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Genetics Notes

Chromosomes and DNA

chromosomeschromosomes

• Structures that contain DNA, look like worms, can be seen during mitosis = chromosomes.

• Chromosomes: made of DNA coiled around protiens.

• Accurate copying of chromosomes during cell division (MITOSIS) is critical.

The structure of Eukaryotic chromosomesThe structure of Eukaryotic chromosomes

Centromere

Chromosome

Sister chromatids

Supercoil within chromosome

Continued coiling within supercoilHistone H1

Nucleosome

DNA

• In DNA, the amount of adenine (A) is always equal to the amount of thymine (T), and the amount of guanine (G) is always equal to the amount of cytosine (C).

• DNA is made of repeating subunits called NUCLEOTIDES.

• three parts: a simple sugar, a phosphate group, and a nitrogenous base.

Phosphate group

Sugar (deoxyribose)

Nitrogenous base

The structure of nucleotidesThe structure of nucleotides

• A nitrogenous base is a carbon ring structure that contains one or more atoms of nitrogen.

• In DNA, there are four possible nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T).

Adenine (A) Guanine (G) Thymine (T)Cytosine (C)

The structure of nucleotidesThe structure of nucleotides

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nucleotide sequencesnucleotide sequences

Chromosome

• The sequence of nucleotides forms a genetic code for an organism.

• The closer the relationship between two organisms, the more similar their nucleotide sequences.

(think CSI)

Replication of DNAReplication of DNA

• Before a cell can divide (mitosis or meiosis), first must make a copy of its

DNA/chromosomes.

• DNA /chromosomes during process called DNA Replication. Or DNA Synthesis

• Without Replication, new cells would only have half the DNA of the parent cell.

• DNA is copied during interphase prior to mitosis and meiosis.

• It is important that the new copies are exactly like the original molecules.

Copying DNACopying DNA

DNA Replication (Synthesis)DNA Replication (Synthesis)

Original DNA

Original DNA

Strand

Original DNA

Strand

Free Nucleotides New DNA

moleculeNew DNA

Strand

New DNA molecule

Think : Watch video clip of DNA replication, then try to narrate with sound off

Replication of DNA

Replication of DNA DNA

Replication

Replication

During each replication cycle, part of the “old” strand of DNA is preserved

• RNA like DNA, is a nucleic acid. RNA structure differs from DNA structure in three ways.

• First, RNA is single stranded—it looks like one-half of a zipper —whereas DNA is double stranded.

RNARNA

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• The sugar in RNA is ribose; DNA’s sugar is deoxyribose.

RiboseRNARNA• Both DNA and RNA contain four nitrogenous

bases, but rather than thymine, RNA contains a similar base called uracil (U).

• Uracil forms a base pair with adenine in RNA, just as thymine does in DNA.

Uracil

Hydrogen bonds Adenine

RNARNA

Comparing nucleic acids

• Double Stranded

• Deoxyribose Sugar

• A T G C

• Only in nucleus

• One kind

• Master Copy

• Single Stranded

• Ribose Sugar

• A U G C

• Can leave nucleus

• 3 kinds (mRNA, rRNA, tRNA)

• Always a copy of DNA, never original

DNA RNA

RNA strand

DNA strand

DNA strand

RNA strand

TranscriptionTranscription

C

During transcription, a molecule of mRNA is manufactured following the DNA code.

DNA Transcription• Steps:

– DNA is “uncoiled and unzipped”

– Free RNA nucleotides are brought in.

– Free RNA nucleotides bind following the same rules as DNA replication (with one exception)

– A-U and G-C

– Ex DNA-AAA TTT GGG CCC

– RNA-UUU AAA CCC GGG

• In prokaryotic cells, which have no nucleus, the mRNA is made in the cytoplasm.

Translation: From mRNA to ProteinTranslation: From mRNA to Protein• Translation: the process of converting the

information in mRNA into a sequence of amino acids in protein.

• Happens at ribosomes (rRNA).

• a ribosome attaches to the mRNA like a clothespin clamped onto a clothesline.

• the role of transfer RNA (tRNA) is to bring the 20 different amino acids dissolved in the cytoplasm to the ribosomes.

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• Each tRNA molecule attaches to only one type of amino acid.

Amino acid

Chain of RNA nucleotides

Transfer RNA molecule

Anticondon

transfer RNAtransfer RNA

• The mRNA codon determines the amino acid that is attached to the tRNA.

Translation Translation

Ribosome

mRNA codon

tRNA

mRNA

anticodon

Amino Acids

1

43

2

• A chain of amino acids is formed until the stop codon is reached on the mRNA strand.

Stop codon

TranslationTranslation

Growing protein

AminoAcids

Peptide bonds

5• Organisms have evolved many ways to protect

their DNA from changes.

Mutations

• In spite of these mechanisms, however, changes in the DNA occasionally occur.

• Any change in DNA sequence is called a mutation.

• Mutations can be caused by errors in replication, transcription, cell division, or by external agents (mutagens).

• Mutations can affect the reproductive cells of an organism.

Mutations in reproductive cells

• If changes happen in the genes or chromosomes of sperm, eggs, or during fertilization, the genes of the offspring will be affected (mutated).

Mutations in reproductive cells

• The mutation may produce a new trait, or result in a protein that does not work correctly.

• Sometimes, the mutation results in a protein that is nonfunctional, and the embryo may not survive.

• In rare cases, a mutation may have positive effects.

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• What happens if powerful radiation, such as gamma radiation, hits the DNA of a nonreproductive cell, a cell of the body such as in skin, muscle, or bone?

• this mutation would not be passed on to offspring.

• However, the mutation may cause problems for the individual.

Mutations in body cells Mutations in body cells

• Damage to a gene may impair the function of the cell.

• When that cell divides, the new cells also will have the same mutation.

• Some mutations of DNA in body cells affect genes that control cell division.

• This can result in the cells growing and dividing rapidly, producing cancer.

• A point mutation is a change in a single base pair in DNA.

• A change in a single nitrogenous base can change a protein because a change in a single amino acid can affect the structure of the protein.

The effects of point mutations

• Examples of point mutations:

– Sickle cell anemia, Tay Sachs disease

The effects of point mutations

Normal

Point mutation

mRNA

Protein

Stop

Stop

mRNA

Protein

Replace G with A

Frameshift mutations• What would happen if a single base were lost

from a DNA strand?

• This new sequence would be transcribed into mRNA. But, the mRNA would be out of position by one base.

• As a result, every codon after the deleted base would be different.

• This would cause every amino acid after the deletion to be changed.

• This is called a frameshift mutation because it shifts the reading frame of codons by one base.

Frameshift mutations

mRNA

Protein

Frameshift mutationDeletion of U

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Chromosomal mutationsA B C D E F G H A B C E F G H

A B C D E F G H A B C B C D E F G H

A B C D E F G H A D C B E F G H

A B C D E F G H

NotesChromosomal Abnormalities

Karyotype: • Chart of chromosome pairs.

• Picture taken during mitosis (metaphase).

• Identifies unusual chromosome numbers or sizes.

• Down Syndrome: (Trisomy 21)• 3 chromosome 21’s• caused by nondisjunction • distinct facial features• mental retardation• reduced life span• 0.12% occurrence

Normal Male Down Syndrome Male

• Klinefelter’s Syndrome:• XXY• male• caused by nondisjunction• low testosterone• tall• no secondary male characteristics• sterility• 0.2% occurrence

Normal Male Klinefelter Male

• Turner’s Syndrome:• X_• female• caused by nondisjunction• sterility• short• slowed sexual development• 0.02% occurrence

Turner femaleNormal Female

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Human Karyotype Lab (nbp 82-83)Purpose: to learn what a normal human karyotpye

looks like and to learn how many chromosomes normal humans have

Pre-lab questions:

Hypothesis: (predict whether your karyotype will be normal, abnormal, male, or female)

MaterialsPartnersProcedure: Follow steps 1-5 from the Karyotype lab 20

1. How many chromosomes do normal humans have?

2. What is a homologous chromosome?

• There must be another form of cell division that allows offspring to have the same number of chromosomes as their parents.

• This kind of cell division, which produces gametes containing half the number of chromosomes as a body cell, is called meiosis.

Why Meiosis?Why Meiosis?

• This pattern of reproduction, involving the production and fusion of haploid sex cells, is called sexual reproduction.

Sperm Cell

Egg Cell

Haploidgametes

(n=23)

Fertilization

Diploid zygote

(2n=46)

Mitosis & Development

Multicellular

diploid adults

(2n=46)

MeiosisMeiosis

MitosisMitosis

Prophase I Metaphase I

Anaphase II

Anaphase I

Metaphase II

Telophase I

Anaphase IITelophase II

During Meiosis, a sperm or egg is formed following this

diagram and the number of chromosomes is reduced by ½

•consists of two divisions,

meiosis I and meiosis II.

• Meiosis I begins with one diploid (2n) cell.

• By the end of meiosis II, there are four haploid (n) cells.

MeiosisMeiosis

• These haploid cells are called sex cells—gametes.

• Male gametes are called sperm.

• Female gametes are called eggs.

• When a sperm fertilizes an egg, the resulting zygote once again has the diploid number of chromosomes.

MeiosisMeiosis

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MeiosisMeiosis

• a source of variation among organisms.

MEIOSIS I

MEIOSIS II

Possible gametes Possible gametes

Chromosome A Chromosome B Chromosome a Chromosome b

Meiosis Use your book (p. 263-267) and the green biology book (p. 82-85) to understand MEIOSIS and answer my questions:

(more realistic diagram p. 267. easiest to draw, p. 270)

1- What is meiosis?2- What kind of cells undergo meiosis?3- Draw a diagram that shows cells undergoing meiosis. You

must include the chromosomes. (start with one cell and end with 4) Best posters get XC

4- How many chromosomes were there at the start of meiosis? How many chromosomes are in each of the 4 new cells at the end of meiosis.

5- after you and a partner completely understand meiosis, explain why meiosis is necessary for sexually reproducing organisms and not for asexually reproducing organisms.

Business 4-20-12• Normal hrs today• STAR test review• to learn:

– punnett squares Genetic Engineering– Evolution Anatomy/physiology

• Notebooks next friday• Problem-Solving lab 11.3 + minilab

11.2 + Karyotype lab = LAB POINTS• DQ’s

Daily questions Unit 3, set 9 (4-20-12)

1. What is evolution?

2. An allele is

3. A dominant allele will..

Answers1- change over time driven by natural selection.

2- one form of a gene

3- always show up in the phenotype (always be transcribed and translated)

The two chromosomes of each pair in a diploid cell are called homologous chromosomes. Each has genes for the same traits.

Homologous Chromosomes

a A

Terminal Axial

InflatedD

Constrictedd

Tall

T

Short

t

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• This fact supports Mendel’s conclusion that parent organisms give one allele for each trait to each of their offspring.

Diploid and haploid cellsDiploid and haploid cells

Chromosome Numbers of Common Organisms

Organism Body Cell (2n)

Fruit fly 8Garden pea 14Corn 20Tomato 24Leopard Frog 26 13Apple 34Human 46Chimpanzee 48Dog 78Adder’s tongue fern 1260

Gamete (n)47

1012

17232439

630

NondisjunctionNondisjunctionMale parent (2n)

Female parent (2n)

Meiosis

Nondisjunction

Meiosis

Nondisjunction

Abnormalgamete (2n)

Abnormalgamete (2n)

Zygote

(4n)

Meiosis Punnett SquaresMeiosis Punnett Squares

The first generationThe first generationDihybrid Cross round yellow x wrinkled green

Round yellow Wrinkled green

All round yellow

Round yellow Round green Wrinkled yellow Wrinkled green9 3 3 1

P1

F1

F2

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• A Punnett square for this cross is two boxes tall and two wide because each parent can produce two kinds of gametes for this trait.

Monohybrid crossesMonohybrid crosses

Heterozygous tall parent

T t

T t

T t

T

t

Heterozygous tall parent

T t

T

t

TT Tt

Tt tt

• The Punnett square shows the possibility of three plants with round seeds out of four total plants, so the probability is ¾ (75%) that the offspring will be round, and ¼ or (25%) that the offspring will be wrinkled.

ProbabilityProbabilityR r

R

r

RR Rr

Rr rr

• Show punnett square video clip

Punnet Square of Dihybrid Cross

Punnet Squares

Title your paper: Punnet Square Practice:Completely answer the following questions1- Cross 2 brown haired parents with the genotypes: Bb and

Bb. Use a punnet square to Determine the genotypic and phenotypic probabilities or %’s of their potential offspring.

2- If D= dark and d=light, Cross a heterozygous dark skinned parent with a light

skinned parent and use a p. square to determine the genotypic and phenotypic probabilities or %’s of their potential offspring.

3- If T=tall and t=short,Cross a Homozygous tall plant with a heterozygous tall plant

and use a p. square to determine the genotypic and phenotypic probabilities or %’s of their potential offspring.