10.chromosome, Gene And Dna.pptx

Chromosome, Gene and DNA Dr. Triwani, M.Kes Medical Faculty Sriwijaya University

WHAT ARE CHROMOSOMES?

Chromosome Structure  Chromosomes are compact spools of DNA.

 Chromosome = DNA and proteins coiled

together into “rod-like” shape  Histones = proteins that help DNA stay coiled

Chromosomes  A chromosome is one of the threadlike "packages" of

genes and other DNA in the nucleus of a cell.  Different kinds of organisms have different numbers of chromosomes.  Humans have 23 pairs of chromosomes, 46 in all: 44 autosomes and two sex chromosomes.  Each parent contributes one chromosome to each pair, so children get half of their chromosomes from their mothers and half from their fathers.

CHROMOSOME

 Chromosomes, the dark structures in this image, are

copied and distributed to the daughter cells as this plant cell reproduces.

Where are the Chromosomes Stay?  You can think of

chromosomes as "DNA packages" that enable all this DNA to fit in the nucleus of each cell.

 In the nuclei of eukaryotic

cells, as they prepare to divide..

 The DNA coils up and

becomes denser than normal, so is more visible. WHY?

Genetic Information  Gene – basic unit of genetic

information. Genes determine the inherited characters.

 Genome – the collection of

genetic information.

 Chromosomes – storage units of

genes.

 DNA - is a nucleic acid that

contains the genetic instructions specifying the biological development of all cellular forms of life

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Chromosome Logical Structure  Locus – location of a gene/marker

on the chromosome.

 Allele – one variant form of a

gene/marker at a particular locus. Locus1 Possible Alleles: A1,A2 Locus2 Possible Alleles: B1,B2,B3

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Human Genome Most human cells contain 46 chromosomes:  2 sex chromosomes (X,Y):

XY – in males. XX – in females.

 22 pairs of chromosomes

named autosomes.

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http://www.ncbi.nlm.nih.gov

PART OF CHROMOSOME  Each 1/2 of chromosome = chromatid  Centromere holds 2 chromatids together

chromatids

centromere

chromosome

What are centromeres for?  Centromeres are required for chromosome

separation during cell division.  The centromeres are attached to microtubules, which are proteins that can pull chromosomes toward opposite ends of each cell (the cell poles) before the cell divides.  This ensures that each daughter cell will have a full set of chromosomes

 Normally, each chromosome has only one centromere.  The position of the centromere relative to the end of the

chromosome helps scientists tell chromosomes apart.  Centromere position can be described three ways: metacentric, submetacentric or acrocentric

 In metacentric chromosomes,

the centromere lies near the center of the chromosome.

 Submetacentric chromosomes

have a centromere that is offcenter, so that one chromosome arm is longer than the other.

 When chromosomes are aligned,

they are oriented so that the short arm, designated "p" (for petite), is at the top, and the long arm, designated "q" (simply for what follows the letter "p"), is at the bottom

Chromosome Numbers  Each species has characteristic # of chromosomes

 Normally, humans have 46 chromosomes in each cell; we

received 23 from our mother and 23 from our father.  2 Categories  Sex Chromosomes- determine gender (2)

 XX = female XY = male  Autosomes- all other chromosomes (44- in 22 pairs)

How Do Scientists Read Chromosomes? To "read" a set of human chromosomes, scientists first use three key features to identify their similarities and differences:  Size. This is the easiest way to tell two different chromosomes apart.  Banding pattern. The size and location of Giemsa bands on chromosomes make each chromosome pair unique.  Centromere position. Centromeres are regions in chromosomes that appear as a constriction. They have a special role in the separation of chromosomes into daughter cells during mitosis cell division (mitosis and meiosis). Using these key features, scientists match up the 23 pairs -- one set from the mother and one set from the father.

Karyotype  Chart showing all of the chromosomes present in each cell of an

individual  To match chromosomes, look at:  Size  Banding  Centromere location

From mom

Homologous pair

From dad

Homologous Pairs (Homologues)  Autosomal chromosomes in pairs  1 from mom, 1 from dad  Same size, shape, and banding pattern  Code for same traits

Eye color

Eye color

MOM

DAD

Note: 22 autosomal homologous pairs, 2 sex chromosomes What gender is this individual?

KIND OF CHROMOSOMES

1. AUTOSOME 2. SEX CHROMOSOME

1. Autosome  Autosome: A chromosome that is not a sex chromosome.  In other words, any one of the chromosomes save the sex

chromosomes.

 People normally have 22 pairs of autosomes in every cell (together

with two sex chromosomes -- an X and a Y in the male and two Xs in the female

 The term "autosome" was coined by Montgomery in 1906.

1. Autosome  An autosomal dominant gene is one on an autosome that is

always expressed, even if a single copy exists.  The chance is 1 in 2 (50%) for passing this autosomal dominant

gene to a particular offspring

2. Sex Chromosomes  The nuclei of human cells contain 22

autosomes and 2 sex chromosomes.  In females, the sex chromosomes are

the 2 X chromosomes.  Males have one X chromosome and one

Y chromosome

2.1 X chromosome  The X chromosome is one of the two sex chromosomes

in humans (the other is the Y chromosome).

 The sex chromosomes form one of the 23 pairs of

human chromosomes in each cell.

 The X chromosome spans about 155 million base pairs

(the building blocks of DNA) and represents approximately 5 percent of the total DNA in cells.

2.1 X chromosome  Each person normally has one pair of sex chromosomes in each

cell.

 Females have two X chromosomes, while males have one X and

one Y chromosome.

 Early in embryonic development in females, one of the two X

chromosomes is randomly and permanently inactivated in somatic cells

 This phenomenon is called X-inactivation or Lyonization.

X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell.

2.1 X chromosome  The X chromosome likely contains between 900 and 1,400

genes.  Genes on the X chromosome are among the estimated 20,000 to 25,000 total genes in the human genome.  There are many genetic conditions related to genes on the X chromosome

2.1 X chromosome The X chromosome carries hundreds of genes but few, if any, of these have anything to do directly with sex. However, the inheritance of these genes follows special rules. These arise because:  males have only a single X chromosome  almost all the genes on the X have no counterpart on the Y; thus  any gene on the X, even if recessive in females, will be expressed in males.

2.2 Y Chromosome  This diagram shows the structure of the

human Y chromosome.  Although 95% of the Y chromosome lies between the pseudoautosomal regions, fewer than 80 genes have been found here.

2.2 Y Chromosome  The Y chromosome is one of the sex-determining

chromosomes in humans and most other mammals  In humans, the Y chromosome spans 58 million base pairs (the building blocks of DNA) and represents approximately 0.38% of the total DNA in a human cell

Using Karyotypes to Predict Genetic Disorders What happens when a person has something different, such as:  Too many or too few chromosomes?  Missing pieces of chromosomes?  Mixed up pieces of chromosomes

Too many or too few chromosomes  To understand how our cells might end up with too

many or too few chromosomes, we need to know how the cells normally get 46 chromosomes.  First we need to understand meiosis. Meiosis is the cell

division process that produces egg and sperm cells (gametes), which normally have 23 chromosomes each.

 If eggs and sperm only have one set of chromosomes, then

how do we end up with 46 chromosomes?

 During fertilization, when the egg and sperm fuse, the

resulting zygote has two copies of each chromosome needed for proper development, for a total of 46.

How can cells end up with too many or too few chromosomes? Some examples of genetic disorders that are caused by an abnormal number of chromosomes are:  Down Syndrome  Turner Syndrome (XO)  Klinefelter Syndrome (XXY)

 Sometimes chromosomes are incorrectly distributed into the egg

or sperm cells during meiosis.  When this happens, one cell may get two copies of a particular chromosome, while another cell gets none.

What happens if a sperm or egg cell with an abnormal number of chromosomes participates in fertilization?  It depends on how many chromosomes the gamete has.  For example, if a sperm with an extra chromosome

fertilizes an egg with a normal chromosome number, the resulting zygote will have 3 copies of one chromosome.  This is called trisomy (pronounced TRY-so-mi).

 If a sperm that is missing a chromosome fertilizes an egg,

then the resulting zygote will have only one copy of that chromosome.  This is called monosomy

 People who are born with an abnormal number of

chromosomes often have genetic disorders because their cells contain too much or too little genetic information.  Scientists can predict genetic disorders by looking for extra or missing chromosomes in a karyotype

Chromosomal conditions are related to the X chromosome  Klinefelter syndrome is caused by the presence of one or more extra

copies of the X chromosome in a male's cells

 Triple X syndrome (also called 47,XXX or trisomy X) results from an

extra copy of the X chromosome in each of a female's cells

 Turner syndrome results when each of a female's cells has one normal

X chromosome and the other sex chromosome is missing or altered.

 other chromosomal conditions often affect sex determination

X-Inactivation  a dense, stainable structure, called a Barr

body is seen in the interphase nuclei of female mammals  The Barr body is one of the X chromosomes.

Its compact appearance reflects its inactivity.  So, the cells of females have only one

functioning copy of each X-linked gene — the same as males.

Gene

s

Gene  A Gene is the functional and physical unit of heredity passed

from parent to offspring.  Genes are pieces of DNA, and most genes contain the information for making a specific protein.

Genes  Genes are the units of heredity in living organism.  They are encoded in the organism's genetic material (usually DNA or

RNA), and control the physical development and behaviour of the organism.

 During reproduction, the genetic material is passed on from the

parent(s) to the offspring.

 Genetic material can also be passed between un-related individuals

(e.g. via transfection, or on viruses).

 Genes encode the information necessary to construct the chemicals

(proteins etc.) needed for the organism to function.

Term "gene"  The gene is shared by many disciplines, including classical

genetics, molecular genetics, evolutionary biology and population genetics.

 Because each discipline models the biology of life

differently, the usage of the word gene varies between disciplines.

 It may refer to either material or conceptual entities.

Two general types of gene in the human genome:

 non-coding RNA genes

 protein-coding genes.

 Non-coding RNA genes represent 2-5 percent of the total and

encode functional RNA molecules.  Many of these RNAs are involved in the control of gene expression, particularly protein synthesis.  They have no overall conserved structure.

 Protein-coding genes represent the majority of the total and

are expressed in two stages: transcription and translation.  They show incredible diversity in size and organisation and

have no typical structure.  There are, however, several conserved features.

Structure of Gene  Introns are regions often

found in eukaryote genes which are removed in the splicing process: only the exons encode the protein

The Structure of Eukaryotic Genes  Eukaryotic genes consist of coding and

non-coding segments of DNA, called exons and introns, respectively.  At first glance it seems to be an unnecessary burden to carry DNA without obvious functions within a gene.

The Structure of Eukaryotic Genes  However, it has been recognized that this has great evolutionary advantages.  When parts of different genes are rearranged on new chromosomal sites during evolution, new genes may be constructed from parts of previously existing genes.

http://www.ncbi.nlm.nih.gov

Genome  A genome is all the DNA contained in an organism or a cell,

which includes the chromosomes plus the DNA in mitochondria (and DNA in the chloroplasts of plant cells).

http://www.ncbi.nlm.nih.gov

Genes  Units of information about heritable traits  In eukaryotes, distributed among chromosomes  Each has a particular locus  Location on a chromosome

How do gene work for heredity?

Genes and Heredity  Heredity is the passing of genes from one generation to the next.

You inherit your parents' genes.

 Heredity helps to make you the person you are today: short or tall,

with black hair or blond, with green eyes or blue.

 Can your genes determine whether you'll be a straight-A student or

a great athlete?

 Heredity plays an important role, but your environment (including

things like the foods you eat and the people you interact with) also influences your abilities and interests.

OUR GENES  Each chromosome is made up

of a long thin thread of DNA that is coiled up like a ball of string as shown in the right

OUR GENES  If the DNA was stretched out it

would look like beads on a string.  Each of these beads of DNA is

called a gene that is a piece of genetic information.  Thousands of genes make up

each chromosome

OUR GENES  Each gene has its own specific

location on the chromosome.  Since the chromosomes come in pairs, there are two copies of each gene in each cell.

OUR GENES  The exception to this rule

applies to the genes carried on the sex chromosomes: the X and Y.  Since males have only one copy of the X chromosome, they have only one copy of all the genes carried on the X chromosome.

OUR GENES  Females have two copies of the X

chromosome in their cells and so they have two copies of the genes carried on the X chromosome.

 So that males and females have the

same number of X chromosome genes "active" in their cells, in females one of the X chromosomes is "switched off" or inactivated.

OUR GENES  The genes on the Y

chromosome are responsible mainly for the development of "maleness" only.

Our genes have an important role in our cells  A gene is a piece of the genetic material that does one particular

job.

 Each gene is a different `packet' of information necessary for our

bodies to grow and work.

 Our genes also contain the information for how we look: the

colour of our eyes, how tall we are, the shape of our nose, etc.

Our genes have an important role in our cells  The DNA "string" between the genes is called "non-coding

DNA", as it appears not to contain the information for gene products that the cells use.

 Studies of this DNA are useful for forensic investigations and

determining biological relationships and it is becoming increasingly clear that the non-coding DNA has a number of different roles to play in the cells.

NOT ALL THE GENES ARE SWITCHED ON ALL THE TIME

 Our bodies have many different types of cells such as those in the skin,

muscle, liver and brain.

 While all of these different types of cells contain the same genes, each cell

requires particular proteins to function correctly.

 Therefore, different genes are active in different cell types, tissues and

organs, producing the necessary specific proteins. Not all the genes in the cell are switched on or are "active" in every cell.

 For example, the genes that are active in a liver cell are different

from the genes that are active in a brain cell. This is because these cells have different functions and therefore require different genes to be active.

 Some genes are only switched on during the development of the

baby.

 After birth they are no longer needed to be active as their "job"

has been completed.

Homologous Chromosomes  Homologous autosomes are identical in length, size,

shape, and gene sequence  Sex chromosomes are nonidentical but still homologous  Homologous chromosomes interact, then segregate from one another during meiosis

Alleles  Different molecular forms of a gene  Arise through mutation  Diploid cell has a pair of alleles at each locus

 Alleles on homologous chromosomes may be same or

different

Studying Human Genetics Studying Human Genetics is much more complicated than using other model systems (e.g. Pea Plants)

 

Humans reproduce slowly, have few offspring, and it is unethical to breed humans for experiments

There are many techniques that are used to study human genetics indirectly

   

Karyotypes Pedigree analysis Linkage maps

Karyotype  Picture of an individual’s chromosomes  Making a Karyotype:  Metaphase chromosomes are fixed and stained

 Chromosomes are photographed through

microscope  Photograph of chromosomes is cut up and arranged to form karyotype diagram

Karyotype

Autosomes

Sex Chromosomes

DNA •DNA: the chemical inside the nucleus of a cell that carries the genetic instructions for making living organisms. •The material inside the nucleus of cells that carries genetic information.

• The scientific name for DNA is deoxyribonucleic acid.

http://www.ncbi.nlm.nih.gov/

TYPES OF DNA

DNA as Carrier of Genetic Information  Although DNA was discovered in 1869 by Friedrich Miescher as a new, acidic, phosphoruscontaining substance made up of very large molecules that he named “nuclein”, its biological role was not recognized  In 1889 Richard Altmann introduced the term “nucleic acid”.

DNA as Carrier of Genetic Information  By 1900 the purine and pyrimidine bases were known  Twenty years later, the two kinds of nucleic acids, RNA and DNA, were distinguished  An incidental but precise observation (1928) and relevant investigations (1944) indicated that DNA could be the carrier of genetic information

DNA as Carrier of Genetic Information  The information for the development and specific functions of cells and tissues is stored in the genes.  A gene is a portion of the genetic information, definable according to structure and function.  Genes lie on chromosomes in the nuclei of cells.

DNA as Carrier of Genetic Information  They consist of a complex long-chained

molecule, deoxyribonucleic acid (DNA).  In the following, the constituents of the DNA molecule will be presented.  DNA is a nucleic acid.

DNA and Its Components  Its chemical components are

nucleotidebases, a sugar (deoxyribose), and phosphate groups.  They determine the threedimensional structure of DNA, from which it derives its functional consequence

A. Nucleotide base

A. Nucleotide base

DNA Structure  In 1953, JamesWatson and Francis Crick recognized

that DNA must exist as a double helix.  This structure explains both important functional aspects: replication and the transmission of genetic information.  The elucidation of the structure of DNA is considered as the beginning of the development of modern genetics.  With it, gene structure and function can be understood at the molecular level.

DNA as a double helix  The double helix is the characteristic structural

feature of DNA.  The two helical polynucleotide chains are wound around each other along a common axis.  The nucleotide base pairs (bp), either A–T or G– C, lie within

Transmission of genetic information  Genetic information lies in the sequence of

nucleotide base pairs (A–T or G–C).  A sequence of three base pairs represents a codeword (codon) for an amino acid.  The codon sequence determines a corresponding sequence of amino acids.  These form a polypeptide (gene product).

Transmission of genetic information  A gene can be defined as a section of DNA

responsible for the formation of a polypeptide (one gene, one polypeptide).  One or more polypeptides form a protein.  Thus, several genes may be involved in the formation of a protein.

Alternative DNA Structures  Gene expression and transcription can be

influenced by changes of DNA topology.  However, this type of control of gene expression is relatively universal and nonspecific.  Thus, it is more suitable for permanent suppression of transcription, e.g., in genes that are expressed only in certain tissues or are active only during the embroyonic period and later become permanently inactive.

Three forms of DNA  The DNA double helix does not occur as a

single structure, but rather represents a structural family of different types.  The original classic form, determined byWatson and Crick in 1953, is B-DNA.  The essential structural characteristic of B-DNA is the formation of two grooves, one large (major groove) and one small (minor groove).

Three forms of DNA  There are at least two further, alternative

forms of the DNA double helix, Z-DNA and the rare form A-DNA.  While B-DNA forms a right-handed helix, Z-DNA shows a left-handed conformation.  This leads to a greater distance (0.77 nm) between the base pairs than in B-DNA and a zigzag form (thus the designation Z-DNA).

Three forms of DNA  A-DNA is rare.  It exists only in the dehydrated state and

differs from the B form by a 20-degree rotation of the perpendicular axis of the helix.  A-DNA has a deep major groove and a flat minor groove (Figures fromWatson et al, 1987).

Three forms of DNA

B -DNA

Z- DNA

A- DNA