in RNA)
The chromosomes are isolated by gently lysing the cells and fixing the chromosomes to a photographic surface
The surface is then immersed in a radioactively-sensitive emulsion containing silver bromide (AgBr)
The radiation released from the tritiated thymidine converts the Ag⁺ ions in silver bromide into insoluble metal grains
Following a period of exposure, excess silver bromide is washed away, leaving the silver grains to appear as small black dots
When the photographic film is developed, the chromosomal DNA can be visualised with an electron microscope

Chromosome Length

John Cairns pioneered a technique for measuring the length of DNA molecules by autoradiography
Previously, chromosome length could only be measured while condensed during mitosis (very inaccurate due to supercoiling)
Cairns used autoradiography to visualise the chromosomes whilst uncoiled, allowing for more accurate indications of length
By using tritiated uracil (³H-U), regions of active transcription can be identified within the uncoiled chromosome

Uncoiled Chromosomes Identified with Autoradiography

Other Discoveries

John Cairns

was further able to use autoradiography to demonstrate key events which occur during chromosomal replication

DNA replication involves formation of a replication bubble (and prokaryotic replication involves a single origin of replication)
DNA replication is bi-directional (it occurs independently at both ends of the replication bubble)

Evidence for the Formation of Replication Bubbles (Prokaryotes)

. Evidence that Replication is Bi-Directional

Chromosome number is a characteristic feature of members of a particular species

Organisms with different diploid numbers are unlikely to be able to interbreed (cannot form homologous pairs in zygotes)
In cases where different species do interbreed, offspring are usually infertile (cannot form functional gametes)
- For instance, a horse (diploid = 64) and a donkey (diploid = 62) may produce an infertile mule (non-diploid = 63)

Chromosome number does not provide a valid indication of genetic complexity, for instance:

Tomatoes (Solanum lycopersicum) have 24 chromosomes and a genome size of 950 million bp, but possess ~32,000 genes
Chickens (Gallus gallus) have 78 chromosomes and a genome size of 1.2 billion bp, but possess only ~17,000 genes

Diploid Chromosome Number Comparisons

Comparison of Chromosome Number, Genome Size and Gene Count

Genome size can vary greatly between organisms and is not a valid indicator of genetic complexity

The largest known genome is possessed by the canopy plant Paris japonica – 150 billion base pairs
The smallest known genome is possessed by the bacterium Carsonella ruddi – 160,000 base pairs

As a general rule:

Viruses and bacteria tend to have very small genomes
Prokaryotes typically have smaller genomes than eukaryotes
Sizes of plant genomes can vary dramatically due to the capacity for plant species to self-fertilise and become polyploid

Comparison of Genome Size in Different Organisms

Variation in Genome Sizes For Different Types of Organisms

Prokaryotic DNA:

Is found freely in the cytoplasm (within a region called the nucleoid)
Is naked (i.e. not bound with proteins and therefore doesn’t form chromatin)
Genomes are compact (contain little repetitive DNA and no introns)
Contains extra-chromosomal plasmids
Is circular in shape

Eukaryotic DNA:

Is contained within a nucleus
Is bound to histone proteins
Genomes contain large amounts of non-coding and repetitive DNA (including introns)
Do not contain plasmids (but organelles such as the mitochondria may contain their own chromosomes)
Are linear in shape

Prokaryotic vs Eukaryotic DNA

Chromosomes are divided into two parts (p and q arms) with a constriction point called a centromere in the middle

The centromere can be located in different positions and this forms the basis for the four different classes of chromosome:

Metacentric – centromere is in middle, meaning p and q arms are of comparable length (e.g. chromosomes 1, 3, 16, 19, 20)
Submetacentric – centromere off-centre, leading to shorter p arm relative to q arm (e.g. chromosomes 2, 4 - 12, 17, 18, X)
Acrocentric – centromere severely off-set from centre, leading to much shorter p arm (e.g. chromosomes 13 - 15, 21, 22, Y)
Telocentric – centromere found at end of chromosome, meaning no p arm exists (chromosome not found in humans)

Types of Chromosomes

Block mutations are changes to segments of a chromosome, leading to large scale changes to the DNA of an organism

Block mutations are commonly caused by transposons, which, by changing positions within the genome, alter gene sequence

Several types of block mutations exist, including:

Duplications – part of chromosome is copied, resulting in duplicate sections (potentially increases gene expression)
Deletions – a portion of the chromosome is removed (along with any genes contained within this segment)
Inversions – a segment of a chromosome is removed and then replaced within the chromosome in reverse order
Translocations – segments of two chromosomes are exchanged (may interrupt gene sequences)

Types of Block Mutations

Human sex determination occurs according to the X - Y system

Females have two copies of the larger X chromosome
Males have one X and one Y chromosome (and hence determine gender in offspring)

Other Systems

X - 0 System

Common in certain insects, including grasshoppers and crickets
Females have two copies of the X chromosome (XX), whereas males only have one chromosome (X0)
Generally, in this method, sex is determined by the overall level of gene expression by the X chromosome (less in males)

Z - W System

Common in birds, some reptiles and certain insects
Males have two copies of the Z chromosome (ZZ), whereas females have one Z and one W chromosome (ZW)
Certain essential female genes appear to reside on the W chromosome (similar to the Y chromosome in human males)

Haplo-diploid System

Common in certain insect species, such as ants and bees
Unfertilised eggs develop into haploid individuals, which are males
Fertilised eggs develop into diploid individuals, which are generally female (all bar the reproductive queen are sterile)

Sex Determination Systems