3.2 Chromosomes Flashcards

Set Details Share
created 2 years ago by Renata_SidorukSołoducha
updated 2 years ago by Renata_SidorukSołoducha
show moreless
Page to share:
Embed this setcancel
code changes based on your size selection



organism whose organelles are
not bound by membranes



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



organism that contain organelles
bound by membranes



technique of
capturing images of radioactive
substances on photographic film


radioactive marker

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



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



proteins associated with
DNA in eukaryotic chromosomes



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



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



the production of
an mRNA molecule from a DNA



polymer of many
amino acids combined by peptide



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



region where sister
chromatids attach



a cell which has
chromosomes in homologous



a cell that has only one
chromosome of each homologous



a diagram showing
the chromosomes of an organism



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



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


Down syndrome

chromosomal anomaly
characterized by 3 chromosomes
in the 21st pair



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



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

card image


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

card image


Micrograph of Bacterial Conjugation via Sex Pili

card image


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
card image


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)
card image


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

card image


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



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



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

card image


Sex Determination in Humans (the XY System)

card image


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

card image


Autosomes vs Heterosomes

card image


Chromosomes Organised by Size

card image


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

card image


Down syndrome

card image


Summary of the Process of Autoradiography

card image



  • 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

card image


Other Discoveries

John Cairns

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

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


Evidence for the Formation of Replication Bubbles (Prokaryotes)

card image


. Evidence that Replication is Bi-Directional

card image

card image

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

card image


Comparison of Chromosome Number, Genome Size and Gene Count

card image


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

card image


Variation in Genome Sizes For Different Types of Organisms

card image


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

card image


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

card image


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

card image


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

card image