DNA structure and function

DNA – is a type of nucleic acid

DNA = deoxyribose nucleic acid. The role of DNA is to pass on genetic information from one generation to the next. It is the chemical that genes are made out of.

DNA- Function

• DNA must be found in every cell (there are exceptions)

• DNA must carry information• DNA must be stable • There are coding and non-

coding regions found on DNA• Coding regions code for genes

(proteins)• Non-coding regions can be

either DNA junk or help regulate protein synthesis

DNA-structure

• A DNA molecule is like a twisted ladder (the double helix). The 2 strands make up a DNA molecule which are the sides of the ladder. These are made up of a phosphate group alternating with a deoxyribose sugar.

DNA continued…

• Connecting the two strands, like the rungs of the ladder, are pairs of bases. There are 4 types of bases in DNA:

A= adenine T = thymineC= cytosine G= guanine

A only bonds with T and C only bonds with G. These pairs are called complementary bases.

DNA

DNA Base pairing rule - A and T pair together - C and G pair together

Nucleotides

• DNA is made up of simple repeating units called nucleotides which consist of:

1) a phosphate group

2) a deoxyribose sugar

3) one of 4 bases (A,G, C, T)

Nucleotides

phosphate

Sugar (deoxyribose)

Base (A, T, C or G)

Chromosomes are found in the nucleus, they are made of DNA. DNA never leaves the nucleus.

Humans have 23 pairs of chromosomes (a total of 46) in every cell in our bodies (except our eggs or sperm – they have a half set of only 23 chromosomes.

Chromosomes are long stretches of DNA, within which lie genes. A single chromosome may contain hundreds of genes.

Below are the locations of some known genes on human chromosomes:

Karyotypes

A karyotype is a picture of the chromosomes in an organism arranged in homologous pairs according to shape and size.

The Karyotype shows:Autosome chromosomes- chromosomes that carry non-sex genesSex chromosomes- Chromosomes that carry sex linked genes

Different living things have different numbers of chromosomes

Genes

Genes are the basic units of heredity in living cells.

They consist of a length of DNA that contains instructions ("codes") for making a specific protein.

Through these proteins, our genes influence almost everything about us, including how tall we will be, how we process foods, and how we respond to infections and medicines.

Genes and alleles

• A gene is a section of DNA that carries the information for a particular protein (characteristic) eg. Eye colour, hair colour, production of a particular enzyme. Now a sequence of bases on a DNA molecule.

• Alleles are different forms of the same gene ie. The gene is eye colour, the alleles are blue, brown and green.

Although most of our cells have the same genes, not all genes are active in every cell. Heart cells synthesize proteins required for that organ's structure and function; liver cells make liver proteins, and so on.

In other words, not all the genes are "switched on" and expressed as proteins within every cell. Within an individual cell, the same genes may be switched on at some times and switched off at other times.

http://www.accessexcellence.org/AE/AEPC/NIH/gene03.html

Different genes are activated in Different genes are activated in different parts of the body different parts of the body

creating the specific proteins that creating the specific proteins that give a particular cell type its give a particular cell type its

character.character...

Tieing it all together Tieing it all together

1. Wrap the 2m of string as tighly and as small 1. Wrap the 2m of string as tighly and as small as possibleas possible

2. Untangle the string and cut into 8 pieces (4 2. Untangle the string and cut into 8 pieces (4 sets of pairs)sets of pairs)

3. Label and name 2 genes on each 3. Label and name 2 genes on each chromosome(1 chromosome per set)chromosome(1 chromosome per set)

4. Write down the base sequence for each 4. Write down the base sequence for each gene.gene.

5. Tweak the base sequence for each gene and 5. Tweak the base sequence for each gene and place this on the other 4 chromosomes in the place this on the other 4 chromosomes in the set. Name these alleles. set. Name these alleles.

Problems with genes

We know a lot about the position of genes on chromosomes by looking at the chromosomes of people with genetic diseases.

Scientists can work out what the gene sequence should be like from healthy people and can see what has gone wrong in someone with a genetic disease. New discoveries are being made often.

The following picture shows human chromosomes 5, 6, 7 and 8 and the positions of genes that we know about so far.

Review Questions:

• 1. Define a gene

• 2. Define an allele

• 3. When may a cell have to divide

• 4. What has to be done prior to a cell starting cell division (there are two things).

Answers

• 1. A gene in a section of DNA/chromosome that code for a particular protein.

• 2. An allele is a different form of the same gene.

• 3. When an organism is growing/repairing or producing sex cells

• 4. DNA must be replicated and the cell must grow in size.

DNA replication

• 1. When would cells need to copy DNA?

• 2. What properties of DNA ensures it is copied accurately?

• 3. Why does DNA need to be copied accurately?

DNA Replication

DNA replicationDNA replicationDNA replicationDNA replication

Chromosomes

Sequence DNA Replication

Put the following sentences into the correct order:•Enzymes add free nucleotides to complement bases on both strands of DNA•DNA is unwound with enzymes•DNA is unzipped between complementary bases with enzymes•Two new strands of DNA are made•This process as known as semi-conservative because each section of DNA is made of one older strand and one newer strand. •6. DNA replication occurs prior to cell division (mitosis and meiosis)

Sequence DNA Replication

• Put the following sentences into the correct order:• 3. Enzymes add free nucleotides to complement bases on

both strands of DNA• 1. DNA is unwound with enzymes• 2. DNA is unzipped between complementary bases with

enzymes• 4.Two new strands of DNA are made• 5. This process is known as semi-conservative because

each section of DNA is made of one older strand and one newer strand.

• 6.DNA replication occurs prior to cell division (mitosis and meiosis)

What is a chromosome?

Genetic materials found inside the nucleus of a cell

Made of protein (histone) and DNA Only seen in a cell during cell division Seen as a mess of spaghetti called

chromatin most of the time.

Each species has a fixed number of chromosomes in the nucleus of each of its cells

Chromosomes always exist in pairs in the body (somatic) cells

Each human somatic cell has 46 chromosomes 23 pairs of homologous chromosomes

Each gametic cell will contain half the number of chromosomes (only one homologous chromosome)

Human gametic cells will have 23 chromosomes

• Members of homologous chromosomes carry same genes

• But the genes on the members of homologous chromosomes may be of different forms

Alleles

Each chromosome will come from one parent.

allele for green eyes

allele for blue eyes

Chromosome forms

Chromosomes can exist in two forms:Replicated and unreplicated

A replicated chromosome has two arms. They are genetically identical and called chromatids

The centromere is a specialised region of the chromosome. It keeps the two chromatids together.

22 pairs are identical in both sexes autosomes

The 23rd pair is different in male and female sex chromosomes

Cell division

Mitosis MeiosisPurpose

To create cells identical to the original cell and to keep the chromosome number the same.Growth and repair

To create gametes (eggs or sperm) that contain HALF the original chromosome number

Where it occurs

In all body cells for growth and repair

In the ovaries in females to make eggsIn the testes of males to make sperm

Cell division - Mitosis

Mitosis occurs in every cell in our body – without mitosis we wouldn’t grow or heal cuts and injuries.

Mitosis is a cell division where the chromosome number is kept the same and two identical cells are produced from one.

Cell division - Meiosis

Meiosis occurs in the ovaries of females to make eggs and the testes of males to make sperm.Two divisions occur, making four cells that have half the number of chromosomes that the original cell had.

Key terms- copy onto describe map

• Somatic cells- non- sex cells, replicate via mitosis.• Gametic cells- sex cells, created via meiosis.• Spindles- proteins that join to chromatids during cell division.• Centrioles- organelles that connects to spindles and

separate the chromatids during cell division.• Haploid- half a set of chromosomes in a cell. Represented

as n. • Diploid- a full set of chromosomes in a cell. Represented as

2n. • Daughter cell- cells that are created as a result of cell

division.

Sequence mitosis and meiosis

Mitosis key terms:•DNA replication •Daughter cells •Genetically identical •Diploid •Spindles •Centrioles•Chromatids •Replicated chromosomes

Meiosis key terms:•DNA replication •Daughter cells •Genetically different •Haploid •Spindles •Centrioles•Chromatids •Replicated chromosomes•Homologous pairs•Independent assortment •Crossing over

Sex Determination

22 pairs are identical in both sexes autosomes

The 23rd pair is different in male and female sex chromosomes

Fertilisation- the process where a sperm and egg fuse together to produce

a single cell called a zygote

Egg23 chromosomes

Sperm23

Zygote 46 chromosomesHaploid (n)

Diploid (2n)

Sex determination

X XX XX XX

Y XY XY

What are the chances of having a boy or a girl???

Everyone has a pair of chromosomes that determine our sex

XX = female XY = maleThe punnett square on the left shows us the probabilities that a man and woman are faced with each time they have a child.

XX = 50%

XY = 50%

If you have had two boys it does not mean you will have a girl next, each time a new zygote is formed the chances or it being male or female are 50 : 50.

Sex determination question

• Describe the combination of chromosomes that determine a male and female

• Complete a punnet square to show the chances of producing either a boy or girl

• Marriam has three boys with Doug. They are expecting another baby to arrive soon. What is the probability of having a girl?

monozygotic (identical) twins

• Twins can occur this way when the zygote splits early on.

• Because the twins come from the same original cell they will be genetically identical

dizygotic ("fraternal") twins

• These twins occur when the mother releases more than one egg during a menstrual period.

• The eggs will be fertilised by different sperm .

• This means they will be genetically different

Yr 11 Quiz

• 1. Define the term allele• 2. Define the term gene• 3. Define the term phenotype• 4. Define the term genotype• 5. State the genotype of a male and female

human• 6. Complete a punnet square with phenotype

ratio to show the chances of offspring being male or female

• 7. A cat has black fur (B)and has a heterozygous genotype. Write down the genotype

• 8. Complete a punnet square to show if two heterozygous black cats bred together. Include a phenotype ratio.

• 9. Define the term haploid• 10. What two mechanisms in meiosis

creates variation?

• 1. A variation of the same gene.• 2. A section of DNA that codes for a particular

protein.• 3. The physical appearance of a trait• 4. the genetic code for a trait made up of two

alleles. • 5. Male = XY, Female =XX

• 6. 1:1 OR 50%:50% X X

X XX XX

Y XY XY

• 7. Bb

• 8. 3:1 OR 75%: 25%

• 9. Half a set of chromosomes

• 10. Independent assortment and crossing over

B b

B BB Bb

b Bb bb

Two new key terms:

• Pure breeding- an individual with a homozygous genotype (either dominant or recessive).

• Carrier- an individual with a heterozygous genotype. Can pass down recessive alleles even though the individual does not express it.

How do we know is an allele is dominant

• Think of the black cat and white cat example:• Breed a black cat and a white cat together. Then

breed two of their offspring together with the same trait.

• If any of their offspring aren’t the same colour as the parents (F1), you know that the colour is dominant.

• This relies on producing many offspring to minimise chance.

A a

a Aa aa

a Aa aa

A a

A AA Aa

a Aa aa

F1 generation (Black cat x White cat)

F2 generation (Black cat x Black cat)

A a

a Aa aa

a Aa aa

a a

a aa aa

a aa aa

F1 generation (Black cat x White cat)

F2 generation (White cat x White cat)

Interpreting Pedigree trees

What information can you gather?

• Which allele is dominant/recessive

• What genotype each individual is (or most).

• What potential offspring may occur between two parents.

Alleles• One of two or more forms of a gene at a given position on a

chromosome. They are caused by a difference in the sequence of DNA.

• A gene which controls eye colour in humans may have two alternative forms – an allele that can produce blue eyes (b), and an allele that produces brown eyes (B). In a plant that occurs in tall and short forms, there may be an allele that tends to produce tall plants (T) and an alternative allele that produces short plants (t).

• The individual genes that form a pair of alleles are located at exactly the same point along a chromosome. There will always be two genes for a characteristic in a cell.

Genotype PhenotypeTT = homozygous dominant TT = tall plant Tt = heterozygous Tt = tall plant tt = homozygous recessive tt = short plant

T

CC

U ur r

y Y

b B

a

t

a

ZZ

AllelesZZ =

homozygous dominant

Uu = heterozygous

aa = homozygous

recessive

Monohybrid crossThe study of single-gene inheritance is done through

monohybrid crosses.- Capital letters represent dominant alleles- Lower case letters represent recessive alleles

e.g. coat colour in guinea pigs

Genotype Phenotype

BB Black

Bb Black

bb white

Homozygous = 2 of the same alleles eg BB or bb

Heterozygous = Bb

Possible fertilisations B b

Bb

A cross between 2 heterozygous black guinea pigs ( Bb x Bb ) expressed as a ratio

Possible fertilisations B b

B BB Bbb Bb bb

Place parents alleles at the top and side of the punnet square

3 black : 1 white

Contemporary Applications of Genetics

• In the exam the questions will be resource based.

• The context will be selected from selective breeding, cloning or genetic modification.

Selective breedingSelective breeding is when we choose animals with good traits for breeding.

Eg. Bulls are chosen to mate with many cows to pass on his good genes.

Selective breeding• Also called artificial selection

The steps for selective breeding are:

• Select the stock or plants that have the best characteristics.

• Breed them with each other.

• Select the best of the offspring and combine them with the best that you already have.

• Continue this process over many generations until you have the desired traits

Selective breeding - Dogs

Cloning• Cloning is the creation of an organism that is an exact

genetic copy of another. This means that every single bit of DNA is the same between the two!

• You might not believe it, but there are human clones among us right now. They weren't made in a lab, though: they're identical twins, created naturally. Below, we'll see how natural identical twins relate to modern cloning technologies.

• How is cloning done?• You may have first heard of cloning when Dolly the

Sheep showed up on the scene in 1997. Cloning technologies have been around for much longer than Dolly, though. The first animal, a tadpole was cloned in 1952. Mice, pigs, cats, and rabbits have also been cloned.

Celebrity Sheep Has Died at Age 6

Dolly, the first mammal to be cloned from adult DNA, was put down by lethal injection Feb. 14, 2003. Prior to her death, Dolly had been suffering from lung cancer and crippling arthritis. Although most Finn Dorset sheep live to be 11 to 12 years of age, postmortem examination of Dolly seemed to indicate that, other than her cancer and arthritis, she appeared to be quite normal. The unnamed sheep from which Dolly was cloned had died several years prior to her creation. Dolly was a mother to six lambs, bred the old-fashioned way.

Image credit: Roslin Institute Image Library, http://www.roslin.ac.uk/imagelibrary/

• Scientists are looking at therapeutic cloning that can be used to generate tissues and organs for transplants.

• To do this DNA would be extracted from the person needing a transplant and inserted into an egg. Once the egg (with the persons DNA) starts to divide, the stem cells that can be transformed into any type of tissue would be harvested. These stem cells would be used to generate an organ or tissue that is a genetic match to the recipient. In theory the cloned organ could then be transplanted without risk of tissue rejection.

Injection of nucleus into egg cell during the cloning process

Blunt end pipette Empty cell

Nucleus put in by sharp ended pipette

Cloning – any

Advantages?

Genetic modification

• This involves moving sections of DNA (genes) from one organism to another so that it produces useful biological products.

• Bacteria is currently used to produce human insulin for diabetes sufferers.

• It also produces human growth hormone for children who aren’t growing properly.

Stages of genetic engineering

• The useful gene is “cut out” by enzymes.

• Particular enzymes will cut out particular parts of DNA

• The DNA of a bacterium is then cut (by enzymes) and the human gene is inserted.

• This splicing of a new gene is again controlled by enzymes.

• The bacteria is then cultivated and soon there are millions of bacteria producing human insulin.

How it occurs

Genetic modification

Comparison

Selective Breeding

• Some people think that it is wrong to manipulate nature.

• Is it right to produce cows that would die if we didn’t milk them, because we have bred them to produce too much milk.

• Is it right to breed pigs with so much meat on them that they have trouble standing up.

• Is it wrong to breed tomatoes or potatoes that are heavy cropping.

Cloning:

• Groups of scientists want to clone human embryos to get replacement tissues and organs for people who need them.

• Using organs from embryos cloned from themselves, would save the lives of people who would otherwise die because they have rejected transplanted organs.

• A lot of people argue that to create a life for spare parts and then kill it is wrong.

• This is the stage that a lot of countries allow abortion at.

Genetic engineering:Genetic engineering: A big problem is the possibility of making A big problem is the possibility of making

designer babies – people may want there child designer babies – people may want there child to be perfect and not carry diseases, need to to be perfect and not carry diseases, need to wear glasses, have the perfect nose etc.wear glasses, have the perfect nose etc.

Changing the genetic make-up of any Changing the genetic make-up of any organisms may effect ecosystems in ways organisms may effect ecosystems in ways laboratory tests can’t predict.laboratory tests can’t predict.

Large seed companies can make money every Large seed companies can make money every year by selling plants that won’t produce fertile year by selling plants that won’t produce fertile seeds, or by producing plants that are resistant seeds, or by producing plants that are resistant to their weed killers.to their weed killers.

G.E. can also be used to produce crops that G.E. can also be used to produce crops that grow in places that they wouldn’t normally and grow in places that they wouldn’t normally and this could save livesthis could save lives

The future • One day in the not-too-distant future, your health care

provider may talk to you about obtaining a single blood sample for DNA analysis, the results of which will be recorded in a computer chip on a wallet-sized plastic card. This card will contain specific aspects of your genetic makeup that can be identified as needed. The genetic information contained there may be used in several ways:

• To predict your risk of developing certain diseases, allowing their earlier diagnosis or possible prevention.

• To more accurately diagnose the cause of symptoms or diseases you may experience.

• To help your health care provider more accurately select the medicine most likely to be of benefit and least likely to cause you harm.

• To help scientists more efficiently discover and develop safer, more effective medicines aimed at the root causes of diseases, not just their symptoms.