WHAT IS DOWN SYNDROME?

· A condition caused when the baby has too many copies of a chromosomes.

DOWN SYNDROME

· A chromosomal disorder resulting from a partial or complete extra copy of chromosome 21

Multiple developmental and health effects including:

i Short stature, distinct facial features

Mild to moderate physical and cognitive impairment

Increased risk of problems involving heart, respiratory, digestive, hearing, vision, and /or

thyroid glands

Trisomy 21 (95% of all cases): three complete copies in all Cells

Mosaicism (1-2% of cases): three copies in some but not all cells

· Translocation (3-4% of cases): partial copy of chromosome 21 attached to

another chromosome

WHAT ARE CHROMOSOMES?

Human cell => 6 billion nucleotide base pairs (-2m)

Wrapped loosely around histone proteins = chromatin

DNA protein highly condensed = chromosome

EUKARYOTIC CHROMOSOMES

Eukaryotes typically have 10 to 50 chromosomes in their body (somatic)cells

Additional/missing chromosomes usually fatal

Humans are diploid (2n)

↳ Two complete sets of chromosomes

↳ One from mom, one from dad

↳ 46 total chromosomes

· Haploid (n)

↳ One set of chromosomes

23 in humans

HOW MANY CHROMOSOMES DO HUMANS HAVE?

Humans are diploid (2n)= 46 total chromosomes

Gametes are n = 23 unique chromosomes (haploid #

The two copies of each chromosome in human cells are homologous

Different versions -same genes in same locations but different DNA Sequence

Different versions (alleles)of a gene may promote different traits le. g. hair type

HOW DO CELLS GET THE RIGHT NUMBER OF CHROMOSOMES?

CELL DIVISION

1. Duplicate cell components

i Organelles

Cytoplasm

Chromosomes

2. Separate the material into two daughter cells

REPLICATION

· Prior to replication, each chromosome composed of a single DNA molecule

After replication, each chromosome composed of 2 identical DNA molecules

Visible as 2 strands connected in middle as chromosome condenses

more

· One chromosome composed of 2 sister chromatids held together

by cohesion proteins

Pair of chromosomes are homologous -Each is a homologue

DNA PACKAGING

Centromere -point of constriction

Replication occurs during S phase

Condensing starts in 62, finishes in prophase

Each sister chromatid has a centromere

FAMILIES: SIMILAR YET DIFFERENT WHY?

ASEXUAL REPRODUCTION

Single Parent

Offspring identical to each other and parent

SEXUAL REPRODUCTION

· Two parents

Offspring are unique

· Offspring are similar to each other and parents

Combine DNA from two individuals

· Combine characteristics of both individuals

BACTERIAL CELL DIVISION:

ASEXUAL REPRODUCTION

· Bacteria divide by binary fission

· Single, circular bacterial chromosome is replicated

Replication begins at the origin of replication and proceeds in two

directions to site of termination

AGAIN, BACTERIA DON'T DO SEX

New chromosomes are partitioned to opposite ends of the cell

· Septum forms to divide the cell into two cells

· No sexual life cycle

Reproduction is clonal

· Not much variation between generations

WHY DO DIPLOID ORGANISMS NEED TO HAVE SPECIALIZED

SEX CELLS?

Sex cells (gametes: sperm or egg)allow traits to be combined from

two organisms

SEXUAL REPRODUCTION

Gametes have only one of each chromosome

· Requires special cell division: Meiosis

· Diploid cells (2n)-p Gametes (n)

Takes place in gonads (testis, ovary)

FEATURES OF MELOSIS 1

· MeioSIS INCLUDES TWO ROUNDS OF DIVISION

Meiosis 1 and meiosis 11

i Each round has prophase, metaphase, anaphase, and telophase

Stages

· Synapsis

↳ Occurs early in prophase I

Homologous chromosomes become closely associated to create

tetrads or bivalents

↳ Does not occur in mitosis!!

FEATURES OF MELOSIS 2

While homologues are paired during prophase I, genetic recombination

I crossing over)occurs

↳ Sites of crossing over are called chiasmata

First meiotic division is termed the "reduction division"

↳ Results in daughter cells that contain one homologue from each

chromosome pair

NO DNA REPLICATION BETWEEN MELOTIC DIVISIONS

Second meiotic division doesn't further reduce the number of chromosomes

↳ Separates the sister chromatids for each homologue

WHY BOTHER?

· For humans with 23 pairs of chromosomes?

· (2)23_ >8 million different possible gametes

· For a couple

↳ possible unique offspring

18 million)x 18 million)= (64, 000, 000, 000, 0001

CROSSING OVER ENABLES EVEN GREATER VARIETY

Exchange of equivalent sections between homologous chromosomes

· Occurs at random locations along chromosome

· Creates new versions of chromosomes

CROSSING OVER

· Genetic recombination between non-sister chromatids

Allows the homologues to exchange chromosomal material

· Alleles of genes that were formerly on separate homologues

can now be found on the same homologue

Chiasmata-site of crossing over

Contact maintained until anaphase I

MELOSIS 11

Resembles a mitotic division

Prophase 11: nuclear envelopes dissolve and new Spindle apparatus

forms

· Metaphase II: Chromosomes align on metaphase plate

· Anaphase 11: Sister chromatids are separated from each other

· Telophase 11: nuclear envelope re-forms around 4 sets of daughter

chromosomes; Cytokinesis follow

ERRORS IN MeioSiS

Nondisjunction -failure of chromosomes to move to opposite poles

during either meiotic division

Aneuploid gamete-gametes with missing or extra chromosomes

Result of nondisjunction event

Most common cause of spontaneous abortion in human

DOWN SYNDROME

The different copies of chromosome 21 for John, Jane, and their

baby are shown here.

HOW IS DOWN SYNDROME DETECTED?

KARYOTYPING

↳ Isolate chromosomes during fetal cell division

Arrange in pairs according to size

MELOSIS VS MITOSIS

Meiosis is characterized by four distinct features:

Homologous pairing and crossing over

Sister chromatids remain joined at their centromeres and segregate

together during anaphase

linetochores of sister chromatids attach to the same pole in meiosis /

DNA replication is suppressed between meiosis 1 and meiosis 11

Meiosis (crossovers and Sister chromatic cohesion lock homologues together. Microtubules connect to the Kinetochores of

Sister chromatids so

that homologues are

pulled toward

opposite poles.

Microtubules pull

the homologous

chromosomes

apart, but sister

chromatids are

held together at

the centromere

Mitosis

Homologues do

not pair;

Kinetochores of

sister chromatids

remain separate:

microtubules

attach to both

kinetochores on

opposite sides of

the centromere

Microtubules

pull sister

chromatids

apart.